Two-electron redox energetics in ligand-bridged dinuclear molybdenum and tungsten complexes

Electron-transfer energetics of bridged dinuclear compounds of the form [(CO)(4)M(mu-L)](2)(0/1-/2-) (M = Mo, W; L = PPh(2)(-), SPh(-)) were explored using density functional theory coupled to a continuum solvation model. The experimentally observed redox potential inversion, a situation where the second of two electron transfers is more thermodynamically favorable than the first, was reproduced within this model. This nonclassical energy ordering is a prerequisite for the apparent transfer of two electrons at one potential, as observed in many biologically and technologically important systems. We pinpoint the origin of this phenomenon to be an unusually unfavorable electrostatic repulsion for the first electron transfer due to the redox noninnocent behavior of the bridging ligands. The extent of redox noninnocence is explained in terms of an orbital energy resonance between the metal-carbonyl and bridging ligand fragments, leading to a general mechanism by which potential inversion could be controlled in diamond-core dinuclear systems.