Electron delocalization in the S1 and T1 metal-to-ligand charge transfer states of trans-substituted metal quadruply bonded complexes

The singlet S-1 and triplet T-1 photoexcited states of the compounds containing MM quadruple bonds trans-M-2(TiPB)(2)(O2CC6H4-4-CN)(2), where (TPB)-P-i = 2,4,6-triisopropylbenzoate and M = Mo (I) or M = W (I'), and trans-M-2(O2CMe)(2)((N[Pr-i])(2)CC = CC6H5)(2), where M = Mo (II) and M W (II'), have been investigated by a variety of spectroscopic techniques including femtosecond time-resolved infrared spectroscopy. The singlet states are shown to be delocalized metal-to-ligand charge transfer (MLCT) states for I and I' but localized for II and II' involving the cyanobenzoate or amidinate ligands, respectively. The triplet states are MoMo delta delta* for both I and II but delocalized (MLCT)-M-3 for I' and localized 3MLCT for II'. These differences arise from consideration of the relative orbital energies of the M-2 delta or M-2 delta* and the ligand pi* as well as the magnitudes of orbital overlap.