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How does the enzyme nitrogenase reduce the inert molecule N2to NH3under ambient conditions that are so different from the energy‐expensive conditions of the best industrial practices? This review focuses on recent theoretical investigations of the catalytic site, the iron–molybdenum cofactor FeMo‐co, and the way in which it is hydrogenated by protons and electrons and then binds N2. Density functional calculations provide reaction profiles and activation energies for possible mechanistic steps. This establishes a conceptual framework and the principles for the coordination chemistry of FeMo‐co that are essential to the chemical mechanism of catalysis. The model advanced herein explains relevant experimental data.

The complex cis-[PtBr2{PPh2(4-catechol)}2] 1has been synthesized by cleavage of the four methyl groups from cis-[PtCl2{PPh2(4-veratrole)}2] using BBr3, followed by work-up in the presence of excess bromide. An X-ray crystal structure of 1·(ethanol)2confirms that the two catechol rings are adjacent to each other and approximately parallel, and therefore well structured to act as double bidentate ligands for adjacent metal atoms on the surface of a nanocrystal. The crystal packing of 1·(ethanol)2involves intermolecular hydrogen-bonding interactions and a parallel fourfold phenyl embrace between PPh2moieties. Density functional calculations have demonstrated that conformational variability of the aryl rings in cis-[PtBr2{PPh2(4-catechol)}2] is energetically feasible, and two conformations of cis-[PtBr2{PPh2(4-catechol)}2] as a complex ligand for Ti atoms on the various surfaces of the anatase and rutile structures of TiO2have been assessed for geometrical commensurability. Three structural models for adsorbates of cis-[PtBr2{PPh2(4-catechol)}2] on TiO2are developed for anatase (110), anatase (101), and rutile (001).

The crystallisation and crystal structures of three new crystal forms of MePh₃P+I₃− are described, bringing to four the number of polymorphs of this compound. Crystallisations were from alcohols, mixed alcohols, and dichloromethane, and are reproducible. There are strong similarities between all four of the crystal packing arrangements, which are solvent-free. In all structures the MePh₃P+ cations associate in two-dimensional nets using edge-to-face, offset-face-to-face, and methyl-to-face motifs, and the I₃− ions lie between these nets, with subtle variations in structure caused by sliding displacements of relatively constant cation-anion assemblies. The related compound EtPh₃P+I₃− has very similar crystal packing. The interactions between the I₃− and MePh₃P+ are mainly C–H⋯I, with some phenyl faces parallel to I₃−. The major electrostatic component of the lattice energy is associated with the occurrence of I₃− ions against the faces of nets of cations, consistent with this structural feature being the most invariant aspect of the crystal packing, while the structural variability is associated with the less stabilising cation⋯cation motifs. It appears that these crystals provide a good example of a system where shape is not a dominant feature in crystal packing, and that the tetramorphs represent almost equi-ergic local minima in a relatively deep section of the lattice energy hypersurface. The crystal packing of (MePh₃P+)₂I₈2− is similar.

Supramolecular phenomena are determined by energies, but a different property, geometry, is commonly measured, and abundant data are available. The sum of van der Waals radii, the conventional distance criterion in assessing geometrical data, needs to be related to the relevant intermolecular potential, with cognisance of the chemistry of the molecular surface. The relationship between distance distributions in crystals and the distance scale of the intermolecular potential is evaluated, and the relevant concepts (and mis-concepts) are elaborated. The position of the minimum in the intermolecular potential, that is the most stabilising distance, is ca. 0.4 Å larger than the van der Waals distance. In crystals a small number of destabilising contacts less than the van der Waals distance can be enforced by a much larger number of longer stabilising distances. Crystal packing analyses with cut-offs at the van der Waals distance are likely to miss key supramolecular features.

Electrospray Fourier transform mass spectrometry of combinations of the cations MePh₃P+ or Ph₄P+ with the anions [Fe(CN)₆]3− or [Fe(CN)₅(NO)]2− in methanol yields a rich collection of aggregates, mainly with net negative charge. 23 anionic aggregates of MePh₃P+ and [Fe(CN)₆]3− range in size from [(MePh₃P+)₂[Fe(CN)₆]3−]− to [(MePh₃P+)₃₇{[Fe(CN)₆]3−}₁₄]5−, while 23 anionic aggregates of Ph₄P+ and [Fe(CN)₆]3− range up to [(Ph₄P+)₃₈{[Fe(CN)₆]3−}₁₅]7−. The compositions of the anionic aggregates tend towards cation/anion ratios that yield a charge density of −0.15 per component species. Structural principles which may be applicable to these gaseous nanoclusters have been investigated via analysis of the crystal packing in four new crystal structures of salts containing phenylphosphonium cations and cyanoferrate anions which have stoichiometries that span the same cation/anion ratios. They are (Ph₄P)₃[Fe(CN)₆](H₂O)(BuOH)(hexane), (Me₂Ph₂P)₃[Fe(CN)₆](H₂O)₂, (MePh₃P)₂[Fe(CN)₅(NO)](H₂O)₂ and (Ph₄P)₂[Fe(CN)₅(NO)]. In the solids there is general dispersal of oppositely charged ions, consistent with the overall electrostatics, but also numerous and prevalent hydrogen bonds between phenyl C–H of the cations and coordinated CN− (and NO) of the anionic metal complexes. Multiple phenyl embraces, often formed by these cations, are not influential in these crystals. Density functional calculation of the energies of the charge-assisted C–H⋯NC hydrogen bonds (ca. 4 kcal mol−1), and estimates of ion⋯ion electrostatic energies, lead to the general conclusion that in the gas-phase ion-aggregates the net electrostatic energies and the net hydrogen bonding energies are dominant and of similar magnitude. Models of the clusters of molecular ions indicate that their diameters are 3–4 nm for [(Ph₄P+)₁₉{[Fe(CN)₆]3−}₈]5− and 4–5 nm for [(Ph₄P+)₃₈{[Fe(CN)₆]3−}₁₅]7−.

