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Inorganic crystal engineering is the modelling, synthesis and evaluation of the properties of crystalline materials obtained from inorganic, organometallic and bioinorganic building blocks. This article will take the reader across this interdisciplinary field of research from supramolecular to materials chemistry and biominerals following the collection of papers presented at the Dalton Discussion on Inorganic Crystal Engineering held at the University of Bologna. The focus will not only be on the preparation and characterisation of supermolecules, co-ordination networks, molecular crystals, nanoporous materials and zeolites but also on the investigation of magnetism and conductivity, of Langmuir–Blodgett films, and of nanoporous, amorphous, polymorphic and pseudo-polymorphic systems. Both directions of foreseen development of the inorganic crystal engineering field as well as some of the major drawbacks will be critically discussed.

The organometallic pseudo-polymorphism that arises from cocrystallisation of organometallic molecules or ions with solvent molecules is discussed, together with the possibility of interconversion between polymorphs and pseudo-polymorphs. Three cases have been investigated: (i) the quantitative preparation of a monohydrated pseudo-polymorph of the supramolecular salt [CoIII(η5-C5H5)2]+[Fe(η5-C5H4CO2H)(η5-C5H4CO2)]−1 by grinding the powder material and subsequently nucleating and crystallising the hydrated form [CoIII(η5-C5H5)2]+[Fe(η5-C5H4CO2H)(η5-C5H4CO2)]−·H2O 2, (ii) the quantitative preparation, nucleation and crystallisation of the elusive anhydrous form of the neutral zwitterion [CoIII(η5-C5H4CO2H)(η5-C5H4CO2)] 4 by thermal dehydration of the hydrated species [CoIII(η5-C5H4CO2H)(η5-C5H4CO2)]·3H2O 3, and (iii) the formation of the salt [RuII(η6-C6H6)2][BF4]25 either by direct crystallisation from water or by desolvation of the pseudo-polymorph [RuII(η6-C6H6)2][BF4]2·MeNO26 obtained from nitromethane. In order to investigate the processes, single crystal and powder X-ray diffraction measurements as well as thermogravimetry and differential scanning calorimetry have been carried out. These results show, inter alia, how elusive polymorphic or pseudo-polymorphic modifications of a given substance can be obtained by non-solution methods.

A crystal synthesis strategy based on a sequence of redox/acid–base/self-assembly/crystallisation processes has been devised and successfully applied to prepare a number of mixed organic/organometallic and organometallic/organometallic crystalline materials. An adequate choice of the building blocks permits design and construction of mono-, two- and three-dimensional superanion framework structures encapsulating organometallic cations. The superanions are generated by partial deprotonation of polyprotic acids from the reaction with the organometallic hydroxides [Cr(η6-C6H6)2][OH] and [Co(η5-C5H5)2][OH], produced in situ by direct oxidation of the neutral complexes [Cr(η6-C6H6)2] and [Co(η5-C5H5)2]. The anionic superstructures are held together by a combination of neutral and interionic O–H· · ·O hydrogen bonds, while the interaction with the organometallic cations is based on a profusion of C–H· · ·O bonds reinforced via charge assistance.

The crystal architecture, stability and behaviour with temperature of the hexafluorophosphate salts [Ru(η5-C5H5)(η6-arene)]+ (arene = benzene 1 or trans-stilbene 2) have been investigated by variable temperature X-ray diffraction and differential scanning calorimetry. Compound 1 shows a reversible high temperature phase transition to a semi-disordered phase at 332 K. It is pseudo-isomorphous with [Cr(η6-C6H6)2][PF6] and with the room temperature form of [M(η5-C5H5)2][PF6] (M = Co or Fe), the latter species also undergoing reversible phase transitions. Crystalline 2 has been characterised in its cocrystallised form [Ru(η5-C5H5)(η6-trans-PhCHCHPh)]- [PF6]·0.5trans-PhCHCHPh. The trans-stilbene molecules fill in the voids formed in the packing when the small [PF6]– anions assemble with the large and asymmetric [Ru(η5-C5H5)(η6-trans-PhCHCHPh)]+ cations. The role of charge assisted C–H(δ+)· · ·F(δ–) interactions is discussed.

