Oxalate bridged triangles incorporating Mo2(4+) units. Electronic structure and bonding

The reactions between [Mo(2)L(2)(CH(3)CN)(6)][BF(4)](2) compounds and [Bu(n)(4)N](2)[O(2)CCO(2)] in CH(3)CN are shown to proceed under kinetic control to the formation of a mixture of molecular triangles and squares. The molecular triangles [L(2)Mo(2)(O(2)CCO(2))](3) I (L = DPhF, PhNCHNPh) and II (L = DAniF, p-MeO-C(6)H(4)NCHNC(6)H(4)-p-OMe) are the major products, and when 0.75 equivalents of [Bu(n)(4)N](2)[O(2)CCO(2)] is employed, they are formed to the exclusion of the square. The molecular structure of II is reported based on a single crystal X-ray determination. The molecular triangles do not enter into an equilibrium with their molecular square counterparts in CH(2)Cl(2), in contrast to their perfluoroterephthalate bridged counterparts. The compounds I and II are orange and have a strong electronic transition at lambda(max) approximately 460 nm assignable to metal-to-ligand charge transfer ((1)MLCT) involving the oxalate bridge. Electronic structure calculations employing density functional theory on model compounds [(HCO(2))(2)Mo(2)(O(2)CCO(2))](3) and [(HNCHNH)(2)Mo(2)(O(2)CCO(2))](3) have been carried out and indicate the frontier occupied molecular orbitals are Mo(2) delta combinations e(4)a(2), and the lowest unoccupied are bridge pi* for the formamidinates and delta* for formates as ancillary ligands. Compounds I and II show quasi-reversible oxidation waves in their cyclic voltammograms and oxidation of II in 2-methyl-THF by reaction with AgPF(6) (1 equivalent) leads to a metal centered EPR signal, g approximately 1.95. The electronic absorption spectrum shows a low-energy broad band centered at 6418 cm(-1), which is assigned to an intervalence charge transfer (IVCT) band of a class III mixed valence ion.