A variety of crystals containing molecules formulated generally as (ArnX)3Y‐Z‐Y(XArn)3with exact or pseudo S6symmetry have been analysed for the presence of intra‐ and intermolecular multiple aryl embraces composed of offset‐face‐to‐face (OFF) and edge‐to‐face (EF) local motifs. Ar is phenyl or substituted phenyl; n=1, 2 or 3; the linkage Z is linear diatomic, linear monatomic, absent (YY bond), or void (Y atoms unconnected). Representative molecular systems analysed are (PhCH2)3Si‐O‐Si(CH2Ph)3, (PhCH2)3Sn‐Sn(CH2Ph)3, (4‐Me‐C6H4CH2)3Ge‐O‐Ge(CH2C6H4‐4‐Me)3, [(PhS)3Fe‐O‐Fe(SPh)3]−, [(PhCH2)3P‐Cu‐P(CH2Ph)3]+and [(Ph3P)3Ag‐CN‐Ag(PPh3)3]+. One characteristic intramolecular motif is a sixfold phenyl embrace (6PE) in which aryl groups fold back to the central domain of the molecule forming a rhombohedral box maintained by (EF)6. When the aryl groups are in exopositions there is an (EF)3motif, like a tractor wheel, at each end of the molecule: [(Ph2N)3Ti‐O‐Ti(NPh2)3] possesses both intramolecular motifs as (EF)3‐(EF)6‐(EF)3. The molecules in this set are hexafaceted, and the crystal packing is generally based on intermolecular EF or OFF motifs with these faces, either from the central (EF)6set or the end (EF)3–(EF)3sets. Three types of subtle crystal packing isomerism occur: 1) the faces of the rhombohedral boxlike molecules slide over each other with variation of the intermolecular motifs, forming dimorphic crystals in space groups R$\bar 3$or P$\bar 1$; 2) the faces of the tractor wheels (EF)3slide over each other or 3) very similar molecules crystallise with the rhombohedral box or tractor wheel structures. In general the molecules considered are shape auspicious rather than shape awkward; solvent is included in a small proportion of the crystals and the crystal packing in these compounds is evidently efficient. Some principles for elaboration of these systems and the design of molecular crystal lattices are considered.

The metallocarbohedrenes are binary molecular clusters containing metal atoms linked by acetylenediide C2 groups. Hundreds of these molecules have been generated, detected and reacted in the gas phase since the prototype, [Ti8(C2)6], was reported in 1992, but none has yet been synthesised pure in bulk: the time gap between detection and preparation increasingly exceeds that of the fullerenes. We report here the results of density functional calculations of geometrical and electronic structure of more than 150 postulated metallocarbohedrenes, stabilised by terminal ligation, in order to recognise the more electronically favourable and less reactive targets. At least 38 metallocarbohedrenes have been identified as having a spin singlet ground state, with a relatively large (> 0.5 eV) energy gap between HOMO and LUMO, and an appropriate HOMO energy. In addition, a considerable number of electronically stable metallocarbohedrenes are predicted to have highly paramagnetic ground states, potentially useful in molecular magnetism. The geometrical principles for enclosing but unstrained coordination of metal sites by terminal ligands are outlined. Mechanisms for rational syntheses are considered in the context of reaction type and precursor selection, including issues of oxidation and reduction, and kinetic versus thermodynamic control. This leads to many diverse reactions suggested for the rational syntheses of metallocarbohedrenes. Some preliminary experimental results are presented.

The radical ion series (SnO)+2-6, (SnO)−2-6, (SnO)0-5Sn+and (SnO)1-6O−have been generated by the high power laser ablation of SnO and SnO2targets positioned inside an ICR cell. In all ablation spectra obtained, and for any particular size Snxcore, the tin-rich clusters (SnO)xSn+were more abundant than the corresponding oxygen-equivalent clusters (SnO)+x, while the oxygen-rich clusters (SnO)xO−were always more abundant than the oxygen-equivalent clusters (SnO)−x. High yields of the ions (SnO)1,3Sn+, (SnO)3,6O−and (SnO)−6suggest high stabilities for these species. Low energy CID studies revealed that loss of neutral (SnO)xunits is the preferred, and for most ions investigated the exclusive, dissociation pathway. Global minima for the smaller cations and anions are proposed on the basis of local density functional theory (DFT) calculations. Calculated dissociation energies for the neutral and charged clusters were found to compare well with effusion cell and FTICR results. DFT also predicts that, for any cluster with the same size Snxcore, IE(SnO)xEA(SnO)x. A correlation between ion abundances and DFT heats of formation is evident, and the ground state geometries provide insight into the evolution of structural versus size trends. Without assistance from the calculations, erroneous conclusions regarding the structures of the experimentally-sampled clusters might have been drawn from the low energy CID results.

Mixtures of [Ph3PNPPh3]+Cl−with CuBr2(or CuBr2+CuCl2) in ethanol/dichloromethane yield crystals containing three‐coordinate copper(II) with mixed chloride and bromide ligands, namely [Ph3PNPPh3]+[CuCl0.9Br2.1]−(1) and [Ph3PNPPh3]+[CuCl2.4Br0.6]−(2). The trigonal‐planar coordination of copper(II) is angularly distorted but unambiguous, as there is no other halide ligand within 6.7 Å of the copper atom. Density functional theory (DFT) calculations on planar [CuClBr2]−show that the energy surface for angle bending is very soft. Crystallisation in the presence of CH3CN yields [Ph3PNPPh3]+[CuCl0.7Br2.3(NCCH3)]−(3), in which there is additional secondary coordination by NCCH3(Cu−N 2.44 Å). DFT calculations of the potential energy surface for this secondary coordination show that it is remarkably flat (<3 kcal mol−1for a variation of Cu−N by 0.8 Å). The crystal packing in 1, 2and 3, which involves multiple phenyl embraces between [Ph3PNPPh3]+ions and numerous C−H⋅⋅⋅Cl and C−H⋅⋅⋅Br motifs, is associated with intermolecular energies that are larger than the variations in intramolecular energies. For reference, the crystal structures of [Ph3PNPPh3+]2[Cu2Cl6]2−(4) and [Ph3PNPPh3+]2[Cu2Br6]2−(5) are described. We conclude 1) that three‐coordinate copper(II) with monatomic halide ligands, although uncommon, can be regarded as normal, 2) that steric control by ligands is not necessary to enforce three‐coordination, 3) that a hydrophobic aryl environment stabilises [Cu(Cl/Br)3]−, and 4) that the energy change in the transition from three‐ to four‐coordinate copper(II) is very small (ca 5 kcal mol−1).

Molecules of the type Ph₃XYXPh₃ commonly crystallise in trigonal and rhombohedral lattices comprised of hexagonal arrays of sixfold phenyl embraces (HA6PE) which are bimolecular concerts of edge-to-face (EF) phenyl···phenyl interactions. In this paper we describe crystal structures containing molecules Ph₃XYXPh₃, also crystallising in the space group R3&cmb.macr; characteristic of the HA6PE lattice, but without the bimolecular (EF)₆ form of 6PE. Instead there are multimolecular concerts of EF and offset-face-to-face (OFF) motifs, building up hexagonal layers and in some cases generating high symmetry cavities. These crystals possess the same molecular symmetry as the HA6PE systems, and crystallise in the same space groups, but have different intermolecular motifs: they are effectively crystal supramolecular isomers of the HA6PE lattice. The specific crystalline compounds analysed are Ph₃SiOSiPh₃·C₆H₆, Ph₃SiNCNSiPh₃ and Ph₃PNPPh₃ +[Nb(CO)₆]−. The crystal packing in (MePh₃P)₂[Cu₄I₆], and in a high symmetry Ni₃ complex crystallised with Ph₃PO, both in space group R3&cmb.macr;c also characteristic of the HA6PE lattice, are supramolecular isomers of the HA6PE lattice because the bimolecular motif is (OFF)₃ rather than (EF)₆. The crystal packing analyses benefit by recognition of the asymmetric sectors of the intermolecular interactions and of cavities, in addition to the conventional asymmetric sectors of the molecular components.