The organic acids cyclobutane-1,2,3,4-tetracarboxylic C4H4(CO2H)4, and trans-acotinic acid C3H3(CO2H)3, have been treated in thf with the organometallic hydroxides [Co(η5-C5H5)2]+[OH]–, [Co(η5-C5Me5)2]+[OH]–, and [Cr(η6-C6H6)2]+[OH]– prepared in situ from the oxidation of the corresponding neutral complexes, to yield the novel organic–organometallic crystalline materials [Co(η5-C5H5)2]+[C4H4(CO2H)3COO]– 1, [Cr(η6-C6H6)2]+[C4H4(CO2H)3CO2]–·H2O 2, and [Co(η5-C5Me5)2]+[C3H3(CO2H)2CO2(H)·C3H3(CO2H)2CO2]–·H2O 3. Self-assembly of the monodeprotonated organic acid C4H4(CO2H)4 generates supra-anionic framework structures held together by O–H· · ·O and O–H· · ·O– hydrogen bonds which accommodate the diamagnetic [Co(η5-C5H5)2]+ and paramagnetic [Cr(η6-C6H6)2]+ cations, respectively. Crystalline 1 does not form single crystals with defined shapes but rather an “enamel” like material which grows parallel to the crystallographic bc plane. The same reaction between trans-acotinic acid and [Co(η5-C5Me5)2]+[OH]–generates a large honeycomb-type structure in [Co(η5-C5Me5)2]+[C3H3(CO2H)2CO2(H)·C3H3(CO2H)2CO2]–·H2O 3. The effect of the size of the templating units [Co(η5-C5H5)2]+ and [Co(η5-C5Me5)2]+ is discussed. The previously unknown structure of the starting material [Co(η5-C5Me5)2]+[PF6]– 4, used for the preparation of [Co(η5-C5Me5)2]+[OH]–, has also been determined.

The reaction of [Ru6C(CO)17] and cycloheptatriene (CHT) gives rise to the formation of several products which have been fully characterized in solution and by single-crystal X-ray diffraction as [Ru6C(CO)15(ENBD)] 1 (ENBD = ethylenenorbornadiene), [Ru6C(CO)14(C7H8)] 2 and [Ru6C(CO)11(η5-C7H9)(µ3-η2∶η2∶η3-C7H7)] 3. Compounds 2 and 3 provide an example of the co-ordinative variability of carbocyclic rings based on the C7-frame. The ligand in 1 contains nine carbon atoms and is formed from C7H9 by an unknown process. The CHT molecule has also been observed to convert to toluene through a ring contraction activated by the cluster to form the known compound [Ru6C(CO)14(η6-C6H5CH3)] 4.

The three ruthenium arene clusters [Ru6C(CO)14(η6-PhCHMePh)] 1, [Ru6C(CO)14(η6-PhCCH2Ph)] 2 and [Ru7C(CO)14(µ3-κ∶η6∶η6-C6H4CH2C6H4)] 3 have been synthesized by reaction of 1,1-diphenylethene with triruthenium dodecacarbonyl in octane under reflux. Compound 1 is a 1,1-diphenylethene hexaruthenium carbido cluster (Ru6C) derivative wherein the organo-group, hydrogenated at the olefinic centre, is bound to the cluster by a single phenyl ring in an η6 manner. Compound 2 is a similar Ru6C derivative wherein the 1,1-diphenylethene ligand remains unsaturated, whilst the third, and minor, product is the first example of a metal carbonyl cluster chelated by a diphenyl ligand and is one of the few examples of an Ru7C ‘spiked’ octahedron. The subtle electronic effects of the Ru6C co-ordination in 2 have been probed by 1H NMR spectroscopy. The solid-state structures of both the hydrogenated derivative 1 and the Ru7C ‘spiked’ octahedral cluster 3 have been determined by single-crystal X-ray analysis. The supramolecular architecture of 3 has been examined in depth.

