A study of structural and bonding variations in the homologous series [Mo2(CN)6(dppm)2]n- (n=2, 1, 0)

Reaction of Mo2Cl4(dppm)(2) (dppm = bis(diphenylphosphino)methane) with 6 equiv of [n-Bu4N][CN] or [Et4N][CN] in dichloromethane yields [n-Bu4N](2)[Mo-2(CN)(6)(dppm)(2)] (1) and [Et4N](2)[Mo-2(CN)(6)(dppm)(2)] (2), respectively. The corresponding one- and two-electron oxidation products [n-Bu4N][Mo-2(CN)(6)(dppm)(2)] (3) and Mo-2(CN)(6)(dppm)(2) (4) were prepared by reactions of 1 with the oxidant NOBF4. Single-crystal X-ray structures of 2.2CH(3)CN, 3.2CH(3)-CN.2H(2)O and 4.2CH(3)NO(2) were performed, and the results confirmed that all three complexes contain identical ligand sets with trans dppm ligands bisecting the Mo-2(mu-CN)(2)(CN)(4) equatorial plane. The binding of the bridging cyanide ligands is affected by the oxidation state of the dimolybdenum core as evidenced by an increase in side-on pi-bonding overlap of the mu-CN in going from 1 to 4. The greater extent of pi-donation into Mo orbitals is accompanied by a lengthening of the Mo-Mo distance (2.736(1) Angstrom in Mo-2(II,II) (2), 2.830(1) Angstrom in Mo-2(II,II) (3), and 2.936(1) Angstrom in Mo-2(III,III) (4)). A computational study of the closed-shell members of this homologous series, [Mo-2-(CN)(6)(dppm)(2)](n) (n = 2-, 0), indicates that the more pronounced side-on pi-donation evident in the X-ray structure of 4 leads to significant destabilization of the delta orbital and marginal stabilization of the delta* orbitals with respect to nearly degenerate delta and delta* orbitals in the parent compound, 2. The loss of delta contributions combined with the reduced orbital overlap due to higher charges on molybdenum centers in oxidized complexes 3 and 4 is responsible for the observed increase in the length of the Mo-Mo bond.