The majority of molecular crystals in the Cambridge Structural Database involve relatively low symmetry molecules, engaged by intermolecular interactions with restricted symmetry, resulting in the preponderant occurrence of a small number of relatively low symmetry crystallographic space groups as expounded by Brock and Dunitz. We are examining some contrasting crystal structure types that contain higher symmetry molecules involved in high symmetry intermolecular motifs and generating high symmetry space groups. This paper considers molecules containing XPh₃ groups, engaged in multiple phenyl embraces with local S₆ (3&cmb.macr;) symmetry, generating crystal lattices with cubic symmetry. The conceptual framework for these analyses also involves the presence, symmetry, and occupancy of cavities in high symmetry lattices, with extensions to crystal trapping of unusual species such as [Cu₂Br₅]2− and [Cu₈Br₁₅]6−. The compounds included in the analyses of crystal packing and symmetry in this paper are (MePh₃P+)₂[Cu₂Br₅]2−, Ph₃POGaCl₃ , Ph₃AsI₂ , N(Ph-2-OMe)₃ , Ph₃SnCl, Ph₃PAuCCAuPPh₃·(C₆H₆)₂ , Ph₃PAu(B₆H₆)AuPPh₃ , (Ph₃PH+)₂[SnCl₆]2−, and (Ph₃MeP+)₆ [Cu₈Br₁₅]6−. Concerts of edge-to-face (EF) and offset-face-to-face (OFF) local interactions between phenyl groups generate bimolecular (EF)₆ = 6PE with symmetry S₆ , hexamolecular (EF)₆ with symmetry S₆ , trimolecular (EF)₃ , hexamolecular (OFF)₆ , and two-dimensional {(OFF)₆}_∞ . Crystal supramolecular analysis includes description of the asymmetric segments of intermolecular motifs, as well as the conventional asymmetric sector of the molecule.

The 1,10-phenanthroline (phen) ligand in metal complexes commonly forms offset face-to-face (OFF ) motifs, and less frequently edge-to-face (EF ) motifs. An investigation of the 335 M(phen) complexes, 159 M(phen)₂ complexes, and 33 M(phen)₃ complexes in the Cambridge Structural Database has revealed that in crystals these primary OFF and EF motifs combine to form concerted motifs, and extended motifs, in a variety of ways. Stacks of phen ligands engaging OFF motifs on both faces are very common for complexes M(phen) and M(phen)₂. Even more common for M(phen)₂ are zigzag chains in which each phen ligand links to neighbours in the chain with an OFF motif. The parallel fourfold aryl embrace (P4AE, comprised of one OFF and two EF ) occurs for complexes M(phen)₂ and M(phen)₃, with some variety in geometry between a single OFF at one extreme to (EF )₂ at the other. This variability in the P4AE is a consequence of the larger surface area of the phen ligand, compared with those of 2,2'-bipyridyl (bipy) ligands or phenyl groups (which also form this motif ), and has been evaluated by calculations of the supramolecular attractive energies. The P4AE associate further, maximising the use of phen surfaces, to form chains of P4AE, chains of P4AE·OFF, and compact two-dimensional nets propagated by both P4AE and OFF motifs. There are examples of three-dimensional nets using these motifs. There is a notable absence of sixfold aryl embraces (6AE, comprised of concerted (EF )₆) amongst M(phen)₃ complexes, and a clear difference with M(bipy)₃ complexes where 6AEs are prevalent. Various M(phen)₂ and M(phen)₃ complexes pack in crystals to form tight hydrophobic domains, often as slabs, segregated from hydrophilic domains containing hydrogen bonding components and anions. The crystallisation and crystal packing of [Co(phen)₃][BF₄]₂·H₂O·EtOH, which exemplifies this pattern, are reported. The implications for crystal engineered enantioselection by [M(phen)₃] complexes are discussed.

Laser ablation of CoP generated 152 binary anionic cobalt phosphide clusters [Co_xP_y]^-, ranging up to [Co₂₅P₁₆]^-. This is a more extensive set of [Co_xP_y]^- clusters than obtained in previous laser ablation of a mixture of Co metal and red phosphorus: the composition maps for the two experiments overlap, but with generally lesser y:x ratios for laser ablation of CoP. The reactivities, reactions, and collisional dissociation of selected ions have been investigated by Fourier transform ion cyclotron resonance mass spectrometry. The majority of the [Co_xP_y]^- ions react with H₂S by addition of (up to three) S atoms, substitution of P₂ by S, and with elimination of HP₂^-. Reaction with NO₂ or N₂O causes addition of one O atom. Collisional activation causes dissociation of P₂, P₄, CoP₂ or CoP₄. The ions [CoP₈]^-, [Co₄P₈]^-, [Co₅P₉]^- and [Co₆P₁₀]^- are unreactive, and for these and the reactive [Co₄P₄]^- ion, density functional investigations of 30 postulated structures have been performed. The probable structures, calculated electron affinities, and electronic structures are reported. The structural principles evident so far are that P atoms, P₂ groups, and P₃ groups bridge the faces, and to a lesser extent the edges, of Co_x polyhedra. The lack of reactivity for relatively P-rich clusters correlates with the absence of co-ordinatively unsaturated metal sites on the clusters: it is postulated that the more reactive species undergo addition at under-co-ordinated Co atoms. The calculated electronic structures generally have close-lying electronic states with unpaired electrons, which explains the overall high reactivity of the [Co_xP_y]^- clusters. The structural differences between these [Co_xP_y]^- clusters and characterised molecules Co_xP_yL_z with ancillary ligands are discussed, as are possible applications of the new [Co_xP_y]^- clusters in synthesis of novel materials.

Supramolecular aggregates of tetraalkylammonium halides (R4NX) are formed by electrospray out of acetonitrile solution. Mass spectrometry reveals 88 charged aggregates for R=Me, Et, Bu; X=Br, I, ranging up to [(Bu4N)39Br42]3−in size. With the objective of improving calculations of intermolecular energies for supramolecular aggregates of ions, calibrated semi‐empirical potentials for inter‐ion interactions have been developed and applied to these aggregates. The accuracy of the calculated energies is supported by the measured collisional dissociation energy of (Et4N+)4(I−)5. Energy optimisations indicate that the probable structures have the halide ions dispersed in a matrix of cations, which, for Bu4N+, can be mutually attractive. The aggregates are structurally fluid, with multiple structures separated by 4–8 kJ mol−1. The energy calculations are entirely consistent with the observed formation of large aggregates, and of multiply charged anions. It is estimated that the cohesive energies of supramolecular assemblies of ions such as these reach about 40 kJ mol−1per constituent ion.