The polycarboxylic acids C6H3(CO2H)3-1,3,5 (trimesic acid, H3tma) and O 2,O 3-dibenzoyl-L-tartaric acid (L-H2bta) reacted in water or thf with [Co(η5-C5H5)2][OH] prepared in situ by oxidation of [Co(η5-C5H5)2] to generate organic superanions self-assembled via negatively charged O–H· · ·O– and neutral O–H· · ·O hydrogen bonds. The resulting organic host accommodates the cations via charge assisted C–Hδ+· · ·Oδ– hydrogen bonds between organometallic and organic components. Crystalline [Co(η5-C5H5)2]+[(H3tma)(H2tma)]–·2H2O 1 was obtained as the major product from acid and base in a 1∶2 stoichiometric ratio. Compound 1 contains a complex hydrogen bonded honeycomb-type structure formed by superanions [(H3tma)(H2tma)]– and water molecules. The mixed salt [Co(η5-C5H5)2]+[Co(H2O)6]2+[tma]3– 2 was obtained as a minor product from the same reaction. In crystalline 2 the water molecules of the aqua complex form hydrogen bonds with the three carboxylic groups of the organic anion resulting in a caged structure that encapsulates the [Co(η5-C5H5)2]+ cation. When dibenzoyl-L-tartaric acid was used the chiral crystal [Co(η5-C5H5)2]+[L-Hbta]– 3 is obtained. The crystal contains chains of O–H· · ·O– hydrogen bonded anions. These results are used to discuss a design strategy for the engineering of organometallic crystals with predesigned structures. Though on a limited data set, the structure of the elusive crystalline hydrate [Co(η5-C5H5)2]+[OH]–·4H2O 4, which is liquid at ambient temperature, is discussed.

The relationship between the molecular and crystal structures of organometallic complexes carrying methylidyne (µ 3 -CH) and methylene (µ-CH 2 ) ligands has been investigated on data retrieved from the Cambridge Structural Database. It has been shown that µ 3 -CH and µ-CH 2 groups participate in intermolecular hydrogen-bonding networks of the C–H · · · O type involving, in most cases, the oxygen atoms of CO ligands as acceptors. The order of decreasing acidity, judged on purely geometrical grounds from the average length of the hydrogen-bonding interactions, is roughly µ 3 -CH > µ-CH 2 . These ligands establish C–H · · · O interactions which are comparable in length to those established by hydrogen atoms bound to sp- and to sp 2 -hybridized carbon atoms consistent with experimental and theoretical evidence.

Several hexaruthenium carbido clusters [Ru6C(CO)14(η6-C6H5C6H4Me)] 1a, [Ru6C(CO)14(η6-MeC6H4C6H5)] 1b, [Ru6C(CO)14(η6-C6H5C6H4Et)] 2a, [Ru6C(CO)14(η6-EtC6H4C6H5)] 2b, [Ru6C(CO)14(η6-C6H5C6H4Ph)] 3 and [Ru6C(CO)14(η6-C6H5C6H3Ph2)] 4 have been synthesized by the reaction of triruthenium dodecacarbonyl and the appropriate polycyclic hydrocarbon. The compounds have been fully characterised by a variety of methods including single-crystal X-ray diffraction for 1a, 1b and 2b. The supramolecular architectures of these three solid-state structures have also been examined. Selectivity of the cluster build-up reaction at different aryl sites is discussed.

Agostic interaction geometries, C–H ⋯ M (M = Li, Group 4 or 5 transition metal or other), have been investigated with data retrieved from the Cambridge Structural Database. Analysis of both neutron- and X-ray-derived structures shows that a strengthening of the H ⋯ M agostic bond is accompanied by a corresponding weakening of the C–H bond. A similarity in behaviour of Li to the heavier transition metals is observed in these agostic interactions. Di- and tri-bridged C–H ⋯ Li geometries involving alkyl groups occur in both intra- and inter-molecular cases.

