EXCITED-STATE DISTORTIONS IN LIGAND TO LIGAND CHARGE-TRANSFER EXCITED-STATES OF METAL DIIMINE DITHIOLATE MIXED-LIGAND COMPLEXES

The electronic absorption and resonance Raman spectra of the metal diimine dithiolate complexes M(mnt)(baba) where M = Ni and Pd, mnt = 1,2-maleonitrile, and baba = biacetylbisaniline are reported. The absorption spectra obtained from a low temperature frozen glass exhibit weakly resolved vibronic structure. The spectra are interpreted by using the time-dependent theory of spectroscopy. The electronic spectra are fit by using a calculation method for electron transfer spectra that includes coupling between potential surfaces (including the coupling of electronic and nuclear motions, i.e. the breakdown of the Born-Oppenheimer approximation). The results of this detailed calculation are compared to those involving uncoupled potential surfaces. Quantitative bond length changes are calculated. The distortions on both ligands that are observed support the assignment of the lowest energy excited state as arising from a ligand to ligand electron transfer transition from the dithiolate ligand to the diimine ligand.