The crystal structures of [Ph₄P]₂[M(mnt)₂] (M = Zn 1, Cu 2 or Ni 3), [Ph₄P][M(mnt)₂] (M = Cu 4 or Ni 5), [Ph₄P]₂[{M(mnt)₂}₂] (M = Co 6 or Fe 7), [Ph₄P]₂[M(mnt)₃] (M = Fe 8 or Cr 10) and [Ph₄P]₃[Cr(mnt)₃] 9 {mnt = maleonitriledithiolate, [S₂C₂(CN)₂]²⁻} have been determined to investigate the packing and supramolecular motifs in crystals containing [Ph₄P]⁺ cations and [M(mnt)_n]^z− anions. Seven different crystal packings are described. Crystals 2 are isomorphous with 3, as are 4 with 5, and 6 with 7, and two dimorphs of 8 were identified (orthorhombic α-8 and monoclinic β-8). The lattices of all compounds with planar (2, 3, 4, 5) or approximately planar (6, 7) anions contain segregated parallel stacks of anions (canted to the stack axis) and columns of embracing [Ph₄P]⁺ cations. The structures of 2 and 3 contain approximately linear sequences of alternating sixfold phenyl embraces (6PE) and offset-face-to-face (OFF ) motifs between cations, while the structures of 4, 5, 6 and 7 contain linear chains of cations engaged in translational fourfold phenyl embraces (4PE). Where the anions are not planar (tetrahedral 1 or octahedral 8–10), OFF interactions between planar subsections of the anions (i.e. mnt ligands) and planar subsections of the cations (i.e. phenyl rings) occur, as a concerted motif involving four ions (1), or as a new sandwich motif (α-8, β-8). This sandwich motif, Ph  · · ·  mnt  · · ·  Ph, occurs for one mnt ligand per anion in α-8, and two mnt ligands per anion in β-8. Calculated energies are reported. Weak C–H  · · ·  S and C–H  · · ·  N hydrogen bonds occur in all structures, and appear to control the details of the commensurability between chains of cations and stacks of anions. Edge-to-face (EF ) motifs between phenyl rings and mnt ligands do not occur. All crystals except one (9) are devoid of solvent, implying an absence of shape awkwardness in crystals of this type. The crystal packing of the related compound [MePh₃P][Ni(mnt)₂] contains columns of stacked parallel fourfold phenyl embracing cations, parallel to stacked anions.

Examination of the Cambridge Structural Database reveals that Ph₄P⁺ cations and PPh₃ ligands in crystals associate through phenyl  · · ·  phenyl non-bonded interactions which are attractive, concerted, and widespread. The strongest and most prevalent interaction is the energetically favourable sextuple phenyl embrace (6PE) in which there are six concerted edge-to-face (EF ) phenyl–phenyl interactions. We have questioned the extent to which substitution on one or more phenyl rings would interfere with XPh₃ as a 6PE tecton. Substitution in the 4-position of Ph does not interfere with the formation of 6PEs. Substituents in other positions can be out of the interaction domain and not affect the 6PE, or if small like methyl they can form 6PE in which some of the EF motifs are replaced by methyl-to-face (MF ). In some cases the substitution of Ph destroys the 6PE. Perfluorination, in (C₆F₅)₃PPtCl₂PtP(C₆F₅)₃, does not obstruct occurrence of chains of intermolecular 6PE. Hetero-molecular 6PE occur, and could be deployed for generation of acentric crystals. The cation (o-NH₂C₆H₄)Ph₃P⁺ can form chains of embraces analogous to the ZZI6PE, which is prevalent for Ph₄P⁺.

The sixfold phenyl embrace (6PE) is manifest in the crystal structures of approximately 30% of the 5089 molecules with PPh₃ ligands (in the Cambridge Structural Database). This paper describes the characteristics and occurrence of the 6PE as a crystal supramolecular motif between metal complexes and clusters with PPh₃ ligands. The number of PPh₃ ligands per molecule, the coordination stereochemistry, and possible crowding of the PPh₃ ligands are included in the analysis. Molecules with multiple PPh₃ ligands can and do form one-, two-, and three-dimensional supramolecular networks constructed with these multiple phenyl embraces. We portray a representative selection of the resulting tectonic lattices based on 6PEs. Variants on the 6PE include a pseudo-embrace in which gold takes the place of a phenyl ring.

We report an analysis of the M–PPh₃ groups in the Cambridge Structural Database. About 60% of the 8663 characterised M–PPh₃ groups do not exhibit the conformation of a threefold rotor. The M–PPh₃ moiety occurs with a variety of conformations for the phenyl rings, ranging from the expected threefold rotor, through conformations where the phenyl rings are parallel or orthogonal to the P–M vector, to conformations where two rings have approximately equal and opposite M–P–C–C torsional angles and are like a flipper rather than a rotor. There are two sub-groups of flipper, in which the third phenyl ring is either approximately parallel or orthogonal to the P–M vector. The rotor conformation of M–PPh₃, and both forms of the flipper conformation, are capable of forming the sixfold phenyl embrace (6PE), in which all six phenyl rings participate, but only the rotor-6PE possesses the concerted cycle of six local edge-to-face Ph  · · ·  Ph interactions. The occurrence of fourfold phenyl embraces in M–PPh₃ compounds is also investigated. Calculated supramolecular energies are presented: a pair of embracing PPh₃ ligands has a strong supramolecular attractive energy, ranging from −11 to −5 kcal mol⁻¹.

In order to understand better the supramolecular chemistry of fluorous molecules, and particularly coordination complexes and organometallic molecules with perfluoroaromatic substituents, the relevant intermolecular potentials have been parametrised. It is shown that density functional calculations using the Perdew–Wang PWC functional and numerical basis sets adequately describe weak intermolecular attractive energies [e.g. (C₆H₆)₂], but that gradient corrected BP or BLYP functionals do not. Due to a dearth of experimental data on intermolecular energies for perfluoroaromatic systems, these density functional calculations of the supramolecular isomers of (C₆F₆)₂ are used to parametrise the Lennard-Jones plus electrostatic potential for fluoroaromatic and derivative molecules. Intermolecular energies for local interactions in the crystal structures of C₆F₆, C₆F₆·C₆D₆, and octafluoronaphthalene are reported. Strong directionality is not observed in the supramolecular forces between fluoroaromatics and the electrostatic components of the intermolecular energies are small, &lt;15% of the total; the polarisation of the C–F bond is also less than expected from electronegativity differences. Perfluorinated molecules and ions such as [B(C₆F₅)₄]^- engage in multiple phenyl embrace motifs, comparable to those of phenylated molecules, despite the reversed polarity of aromatic C–F relative to C–H bonds (and the opposite quadrupole moments of C₆F₆ and C₆H₆). Examples of the sixfold perfluorophenyl embrace (6P^FE), the hexagonal array of sixfold perfluorophenyl embraces (HA6P^FE), and fourfold perfluorophenyl embraces (4P^FE) are described. The intermolecular attractive energies of some of these motifs comprised of concerted edge-to-face and offset-face-to-face interactions are in the range -5.5 to -11.7 kcal mol^-1. It is concluded that there are no major differences between the supramolecular motifs and energies of perfluorinated- and hydroaromatics.