Single-crystal X-ray diffraction data have been collected for [Fe2Os(CO)12] at 120, 223, 288, 292 and 323 K. The two studies at ambient temperature (288 and 292 K) reveal a ≈12 : 1 disorder of the metal triangle as previously reported. At the two lowest temperatures there is no evidence of disorder, while at 323 K the ratio of the major : minor component of disorder decreases significantly to ≈1.4 : 1. Data collected on the same crystal specimens indicate unequivocally that this disorder is dynamic in nature. Two-dimensional exchange and one-dimensional variable-temperature 13C magic angle spinning (MAS) NMR spectroscopy showed that carbonyl exchange is rapid above 306 K in the crystalline solid. Two independent exchange processes of similar energy are observed. The first is consistent with the crystallographic evidence, and involves an ‘in-plane’ rotation of the Fe2Os triangle in steps of 60° within a relatively rigid icosahedral carbonyl manifold. The second involves localised axial–equatorial exchange in the Os(CO)4 group. The Os LIII and Fe K edge X-ray absorption fine structure spectra are consistent with identical structures being present in tetrahydrofuran solution and in the solid phase.

The electronic structure of [Fe3(CO)12] has been investigated by means of extended-Huckel calculations and compared to that of [Ru3(CO)12], showing that the bridged structure, observed in the solid state, arises because of electronic reasons, namely a weaker metal–metal repulsion in the case of the lighter cluster. This is associated with the bridge-formation process and becomes determining for the energetic balance in compounds of heavier metals. The possible existence of crystal structures which are alternative to the observed disordered one known for [Fe3(CO)12] has been explored by means of the atom–atom pairwise potential-energy method. Alternative ordered molecular arrangements have been generated and compared with the experimental crystal structure in terms of packing efficiency and cohesive energy.

The ion organization in crystalline salts formed by transition-metal carbonyl cluster anions containing halide ligands has been investigated. It has been shown that the presence of the heteroligand(s) drives either towards the ‘trapping’ of two halogen atoms within a cage of counter ions or towards the formation of anion rows insulated by cation rows. The formation of either type of pattern appears to be due to the size and mode of bonding of the halogen atoms, the first type being preferred with small terminal halogens or with large halogens in bridging bonding mode, while the second is observed with large halogens in terminal bonding mode.

Thermolysis of [Ru3(CO)12]1 in octane with guaiazulene (7-isopropyl-1,4-dimethylazulene) gave [Ru3(CO)7(µ3-η5:η3:η3-C15H18)]2 and [Ru4(CO)9(µ3-η5:η3:η3-C15H18)]3. The products 2 and 3 have been characterised by spectroscopic techniques and by single-crystal X-ray diffraction. Comparison is made between the two structures of the cluster compounds. Proton NMR and crystal structural data indicate an increased degree of metal–hydrocarbon interaction in 3.

The heptanuclear clusters [Ru7C(CO)16(C9H8)] and [Ru7C(CO)16(C12H12)] have been isolated from the reaction between [Ru3(CO)12] and isopropenyl- or 1,3-diisopropenyl-benzene and their crystal structures determined: the Ru7 cores represent rare examples of spiked octahedra; the organo-ligands are bound through all four π bonds to a metal face and through two σ bonds to the ruthenium spike, donating a total of ten electrons.

The molecular organization in crystals of the prototypical organometallic molecule [Cr(CO)6] has been investigated by means of packing-potential-energy calculations and computer graphics analysis. The atom–atom pairwise-potential-energy method has been used to study the interaction energy between molecular pairs and the molecular self-recognition process which leads to crystal construction. Alternative crystal structures have been generated and compared with the experimentally observed structure in terms of packing cohesion.

A series of Ru4 butterfly cluster complexes have been synthesized and investigated by spectroscopic and diffraction methods. The solid-state structure of [Ru4(CO)12(µ4-η2 : η2-C6H8)]1 and of the two isomers of [Ru4(CO)9(µ4-η2 : η2-C6H8)(η6-C6H6)] with the benzene ring located on the butterfly hinge (2) or on the wingtip (3), have been characterized. The three complexes are monoclinic, 1: space group P21/c, a= 9.779(8), b= 22.163(14), c= 10.069(6)A, β= 90.23(9)°, Z= 4; 2: space group Cc, a= 10.010(3), b= 15.532(4), c= 15.384(4)A, β= 93.50(2)°, Z= 4; 3: space group P21/c, a= 16.655(3), b= 15.709(3), c= 17.831(4)A, β= 91.44(3)°, Z= 8. The molecular structures of the complexes have been investigated in relation with the respective crystal structure and, in the case of 2, compared with the polymorphic modification described previously.

