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The 'POCOP' pincer ligand, [2,6-(OPR 2 ) 2 C 6 H 3 ], has been attached to titanium in both Ti(III) and Ti(IV) complexes for the first time. Using a lithium-halogen exchange route [2,6-(OPR 2 ) 2 C 6 H 3 ]Li ([RPOCOP]Li) can be synthesised. Both the iso -propyl and tert -butyl derivatives can be made, but only the latter isolated. These can be reacted with the Ti(III) and Ti(IV) synthons to make a range of [POCOP]TiCl x species. In the presence of Ti(IV), THF and [RPOCOP]Li, an unprecedented ligand rearrangement occurs. ( t Bu POCOP)TiCl 2 , 1, can be derivatised with alkylating agents to make bis-methyl, phenyl and neopentyl complexes. The last of these can activate H 2 to make a rare example of a titanium chlorohydride, with the metal pincer fragment staying attached. EPR has been used to characterise the paramagnetic complexes and locate their electron spins, which is further validated with DFT calculations. This opens the door for this archetypical pincer ligand to be used with early transition metals.

Asymmetrically-bound pyrrolide-based bis-PNP pincer complexes of zirconium and hafnium have been formed. The [κ2-PNPPh][κ3-PNPPh]MCl2 species are in direct contrast to previous zirconium PNP pincer complexes. The pincer ligands are fluxional in their binding and the energy barrier for exchange has been approximated using VT-NMR spectroscopy and the result validated by DFT calculations.

The pentamethylcyclopentadienyl (Cp*) antimony(iii) cations [Cp* 2 Sb][B(C 6 F 5 ) 4 ], [Cp 2 *Sb][OTf], [Cp*SbCl][B(C 6 F 5 ) 4 ] and [Cp*Sb][OTf] 2 have been isolated and structurally characterised. [Cp*SbCl] + forms dimers in the solid state via an intermolecular Sb-Cl interaction. Initial screening shows that [Cp*SbCl][B(C 6 F 5 ) 4 ] is significantly Lewis acidic and can catalyse the dimerisation of 1,1-diphenylethylene; [Cp 2 *Sb][B(C 6 F 5 ) 4 ] exhibits negligible Lewis acidity. Highly unstable [Cp*SbF][B(C 6 F 5 ) 4 ] could not be isolated, but stabilisation with the IMes ligand allowed isolation of [Cp*SbF(IMes)][B(C 6 F 5 ) 4 ]. Fluorodechlorination of CH 2 Cl 2 and PhCCl 3 was observed in the presence of crude [Cp*SbF][B(C 6 F 5 ) 4 ] in solution. A computational mechanistic investigation suggests that the latter proceeds via a carbocation intermediate.

We report the direct carbonyl cleavage in a κ 3 -P',(η 2 -C,O),P'' ligand by a monomeric cobalt centre through metal-ligand cooperativity. C-O cleavage leads to the formation of a PC carbene P(O) pincer ligand with a central alkylidene anchor. A DFT analysis, supported by kinetic studies, suggests that decoordination of a pincer phosphino arm to generate a kinetically accessible free phosphine may be critical in transfer of the oxide to a phosphorus position. Thus, oxide transfer to bisphosphino bidentate co-ligands was also found to be viable, allowing access to the previously reported PC carbene P pincer complex (2), where bisphosphines were used as oxide sequestering agents.