Analysis of the 752 crystals in the Cambridge Structural Database containing [Ph₃PNPPh₃]⁺ ([PPN]⁺) cations has revealed mutually attractive interactions between the cations leading to the formation of supramolecular motifs. The cation is flexible due to P–N–P bending: calculation of single point energies for an idealised [H₃PNPH₃]⁺ fragment show a flat energy well for the cation, with there being less than 1 kcal mol⁻¹ difference in energy between P–N–P angles of 130–180°. The most populated conformation is that with a P–N–P angle in the range 140–145°. The types of inter-cation interaction can be classified by a combination of the N–P  · · ·  P angle, the N–P  · · ·  P–N torsion angle and the four intermolecular P  · · ·  P distances between neighbouring cations. Interactions with an N–P  · · ·  P angle of greater than 125° indicate a sixfold phenyl embrace (6PE), whilst those at more acute angles form expanded phenyl embraces with neighbouring cations either parallel (PEPE) or orthogonal (OEPE). The PEPE and OEPE are differentiated by torsion angle or intermolecular P  · · ·  P separations. Computation of the energies of attraction between the cations gives values in the ranges 7.0–10.5, 7.9–11.1 and 8.6–13.0 kcal mol⁻¹ for the 6PE, OEPE and PEPE respectively. The individual embraces combine to form zigzag chains of cations leading to either columnar or layered structures. The crystal lattice formed does not depend on anion size or charge.

The positive- and negative-ion electrospray spectra of methanolic solutions of a water-soluble platinum (II) phosphine complex, chlorotris(1,3,5-triaza-7-phosphaadamantane)platinum (II) chloride, have been shown to contain a series of platinum (II) cations and anions. Mono-platinum ions such as [Pt(TPA)3Cl]+and [Pt(TPA)2Cl2]Cl−(TPA = 1,3,5-triaza-7-phosphaadamantane) were the most abundant ions but poly-platinum ions were also observed. The ions have been subjected to two different dissociation experiments and the results for skimmer and on-resonance collision-induced dissociations compared. Substitution reactions such as halide exchange and phosphine exchange have been monitored in solution. The gas-phase reactions of hydrogen sulfide with selected ions have been studied.

By analysis of crystal packing we have identified a crystal supramolecular motif that is a two-dimensional net of terpy embraces formed by metal complexes [M(terpy)2]2+ (terpy=2,2′:6′,2″-terpyridyl) and similar meridional [M(N3-tridentate)2] complexes. The terpy embrace involves two complexes attracted by one offset-face-to-face (off) and two edge-to-face (ef) interactions by the outer pyridyl rings of the ligand. In many crystals containing small monoanions there is a two-dimensional net of these embraces, in which each complex forms eight ef and four off interactions with its neighbours. The principal axes of the complexes are normal to the layer, which is exactly or approximately planar, and can occur with high (tetragonal) or low crystal symmetry. Grooves that occur on the layer surfaces, formed between parallel central pyridyl rings of the ligands, run in orthogonal directions on the two surfaces of each layer. Anions and solvent molecules in the crystals are usually disordered, in or near the grooves. The net attractive energy of the terpy embrace for a pair of [M(terpy)2]2+ is calculated to be ca. 15 kJ mol-1: in the two-dimensional net the attractive cation···cation energy per cation is ca. 29 kJ mol-1. Inclusion of the anions associated with one layer increases the attractive energy per [M(terpy)2]2+ to the order of 130 kJ mol-1. A variety of ligands, which are minor or major modifications of terpy, also form this supramolecular motif. Hydrogen bonding involving NH functions of these ligands, solvent, and/or anions, does not in general disrupt the motif. In one instance where the [M(N3-tridentate)2] complex is uncharged there is mutual interpenetration of contiguous layers. These infinite two-dimensional nets of octahedral metal complex sites formed as crystal supramolecules are analogous to the two-dimensional gridlike supermolecules formed by extended oligo-chelating ligands. Opportunities for crystal engineering are discussed.

Carbon anions Cn- (n=4–13, 15–25 odd values only), generated by laser ablation of graphite or petroleum coke and isolated by Fourier transform ion cyclotron resonance mass spectrometry, have been reacted with the sulfur-containing molecules H2S, C6H5SH and S8. The Cn- are unusually reactive with these molecules and generate anions of the types CnS-, CnS2-, HCnS-, CnH3S-, CnC6H5SH2-, and C4S3-. The structures of CnS-, CnS2- and HCnS- (n=4–9) have been investigated by density functional calculations; the lowest energy isomers have linear structures with the hetero atom(s) at the end(s) of the chain. C4S3-,0 are linear SC4S1S species with a single S1S bond. The thioformyl molecule HCS, recently proposed to be involved in the collision of the comet Shoemaker–Levy with Jupiter, is believed to be a product in the reaction of C9- with H2S.

Stable carbon phosphide anions have been prepared by the reaction of small carbon anions (Cn−n= 3–9) with gaseous P4. The Cn−anions, produced by laser ablation, were trapped and reacted in a Fourier transform ion cyclotron resonance mass spectrometer. The compositions of the anions formed were CnP, CnP2, C4P4, CnP5. All the anions for n= 3–9 are covered in this paper and have been investigated theoretically using density functional calculations, incorporating gradient corrections and numerical basis sets. The [CnP]−and [CnP2]−anions are calculated to be linear, with the P atoms at one and both ends of the Cnchain, respectively. The [C4P4]−anion is the only ion containing four P atoms, and never has more than minor intensity; three structures close in energy are calculated for [C4P4]−, with the most stable possessing a CP4mononocycle at the end of the C4chain. The [CnP5]−anions observed when n= 4, 6 or 8 have a lowest energy structure with one phosphorus atom at one end of the chain and a P4ring at the other end.