The molecular and crystal structure of [Fe3(CO)12] has been reinvestigated from single-crystal X-ray diffraction data collected at 100, 160, 250 and 320 K. The asymmetric bridging carbonyl ligands become progressively more symmetric as the temperature is decreased. At the lowest temperature the molecule possesses nearly exact C2v, symmetry. The crystal structure has been examined in the two limiting hypothetical P21 and P1 crystals composing the disordered P21/n structure.

The hexanuclear cluster [Ru6C(CO)14(η6-arene)]1(arene = C6H6, C6H5Me, C6H4Me2-1,3 or C6H3Me3-1,3,5) reacts with Me3NO–CH2Cl2 in the presence of cyclohexa-1,3-diene or cyclohexa-1,4-diene to yield compound [Ru6C(CO)12(η6-arene)(µ-η2:η2-C6H8)]2 in which the diene spans one metal edge of the octahedral cluster unit. Further treatment of 2 with Me3NO–CH2Cl2 affords the bis(arene) derivative [Ru6C(CO)11(η6-arene)(C6H6)]. On the basis of spectroscopic data and single-crystal X-ray data it appears that these bis(arene) derivatives exist in two isomeric forms, viz. [Ru6C(CO)11(η6-arene)(µ3-η2:η2:η2-C6H6)]3 and [Ru6C(CO)11(η6-arene)(η6-C6H6)]4. Evidence that compound 3 is formed initially, followed by isomerisation to 4 is presented. The structures of [Ru6C(CO)11(η6-C6H5Me)(µ3-η2:η2:η2-C6H6)]3b and [Ru6C(CO)11(η6-C6H4Me2-1,3)(µ3-η2:η2:η2-C6H6)]3c have been determined by single-crystal X-ray diffraction. Compound 3b is orthorhombic, space group Pnma, a= 9.035(5), b= 14.796(2), c= 20.534(4)A, Z= 4, 1770 unique observed reflections [I > 2σ(I)], R= 0.020; 3c is monoclinic, space group P21/n, a= 18.27(2), b= 9.729(2), c= 34.39(3), A, β= 94.94(6)°, Z= 8, 7833 unique observed reflections [I > 2.0σ(I)], R= 0.058, R′= 0.062.

The action of MeSO3CF3 on [Fe2(cp)2(CO)2(µ-CO){µ-C(CN)SMe}]1(cp =η5-C5H5) yields [Fe2(cp)2(CO)2(µ-CO){µ-C(CN)SMe2}][SO3CF3]2. The sulphonium cation is also obtained, trapped in the unusual triflate complex Fe(OSO2CF3)4{Fe2(cp)2(CO)2(µ-CO)[µ-C(CN)SMe2}2]3, by treating the thiolcarbamate [Fe2(cp)2(CO)2(µ-CO){µ-C(CN)N(Me)C(O)SMe}]4 with MeSO3CF3. The X-ray crystal structure of 3 shows the iron(II) centre co-ordinated by four O-bonded triflate anions and by two, mutually trans-disposed, N-co-ordinated organometallic cations in an octahedral geometry.

The activated cluster [Os3(CO)7(µ3-η2:η2:η2-C6H6)(C2H4)(MeCN)] reacts with alkynes RC2R′(R = R′= H, Ph or Me; R = H, R′= Ph; R = Me, R′= Et) to produce the new clusters [Os3(CO)7(η6-C6H6)(µ3-η2-RC2R′)] in which the benzene molecule has migrated to a single osmium atom. In contrast, reaction of the ruthenium cluster [Ru3(CO)9(µ3-η2:η2:η2-C6H6)] with similar alkynes RC2R′(R = R′= H, Ph or Me; R = Ph, R′= H) yields the new clusters [Ru3(CO)7(η6-C6H6)(µ3-η2-RC2R′CO)] in which carbonyl insertion has occurred. The molecular structures of [Os3(CO)7(η6-C6H6)(µ3-η2-C2Me2)] and of [Ru3(CO)7(η6-C6H6)(µ3-η2-PhC2PhCO)] have been established by single-crystal X-ray analysis. The C6H6 ligand is η6-bonded to a single metal atom, while the C2Me2 and the PhC2PhCO ligands lie above the metal triangles and are bonded to the three metal atoms via two σ interactions and one π interaction.

