Investigations of time-dependent delocalization and the correlation of carbonyl IR shifts with ΔE1/2 values 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 (C5H4R)Mn(CO)(2)PPh3 and variations thereof. Six dinuclear complexes are each oxidized in two one-electron steps with E-1/2 separations (Delta 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, Delta rho, that estimates the relative charges at the two redox sites and which correlates linearly with the Delta E-1/2 values. This correlation extends to mixed-valent Cr(I)Cr(O) systems linked by a biphenyl bridge. The doubly linked system [(Fulv){Mn(CO)(2)}(2)(mu-dppm)](-), 1(+), displays trapped valence in its IR spectra, but delocalized valence in its ESR spectra, implying a time-dependent delocalization process.