Multiple phenyl embraces are concerted supramolecular motifs based on intermolecular phenyl–phenyl attractions, mainly in the edge-to-face (ef) geometry. Variations of the six-fold phenyl embrace (6PE) and higher order multiple phenyl embraces involving eight or twelve phenyl groups are described. They occur in crystals of metal complexes with three or four Ph3 ligands. The enlarged 6PE (E6PE) involves phenyl groups from three Ph3X ligands in each of two molecules, and is formulated generally as (XPh)3···(PhX)3: the H6PE, or (XPh)3···(Ph3X), is a hybrid of the E6PE and the 6PE [which is (XPh3)···(Ph3X)]. The 8PE uses {M(XPh2)2}···{(Ph2X)2M} and occurs in two variants according to whether the XMX planes on the two molecules are approximately parallel (P8PE) or orthogonal (O8PE). Molecules [M(PPh3)3]z can form a 12PE, which uses {M(XPh2)3}···{(Ph2X)3M} and includes ten intermolecular ef interactions. The crystal packing in various compounds containing [M(PPh3)3]z or [M(PPh3)4]z is described, and involves either multiple instances of one embrace type in high symmetry crystal lattices or combinations of various multiple phenyl embraces in lower symmetry crystal lattices. The current hierarchy of multiple phenyl embraces is summarised.

The inclusion compounds of 2,8-dimethyltricyclo[5.3.1.13,9]dodecane-syn-2,syn-8-diol,3, with ferrocene and with squalene have been prepared. The crystal structures of these helical tubulate compounds: (3)3·(ferrocene)0.75 [P3121,a=b=13.7480(6),c=7.0312(5) Å,Z=1,R=0.038] and (3)3·(squalene)0.23 [P3121,a=b=13.677(1),c=7.0533(9) Å,Z=1,R=0.042] are described.

The discovery thatexo-2,exo-6-dihydroxy-2,6-dimethylbicyclo [3.3.1]nonane (2) forms novel multimolecular canal-type inclusion complexes has led to a systematic study of related molecular structures. This synthetic programme has shown that diol (2) is the prototype of a family of host diols all with the same crystallographic space groupP3121, but with distinctly different canal shapes and dimensions. The structures of the first four members of this new family of hosts are described and contrasted.

Alicyclic oil 's of specific design crystallise with a lattice enclosing a series of parallel helical canals which trap a wide range of guest molecules. Typical structures of these helical tubulate compounds are presented. The canal sizes and geometries vary considerably depending on the actual diol utilised. Lattices with smaller canals retain the helical tubuland structure in the absence of guests, whereas those with larger canals collapse to a close-packed structure.

In a further exploration of the structural factors causing alicyclic diols such asexo-2,exo-6-dihydroxy-2,6-dimethylbicyclo[3.3.1]nonane (1) to adopt the helical tubuland inclusion host structure, the title compound (8) has been synthesized and its crystal structure determined. This 9-oxa analogue of (1) has a totally different structure [C10H18O3; orthorhombic;P21212;a 18.989(4),b 19.064(3),c 14.256(3) Å;Z 20; with finalR 0.065] due to involvement of the ether oxygen atom in the hydrogen bonding network. The structure of the diol (8) approximatesP¯421c symmetry, and 16 of the 20 molecules per cell create a hostlike structure of this symmetry, which consists of tightly bound hydrogen bonded pillars parallel toc. The remaining guestlike molecules occupy inequivalent pseudo ¯4 sites separated byc/2 to form weakly bound columns parallel toc. Hydrogen bonds also occur between the pillars and columns. An ordered structure with sensible intermolecular contacts can be formed inP212121 with thec axis doubled. A difference betweena andb axial lengths correlates with a difference in occupancies of pseudo-¯4-related sites for the guest-like component of the structure.

The X-ray crystal structures of helical tubulate inclusion compounds of the diol host 1 reveal considerable variation in canal size depending on the specific guest entrapped. Examination of this data reveals how such flexibility is achieved through changes in the host orientation and hydrogen bonding angles. The sum of these effects makes 1 a potent inclusion host for a wide range of small molecules and depends little on the guest functional group. Representative helical tubulate stoichiometries, packing, and interactions are explored. The guest-free diol exists with the same lattice type as an empty microporous solid.

A new family of co-crystalline materials formed between helical tubuland diols and phenols has been discovered. Preliminary details of the X-ray structures of (1)(p -chlorophenol), (2).(p -methoxyphenol), and (1).(hydroquinone)0.5 are presented, and the crystal engineering aspects of these compounds are discussed.

Questions about the geometrical structures, electronic structures and stabilities of [MnxSy]- clusters observed in the gas phase have been approached using density functional calculations of electronic structure and energy, optimised geometrical structure, and electron affinity. Results are presented for 61 postulated isomers for 23 different compositions ranging in size from [MnS2] to [Mn15S15], and general structural types and principles are identified. Structures based on stacks of Mn3S3 triangles are thermodynamically favourable, as are regular Mnx polyhedra with (3-S) caps. Structures with doubly-bridging S are frequently favourable, but there is no evidence that (4-S) capping confers extra stability. Trigonal MnS3 local co-ordination is desirable. Bonding between Mn atoms occurs throughout, at distances from ca. 2 Å in the smallest underco-ordinated structures up to ca. 2.5 Å. Electronically these clusters are characterised by a band of closely-spaced bonding molecular orbitals, composed mainly of Mn 3d atomic orbitals, at and below the Fermi level. The HOMO–LUMO gaps are generally very small, and low-lying electronic states with associated magnetic and optical properties are expected. The electron affinities generally increase with cluster size, up to ca. 3.5 eV.

Laser ablation of solid MnS yields in the gas phase polysulfane cations [MnSn]+ (4  n  11), and anionic clusters [MnxSx]- and [MnxSx+1]- (2  x  22), together with clusters [MnxSxO]- (4  x  22) formed from adventitious water. These ions were detected and investigated further by Fourier-transform ion cyclotron resonance mass spectrometry. There are no major discontinuities in abundance or especially stable structures in the series [MnxSx]- and [MnxSx+1]-. There are some differences in the distributions of ions formed by the pink and green forms of solid MnS, with the pink form generating more sulfur vapour and polysulfane cations, and the green form more suitable for the formation of cluster anions. The cations [MnSn]+ lose S2 on collisional activation. The cluster anions [MnxSy]- are generally unreactive and not oxidised by N2O, but on reaction with H2S (g) they undergo protonation or add one or two S atoms to form also the new products [MnxSx+2]- (4  x  8). The compositions [MnxSx]- and [MnxSx+1]- represent a well defined region of stability for anions, and for the neutral clusters from which they are believed to be formed by electron attachment.