Treatment of the functionalized cluster [Os3(CO)10(MeCN)2] with trimethylsilylacetylene quantitatively affords [Os3(CO)10(HCCSiMe3)]1 which in refluxing heptane undergoes decarbonylation and hydrogen migration yielding [Os3H(CO)9(CCSiMe3)]2 the structure of which was determined by an X-ray study [triclinic, space group P, a= 9.201(2), b= 13.127(2), c= 9.118(7)A, α= 107.31(3), β= 98.57(3), γ= 93.18(2)°, Z= 2]. Desilylation of 2 affording [Os3H(CO)9(C2H)] takes place either on silica TLC plates or on stirring in hot methanol. The compound [Os3H(CO)10(CCSiMe3)]3 has been prepared through two different routes: (i) reaction of [Os3H(CO)10Cl] with Li[CCSiMe3]; (ii) reaction of [Os3(CO)11(MeCN)] with HCCSiMe3. It has been characterized by X-ray diffraction studies [monoclinic, space group P21/c, a= 13.338(5), b= 9.141(4), c= 18.902(7)A, β= 109.99(3)°, Z= 4]. On refluxing 3 in heptane, decarbonylation occurs with formation of 2.

The molecular organization in crystals of [Co2(CO)8] and [Fe2(CO)9] has been investigated by means of packing potential-energy calculations and computer graphic analysis. It is shown that the two molecules pack in nearly identical ways in their crystals. However, while crystalline [Fe2(CO)9] is compact, solid [Co2(CO)8] presents large empty channels (van der Waals width between 2.4 and 4.5 A, at room temperature) corresponding to the approximate location of possible residual electron density (whether due to Co–Co bonding electrons or to lone pairs on the Co atoms). Alternative crystal packings for [Co2(CO)8] have been generated, using improved atomic potential parameters adapted to carbonyl compounds. On purely steric grounds, it is shown that the [Co2(CO)8] molecule is compatible with more close-packed crystal arrangements than the one actually observed. In two alternative triclinic and monoclinic crystal structures the ‘empty’ bridging sites are shown to be available for molecular interlocking.

The molecular structure of the prototypical hexaruthenium carbidocarbonyl cluster [Ru6C(CO)17] has been redetermined on crystals obtained by two different routes. Form I is monoclinic, space group P21/n, a= 9.19(1), b= 32.043(9), c= 9.598(4)A, β= 111.93(3)°, Z= 4, R= 0.076, R′= 0.079, for 2764 [Io > 2σ(Io)] out of 4755 absorption-corrected reflections; form II is also monoclinic, space group P2/a, a= 17.668(2), b= 9.335(1), c= 24.057(7)A, β= 97.96(2)°, Z= 6, R= 0.023, R′= 0.028, for 7656 [Io > 2σ(Io)] out of 10 288 absorption-corrected reflections. The structural analysis shows that the complex possesses three different molecular structures in the solid state (I, IIA and IIB), differing essentially in the rotameric conformation of the tricarbonyl units above and below the equatorial plane containing bridging and semibridging ligands. The conformation is staggered in form I, while II contains two nearly eclipsed conformers. The relationship between the structure of the three isomers and of their crystals has been investigated.

The structure of [Mo(C6H5Me)(CO)3] has been determined by single-crystal X-ray diffractometry. The complex shows a staggered orientation of the tricarbonyl group with respect to the arene atoms, differing from [Cr(C6H5Me)(CO)3] where the tricarbonyl group is eclipsed. The dynamic behaviour evidenced by 13C cross-polarization magic angle spinning n.m.r. spectroscopy in the solid state is explored by means of atom–atom pairwise potential-energy calculations and thermal motion analysis.

