Investigations of Time-Dependent Delocalization and the Correlation of Carbonyl IR Shifts with ΔE1/2Values for Hydrocarbon-Linked Class II and Class III Mixed-Valent Complexes

A series of fulvalenediyl-bridged dimanganese complexes has been studied by electrochemistry and by IR, optical, and ESR spectroscopic methods. The redox site is based on the Mn(I) moiety (C 5 H 4 R)Mn(CO) 2 PPh 3 and variations thereof. Six dinuclear complexes are each oxidized in two one-electron steps with E 1/2 separations (Δ E 1/2 ) of between 80 and 450 mV. Each of the monocations exhibits trapped valence by IR spectroscopy. Analysis of the carbonyl stretching frequencies of the mixed-valent systems allows calculation of a charge distribution parameter, Δρ, that estimates the relative charges at the two redox sites and which correlates linearly with the Δ E 1/2 values. This correlation extends to mixed-valent Cr(I)Cr(0) systems linked by a biphenyl bridge. The doubly linked system [(Fulv){Mn(CO) 2 } 2 (μ-dppm)] -, 1 +, displays trapped valence in its IR spectra, but delocalized valence in its ESR spectra, implying a time-dependent delocalization process.