The crystal structures of five compounds with phosphonium cations and bromocadmate anions have been determined, and analysed in terms of the supramolecular interactions which determine the lattice types. The salts [MePh3P]2[CdBr4] and [MePh3P]2[CdBr4]·CH2 Cl2 crystallise in a large rhombohedral lattice in which dimers of cations in sextuple phenyl embraces (SPE) form a pseudo-diamondoid array, with the anions (and CH2Cl2) contained in cavities surrounded by 12 cations. The same hexagonal array of sextuply embracing cations occurs also in [MePh3P]2[CdI4] and [MePh3P]2[Cu4I6], with the dimensions of the array of cations quite variable according to the size and shape of the anion. This newly recognised lattice type (HASPE, hexagonal array of sextuple phenyl embraces) is maintained principally by the strongly attractive SPEs and by attraction between the anion and surrounding cations. The HASPE is a crystal structure type with general occurrence. The cations in the crystal structures of [Ph4P]2[CdBr4] and [Ph4P]2[Cd2Br6] occur as zigzag infinite sextuple phenyl embraces (ZZISPEs), chains of cations which are orthogonal and parallel respectively. Determination of the crystal structure of [HPh3P]2[CdBr4] reveals an absence of specific cation–cation interactions.

In order to be able to predict and then synthesize new lattice inclusion compounds with the helical tubuland diol host structure, the hydrogen bonding modes of a range of bicyclic and tricyclic diols have been studied using X-ray crystallography. Several distinct types of lattice structure have been recognized. Steric factors play a major role in determining which of these is produced in a given case. Establishment of a series of structural rules provides a window of opportunity for duplication of the helical tubuland host lattice by deliberate design and synthesis. New inclusion compounds resulting from this approach are presented.

A large number of new and unexpected molecules, fundamentally inorganic and containing just metal and sulfur, or metal and carbon, can be synthesised in the gas phase, and their reactivities investigated. In the absence of bulk samples and definitive characterisation data they can be very profitably investigated using density functional calculations. A valuable perspective places these new and unprotected molecules in the context of related non-molecular solids, and terminally ligated molecules. The Fe7MoS9 cluster which effects the mild reduction of N2 at the active site of nitrogenase is similarly undercoordinated, floppy, and reactive. Density functional calculations reveal the characteristics of the Fe7MoS9 site and elucidate postulated mechanisms for the reduction.

The compound 9-benzamido-6,7,8,9,10,11-hexahydro-5,9:7,11-dimethano-5H-benzocyclononen-7-ol,3, has been prepared, and found to crystallise as inclusion complexes (2·G where the guest G is ethyl acetate or carbon tetrachloride. The host molecule contains a benzo group as part of a rigid polycyclic framework, on which there are hydroxy and benzamido substituents. The crystal structure of the complex with CCl4 contains the host molecules hydrogen-bonded in layers, with the CCl4 molecules trapped between the layers. Two types of host-host hydrogen bonds, OH to amide carbonyl O, and amide NH to hydroxyl O, maintain the host layers. The benzo groups protrude normal to these host layers, and six such groups provide the closest surroundings of the CCl4, which is constrained to two disordered orientations at the one location. This is a layer clathrate structure.

The alicyclic diols (1–5) constitute the first members of a family of novel helical tubuland hosts crystallising in space group P3121 but possessing quite different canal shapes and dimensions. Consideration of their structural data has revealed two distinct sub-classes of these materials. The molecular features necessary for a diol to crystallise with the helical tubuland structure are defined and discussed.

The sixfold phenyl embrace (6PE) which is a widespread supramolecular motif for Ph4P+ cations in crystals can occur as a zig-zag chain motif (ZZI6PE) in which each Ph4P+ engages two 6PE, and these ZZI6PE allow further multiple phenyl embraces at each Ph4P+ cation. The Cambridge Structural Database has been explored for occurrences of the two- and three-dimensional networks of multiple phenyl embraces involving the common cation Ph4P+. A prevalent motif involves parallel ZZI6PE chains linked by fourfold phenyl embraces (the parallel variant, P4PE) to form skewed hexagonal nets, called ZZI6PE/P4PE layers. Another type has ZZI6PE chains running in orthogonal directions and linked by the orthogonal variant of the 4PE: these are the orthogonal ZZI6PE/O4PE three-dimensional nets. An expanded layer involves eight-rings of Ph4P+ built from finite sequences of 6PE. The compounds [Ph4P+]2[Te4]2– and [Ph4P+]4[(S6)Cu(S8)Cu(S6)]4– contain more elaborate three-dimensional nets comprised of 6PE, 4PE and 3PEs. These nets are more highly concerted crystal supramolecular motifs. The nets described are different from those based on 4PE, and the difference is related to the charge of the anion. Anions are usually located over or in the cycles of cations in the layered motifs, and in cavities of the three-dimensional motifs. The concerted supramolecular motifs of cations, rather than cation–anion attractions, are believed to be the dominant factors in the crystal supramolecularity. Calculated energies for the motifs are reported. It is concluded that a single Ph4P+ cation can participate in up to four multiple phenyl embraces, and that the maximum attractive interaction energy per Ph4P+ cation is ca. 9 kcal mol–1. This is comparable with the energies of hydrogen bonds.

Tetraphenylphosphonium cations in crystals associate with attractive fourfold phenyl embraces (4PE), in which two phenyl rings from each cation engage in concerted edge-to-face and offset-face-to-face local interactions. The names of the orthogonal (O4PE) and parallel (P4PE) subclasses refer to the relationship between the relevant C–P–C planes in the two cations. One-dimensional ladder networks with O4PE along the sides and P4PE as rungs, and two-dimensional networks with O4PE in both directions, P4PE in both directions, or O4PE in one direction and P4PE in the other have been explored. The layers can be planar or corrugated. Crystalline [Ph4P+][Cl3Te(OCH2CH2O)]– contains a zeolite-like network of cations involved in 4PE, with the anions in channels. Most of the crystals included in this survey contain mono-negative anions, ranging from Cl3– to metal complexes with chelate ligands, and include structurally non-molecular anions such as [{Cu4I5ReS4}2–]∞. The crystalline compound [Ph4P+]2[C60]–[I–]x contains a high symmetry two-dimensional network of cations in P4PE, which functions as an effective host lattice completely encapsulating the [C60]–, with phenyl  · 4;· 4;·  fullerene interactions which are both face-to-face and edge-to-face.