The dynamic motion that the carbonyls [Co2(CO)8], [Fe3(CO)12], and [Co4(CO)12] undergo in the solid and solution has been examined. Evidence is offered to suggest that the fluxional behaviour and the isomerisation the molecules undergo may be best understood in terms of the polyhedral model in which the metal unit librates within the CO polyhedron. Support for this approach comes from an examination of the anisotropic displacement parameters of the metal atoms and the usefulness of this method to the understanding of molecular motion in the solid is stressed.

The crystal packings of [V(CO)6], [Cr(CO)6], [Mn2(CO)10], [Fe(CO)5], [Fe2(CO)9], [Co2(CO)8], and [Ni(CO)4] have been examined. It is shown that each molecule except [Ni(CO)4] is enclosed in a 12-molecule shell. Two basic packing motives are recognized, namely the cubo-octahedron and the anti-cubo-octahedron. Crystal and molecular qualifiers, such as molecular volumes, and packing coefficients have been calculated. The atom–atom pairwise potential-energy method has been used to estimate the separate contributions to the total packing potential energy (p.p.e.) of each kind of intermolecular interaction; given the same number of metal atoms, a CO group contributes a fairly constant amount to the p.p.e. The CO-group intermolecular mutual orientations have also been examined, showing that the CO axes are preferentially orthogonal in projection.

X-Ray diffraction studies from compounds obtained by reaction of transition-metal salts, dissolved in tetrahydro-furan, with potassium–graphite (C8K), show that titanium, manganese, iron, cobalt, copper, and zinc intercalate in graphite. It has been established that lamellar compounds of stages 1–5 (i.e. with intercalated transition metal layers separated by 1–5 graphite layers respectively) are formed. The introduction of the metal layers leads approximately to the same increase in interlayer spacing for all stages. Values ranging from 0.56 to 0.58 nm were found for the thicknesses of the filled layers.

The cluster anion [Os8H(CO)22]–(1) has been obtained from the reaction of [Os3(CO)12] in BuiOH and X-ray analysis of its [PMePh3]+ salt shows that the anion has an unusual fused tetrahedral metal-core geometry related to that of [Os6(CO)18]. Reaction of (1) with iodine gives [Os8H(CO)22I](2) in which an overall change in cluster geometry has occurred; the reaction is reversible and heating (2) with iodide regenerates (1). Reaction of (1) with concentrated H2SO4 gives [Os8H2(CO)22] which reacts with P(OMe)3 to give [Os8H2(CO)22{P(OMe)3}] and with halide ions to give eventually [Os8(CO)22]2–. Crystals of [PMePh3][Os8H(CO)22] are triclinic, space group P, with a= 16.775(3), b= 1 3.509(2), c= 11.267(2)A, α= 89.88(1), β= 103.97(1), γ= 96.99(1)°, and Z= 2. Refinement of atomic parameters using 5 615 absorption-corrected data converged at R= 0.0359. Crystals of [Os8H(CO)22I]·H2O are monoclinic, space group P21/c, with a= 14.386(2), b= 11.037(1), c= 24.629(3)A, β= 94.30(2)°, and Z= 4. Refinement of atomic parameters using 4 644 absorption-corrected reflections converged at R= 0.0569.

The new mixed-metal carbido-carbonyl cluster species [Rh6C(CO)13{Au2(PPh3)2}] has been obtained by reaction of [Rh6C(CO)13]2– with [Au(PPh3)Cl]. The molecular structure has been determined by single-crystal X-ray diffractometry: orthorhombic, space group Fdd2, a= 22.335(4), b= 31.710(6), c= 14.947(7)A, Z= 8, R= 0.045, and R′= 0.047. The cluster is a distorted Rh6 octahedron with two adjacent triangular faces capped by Au(PPh3) moieties which also establish an Au–Au bond; the CO-ligand stereogeometry is related to that of the parent dianion. The 31P and 13C solution n.m.r. spectra are consistent with a fluxional behaviour of both the CO groups and the two Au(PPh3) moieties over the Rh6 core. The compound reacts easily with CO (1 atm, 101 325 Pa; room temperature) yielding the bicapped trigonal prism [Rh6C(CO)15{Au(PPh3)}2].

