Ligand dependence of metal-metal bonding in the d(3)d(3) dimers M2X9n- (M-III = Cr, Mo, W; M-IV = Mn, Tc, Re; X = F, Cl, Br, I)

The ligand dependence of metal-metal bonding in the d(3)d(3) face-shared M2 X9n- (M-III = Cr, Mo, W; M-IV = Mn, Te, Re; X = F, Cl, Br, I) dimers has be en investigated using density functional theory. In general, significant di fferences in metal-metal bonding are observed between the fluoride and chlo ride complexes involving the same metal ion, whereas less dramatic changes occur between the bromide and iodide complexes and minimal differences betw een the chloride and bromide complexes. For M = Mo, Tc, and Re, change in t he halide from F to I results in weaker metal-metal bonding corresponding t o a shift from either the triple metal-metal bonded to single bonded case o r from the latter to a nonbonded structure. A fragment analysis performed o n M2X93- (M = Mo, W) allowed determination of the metal-metal and metal-bri dge contributions to the total bonding energy in the dimer. As the,halide c hanges from F to I, there is a systematic reduction in the total interactio n energy of the fragments which can be traced to a progressive destabilizat ion of the metal-bridge interaction because of weaker M-X-bridge - bonding as fluoride is replaced by its heavier congeners. In contrast, the metal-me tal interaction remains essentially constant with change in the halide.