The cluster [Zn10S7(py)9(SO4)3]·3H2O has been synthesized by reaction of the product of thermolysis of [NMe4]4[Zn10S4(SPh)16] [believed to be Zn10S4(SPh)12] with pyridine containing Na2SO4. The compound was characterised by X-ray crystallography: monoclinic space group P 21 /c, a 4;= 4;11.279(6), b 4;= 4;23.419(6), c 4;= 4;27.26(1) Å, β 4;= 4;107.74(2), Z 4;= 4;4. This cluster is unprecedented in its architecture, and for its combination of sulfide, sulfate and pyridine ligands. It has pseudo-three-fold symmetry. In the core the ten Zn atoms are arrayed as a central Zn4 tetrahedron on the three-fold axis, crowned with a slightly puckered Zn6 hexagon around the axial Zn atom of the Zn4 tetrahedron. The Zn4 tetrahedron contains µ4-S2–. The axial Zn atom of the Zn4 tetrahedron is connected to the Zn6 hexagon through three µ3-S2– ligands. Another three µ3-S2– ligands connect the base of the Zn4 tetrahedron to the Zn6 hexagon. Each SO42– ion bridges three Zn atoms: two from an edge of the hexagon and one basal Zn atom of the tetrahedron. All Zn atoms have distorted tetrahedral co-ordination, completed by terminal pyridine ligands on all except the axial Zn atom. The crystal supramolecularity is a stacking of the clusters such that three axially directed pyridine ligands (on the basal Zn4 atoms) nestle amongst six equatorially directed pyridine ligands (on the Zn6 hexagon) of an adjacent molecule. Water molecules are hydrogen bonded to SO42– and µ3-S atoms. The Zn–S connectivity in this cluster is different from that in the cubic or hexagonal lattices of ZnS, and from that in known M8S(SR)16, M17S4(SR)28 and M10(SR)10 clusters formed by Group 12 metals.

Tris(bidentate ligand)metal complexes such as [M(bipy)3] commonly associate in crystals as columns of complexes with collinear three-fold axes, in order that three heterocyclic rings of one complex can engage in six vertex-to-face interactions with three heterocyclic rings from the neighbour. This concerted supramolecular motif is called the sextuple aryl embrace (SAE), and is analogous to the sextuple phenyl embrace formed by XPh3 and XPh4 molecules. The three oxidation states of [Ru(bipy)3]z, z 4;= 4;0, 2+, 3+, were found to crystallise with SAEs, as do most other [M(bipy)3]z complexes with small symmetrical anions. Expansion of crystal lattices to accommodate anions was found not to disrupt the infinite columns of SAEs. A pair of tris(bidentate ligand)-metal complexes must have opposite chirality to participate in an SAE, and in this way the SAE differs from the otherwise similar homochiral helicate complexes. A small number of tris(tridentate ligand) complexes demonstrate a similar crystal supramolecular motif.

The sextuple phenyl embrace (SPE) occurs widely between molecules with XR3 (R 4;= 4;aryl) moieties, and has at least quasi-three-fold symmetry. The SPE can have exact 3&cmb.macr; symmetry, and therefore can be aligned with the principal axis of trigonal crystal lattices. Using the Cambridge Structural Database we report that this does occur in the crystals of a chemically diverse set of compounds, ranging from P(C6H4Me-4)3 and Ph3CCO2H, through salts such as [Ph3PH+]2[Ga2Cl6]2– and [Ph3PMe+]2[Cu4I6]2–, small molecules such as Ph3PAlMe3 and Ph3POs(CO)3PPh3, to larger molecules such as Ph3SiOTiN(CH2CH2O)3 and Ph3PCu(µ-SPh)3U(µ-SPh)3CuPPh3(thf )6 (thf 4;= 4;tetrahydrofuran). The XR3 moieties and the SPEs occur in hexagonal nets, and this generic crystal structure type (in space groups R3&cmb.macr;, P3&cmb.macr;, R3&cmb.macr;c, P3&cmb.macr;c1) is named the hexagonal array of sextuple phenyl embraces, HASPE. The hexagonal nets can be planar, or puckered by expansion or compression along the trigonal axis. The linkages around these hexagonal nets can be further multiple phenyl embraces, or may be elongated substantially. The non-embracing sections of the molecules can occupy the centres of the hexagons. The HASPE lattice type shows considerable flexibility in order to accommodate diverse components, but the integrity of the SPEs is maintained, attesting to their strong contribution to the lattice energy. Lower symmetry portions of molecular structure, and lower symmetry supramolecular motifs such as the CO2H  · 4;· 4;·  HO2C dimer, are forced to disorder by the dominant hexagonal array of SPEs. Quasi HASPE lattices with lower crystal symmetry have been recognised.

All of the lanthanides except promethium have been treated as monocations Ln+ with S8(g), in an ion cyclotron resonance mass spectrometer. A rich array of 213 new [LnSx]+ ions is described. Each lanthanide generates sequences of products [LnSx]+ in which x generally increases with time. Dominant intermediate ions in these sequences occur with x 4;= 4;2 (La, Gd), 3 (Eu, Yb), 4 (Nd, Sm, Tb, Dy, Ho, Er, Tm, Lu), 6 (La, Ce, Pr), 9 (Yb), 10 (Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu), 11 (Eu), 12 (La, Ce, Pr, Gd), 14 (La) and 18 (Nd). The largest product observed is [LnS21]+ for La and Pr, and the size of the largest ion drops towards the end of the lanthanide series. There is a remarkable similarity between the reactions of Eu+ (f 7s1) and Ca+ (s1) with S8: both form [MS3]+ rapidly, then add S8 rapidly to form an isomer which can easily dissociate S8, and also form more slowly a second isomer [MS11]+ which is more stable. Other M+ with s1 ground-state configurations do not behave similarly. The smaller [LnSx]+ probably contain co-ordinated S2, S3 and S4, while some of the larger [LnSx]+ may contain associated S8 molecules not bonded directly to Ln. Reaction rates correlate approximately with the occurrence of a ground or low excited state with two unpaired non-f electrons.

All of the lanthanides except promethium have been treated as monocations Ln⁺ with S₈(g), in an ion cyclotron resonance mass spectrometer. A rich array of 213 new [LnS_x]⁺ ions is described. Each lanthanide generates sequences of products [LnS_x]⁺ in which x generally increases with time. Dominant intermediate ions in these sequences occur with x = 2 (La, Gd), 3 (Eu, Yb), 4 (Nd, Sm, Tb, Dy, Ho, Er, Tm, Lu), 6 (La, Ce, Pr), 9 (Yb), 10 (Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu), 11 (Eu), 12 (La, Ce, Pr, Gd), 14 (La) and 18 (Nd). The largest product observed is [LnS₂₁]⁺ for La and Pr, and the size of the largest ion drops towards the end of the lanthanide series. There is a remarkable similarity between the reactions of Eu⁺ (f ⁷s¹) and Ca⁺ (s¹) with S₈: both form [MS₃]⁺ rapidly, then add S₈ rapidly to form an isomer which can easily dissociate S₈, and also form more slowly a second isomer [MS₁₁]⁺ which is more stable. Other M⁺ with s¹ ground-state configurations do not behave similarly. The smaller [LnS_x]⁺ probably contain co-ordinated S₂, S₃ and S₄, while some of the larger [LnS_x]⁺ may contain associated S₈ molecules not bonded directly to Ln. Reaction rates correlate approximately with the occurrence of a ground or low excited state with two unpaired non-f electrons.