Salts of the green-brown anions [Co4(CO)11X]–(X = Br, I, or SCN) have been obtained at –70 °C starting from [Co4(CO)12] and tetrasubstituted ammonium ([NEt4] Br or [NEt4]I) or phosphonium {[PMePh3]Br, [N(PPh3)2]I, or [N(PPh3)2][SCN]} salts. Using sodium alkoxides or propylamine, formation of purple-red solutions containing, on the basis of the i.r. spectra, similar monosubstituted anions (X = COOMe, COOPri, or CONHPrn) has been observed. The transformation of [Co4(CO)12] into [Co2(CO)8] has been studied in PriOH at room temperature under cabron monoxide. The considerable acceleration of the rate of this reaction by halide ions is in agreement with the ready decomposition observed for all the [Co4(CO)11X]– anions in Lewis-base solvents.

The title complex has been prepared by direct reaction of [Co3C(CO)9Cl] with [PPh4][Rh(CO)4] in the molar ratio 1 : 4 in tetrahydrofuran solution, and characterized by chemical and crystallographic methods. The anion belongs to the family of octahedral carbide-carbonyl clusters (86 valence electrons) with 13 ligands. The Co atoms are found to be contiguous with no trace of Co/Rh disorder. The ligand distribution differs from that of the analogous homometallic species [Co6C(CO)13]2– and [Rh6C(CO)13]2–. The factors controlling the ligand arrangements in the three species are discussed. The compound [PPh4]2[Co2Rh4C(CO)13] crystallizes in the triclinic space group P, with a= 13.358(3), b= 21.981(2), c= 11.526(1)A, α= 94.779(9), β= 107.62(1), γ= 78.35(1)°, and Z= 2.6 273 Reflections were used to solve and refine the structure to R= 0.048, R′= 0.058.

The anion [Co13C2(CO)24]3– has been obtained by oxidation of the corresponding paramagnetic tetra-anion with iodine in MeCN. Its molecular structure has been determined by single-crystal X-ray diffractometry, showing that the two anions are isostructural. The metal-atom polyhedron contains two trigonal-prismatic cavities in which two interstitial carbon atoms are accommodated. Average bond distances are Co–Co 2.575 A and Co–C(carbide) 1.97 A. The effect of the removal of the unpaired electron from the parent tetra-anion is discussed. A comparison of structural parameters is reported.

The high nuclearity mixed-metal cluster monoanions [Os10C(CO)24Cu(NCMe)]–(1) and [Os10C(CO)24Au(PPh3)]–(2) have been obtained by reaction of the carbido-dianion [Os10C(CO)24]2–(3) with one equivalent of [Cu(NCMe)4][BF4] or Au(PPh3)Cl, respectively, in CH2Cl2. Addition of excess [Cu(NCMe)4][BF4] to either (1) or (3) gives the di-copper species [Os10C(CO)24{Cu(NCMe)}2](4). X-Ray analysis of the [PPh3Me]+ salts of (1) and (2) show that the Cu and Au ligands have added to capping tetrahedra of the dianion (3) in µ3- and µ-bridging positions, respectively. Crystals of [PPh3Me][Os10C(CO)24Cu(NCMe)] are triclinic, space group P, with a= 16.452(3), b= 13.881(3), c= 13.290(3)A, α= 82.1 2(3), β= 93.23(3), γ= 96.40(3)°, Z= 2; refinement of atomic parameters using 4 325 absorption corrected data converged at R 0.0492. The salt [PPh3Me][Os10C(CO)24Au(PPh3)] also crystallises in the triclinic space group P, with a= 17.296(4), b= 15.403(3), c= 15.191(3)A, α= 107.31(3), β= 98.28(3), γ= 88.99(3)°, Z= 2; refinement of atomic co-ordinates using 5 314 absorption corrected data gave R 0.0551.