| Abstract: | Several edge-sharing bioctahedral compounds containing tungsten, tantalum or niobium M₂⁶⁺ cores have been prepared and structurally characterized. These include W₂Cl₆(dppm)₂, W₂Cl₆(dmpm)₂, W₂(SPh)₂Cl₄(dppm)₂, W₂(SPh)Cl₅(dppm)₂, W₂(SePh)₂Cl₄(dppm)₂, W₂(SePh)Cl₅(dppm)₂, Nb₂Cl₆(SMe₂)₄, Ta₂Cl₆(SMe₂)₄, Ta₂Cl₆(dto)₂, Nb₂Cl₆(dto)₂, Nb₂Cl₆(dene)₂, Ta₂Cl₆(dene)₂ and Nb₂Cl₆(depe)₂ where dppm is bis(diphenylphosphino)methane, dmpm is bis(dimethylphosphino)methane, dto is 3,6-dithiaoctane, dene is N,N,N',N'-tetraethylethylenediamine, and depe is 1,2-bis(diethylphosphino)ethane. The quadruply bonded tungsten dimer, W₂Cl₄(dppm)₂, has also been prepared and structurally characterized. Oxidative-addition reactions on this compound resulted in the formation of the aforementioned edge-sharing bioctahedral compounds of tungsten with exception of W₂Cl₆(dmpm)₂ which was formed from an oxidation-addition reaction involving W₂Cl₄(dmpm)₂ generated in situ. The two electron oxidation of the W₂⁴⁺ species results in the conversion of the tungsten-tungsten quadruple bond into a tungsten-tungsten electron-rich single bond where the orbitals involved in metal-metal bonding are filled in the order of σ<<π<<δ* rather than σ<<π<δ. The confirmation of this ordering is seen by comparing W₂Cl₆(dppm)₂ and W₂Cl₆(dmpm)₂ with a series of M₂Cl₆(dppm)₂ and M₂Cl₆(dmpm)₂ compounds containing second and third row group 5 to group 8 metal atoms. The niobium and tantalum compounds were prepared from ligand substitution reactions on M₂Cl₆(SMe₂)₃ (M = Nb, Ta). Interesting and unusual structural features were found for the sulfur containing compounds, Nb₂Cl₆(SMe₂)₄, Ta₂Cl₆(SMe₂)₄, Ta₂Cl₆(dto)₂ and Nb₂Cl₆(dto)₂, where the latter compound contains a typical metal-metal bond length for a double bond, while the former three compounds contain relatively long metal-metal distances. While the exact nature of the metal-metal bonding in Nb₂Cl₆(SMe₂)₄, Ta₂Cl₆(SMe₂)₄ and Ta₂Cl₆(dto)₂ remains uncertain, interpretation of the molecular orbital calculations has shown a relationship between metal-metal bond length and Cl[subscript ax]-M-Cl[subscript ax] bond angle which is not related to steric repulsion between axial ligands. Preparation and structural information of edge-sharing bioctahedra are presented. |
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