The bis(carboranyl)phosphines [μ-2,2'-PPh-{1-(1'-1',2'-closo-C 2 B 10 H 10 )-1,2-closo-C 2 B 10 H 10 }] (I) and [μ-2,2'-PEt-{1-(1'-1',2'-closo-C 2 B 10 H 10 )-1,2-closo-C 2 B 10 H 10 }] (1) have been prepared and spectroscopically and structurally characterised. Crystallographic and DFT computational studies of 1 suggest that the orientation of the ethyl group, relative to the bis(carborane), is the result of intramolecular dihydrogen bonding. This orientation is such that the magnitudes of the 2 J PH coupling constants are approximately equal but of opposite sign, and fast exchange between the methylene protons in solution leads to an observed 2 J PH close to zero. The steric properties of I, 1 and their derivatives [μ-2,2'-P(Ph)AuCl-{1-(1'-1',2'-closo-C 2 B 10 H 10 )-1,2-closo-C 2 B 10 H 10 }] (2) and [μ-2,2'-P(Et)AuCl-{1-(1'-1',2'-closo-C 2 B 10 H 10 )-1,2-closo-C 2 B 10 H 10 }] (3) have been assessed by Tolman cone angle and percent buried volume calculations, from which it is concluded that the bis(carboranyl)phosphines I and 1 are comparable to PCy 3 in their steric demands. The selenides [μ-2,2'-P(Ph)Se-{1-(1'-1',2'-closo-C 2 B 10 H 10 )-1,2-closo-C 2 B 10 H 10 }] (4) and [μ-2,2'-P(Et)Se-{1-(1'-1',2'-closo-C 2 B 10 H 10 )-1,2-closo-C 2 B 10 H 10 }] (5) have also been prepared and characterised. The 1 J PSe coupling constants for 4 and 5 are the largest reported so far for carboranylphosphine selenides and indicate that I and 1 are very weakly basic.

The synthesis of a new endohedral ten-vertex Zintl ion cluster, [Fe@Sn10](3-), isoelectronic with [Fe@Ge10](3-), is reported. In an attempt to place this new cluster within the context of the known structural chemistry of the M@E10 family (M = transition metal, E = main group element), we have carried out a detailed electronic structure analysis of the different structural types: viz bicapped square antiprismatic ([Ni@Pb10](2-), [Zn@In10](8-)), tetra-capped trigonal prismatic ([Ni@In10](10-)) and the remarkable pentagonal prismatic [Fe@Ge10](3-) and [Co@Ge10](3-). We establish that the structural trends can be interpreted in terms of a continuum of effective electron counts at the E10 cage, ranging from electron deficient (<4n + 2) in [Ni@In10](10-) to highly electron rich (>4n + 2) in [Fe@Ge10](3-). The effective electron count differs from the total valence electron count in that it factors in the increasingly active role of the metal d electrons towards the left of the transition series. The preference for a pentagonal prismatic geometry in [Fe@Ge10](3-) emerges as a natural consequence of backbonding to the cage from four orthogonal 3d orbitals of the low-valent metal ion. Our calculations suggest that the new [Fe@Sn10](3-) cluster should also exhibit structural consequences of backbonding from the metal to the cage, albeit to a less extreme degree than in its Ge analogue. The global minimum lies on a very flat surface connecting D4d, C2v and C3v-symmetric minima, suggesting a very plastic structure that may be easily deformed by the surrounding crystal environment. If so, then this provides a new and quite distinct structural type for the M@E10 family.

Transition-metal complexes containing (C-C)→M σ-interactions have potential applications in both catalysis and the activation and cleavage of C-C bonds. Fully characterising the bonding and interactions in complexes containing such (C-C)→M σ-interactions is vital to understand their chemical behaviour. As a result a high-resolution experimental X-ray charge density study has been undertaken on [Rh(Binor-S)(PCy(3))][HCB(11)Me(11)] (Binor-S = 1,2,4,5,6,8-dimetheno-s-indacene) which contains a (C-C)→Rh interaction. The data are analysed using Bader's "Atoms in Molecules" (AIM) approach with particular attention paid to the interactions around the rhodium centre. The results provide clear evidence for the σ(C-C)→Rh interaction in the solid-state which is classified as a weak covalent interaction. These results are supported by theoretical calculations.

The electronic structure of so-called 'xenophilic' clusters, which contain both organometallic fragments and Werner-type paramagnetic transition metal centres, presents a challenge to simple theories of bonding. Density functional theory shows clearly that the cluster Mn(2)(thf)(4)(Fe(CO)(4))(2) is best described as an exchange-coupled Mn(II)(2) dimer, the closed-shell organometallic [Fe(CO)(4)](2-) fragments acting simply as bridging ligands. The high-spin configuration of the Mn(II) ions leads to single occupation of the Mn-Fe σ* orbitals and therefore substantially weaker metal-metal bonding than in conventional low-valent organometallic clusters. The transition metal fragments are effective mediators of superexchange (J(calc) = -44 cm(-1)), leading to the measured effective magnetic moment of ~5 μ(B) at 300 K, considerably lower than the limiting value of 8.37 μ(B) for two uncoupled S = 5/2 Mn(II) centres.