Electronic spectra and structures of d2 molybdenum-oxo complexes.: Effects of structural distortions on orbital energies, two-electron terms, and the mixing of singlet and triplet states

Molybdenum-oxo ions of the type [(MoOL4Cl)-O-IV](+) (L = CNBut, PMe3, 1/2Me(2)PCH(2)CH(2)PMe(2)) have been studied by X-ray crystallography, vibrational spectroscopy, and polarized single-crystal electronic absorption spectroscopy (300 and ca. 20 K) in order to investigate the effects of the ancillary ligand geometry on the properties of the Moequivalent toO bond. The idealized point symmetries of the [(MoOL4Cl)-O-IV](+) ions were established by X-ray crystallographic studies of the salts [MoO(CNBut)(4)Cl][BPh4] (C-4nu), [MoO(dmpe)(2)Cl]Cl.5H(2)O (C-2nu), and [MoO(PMe3)(4)Cl][PF6] (C-2nu); the lower symmetries of the phosphine derivatives are the result of the steric properties of the phosphine ligands. The Moequivalent toO stretching frequencies of these ions (948-959 cm(-1)) are essentially insensitive to the nature and geometry of the equatorial ligands. In contrast, the electronic absorption bands arising from the nominal d(xy) --> d(xz), d(yz) (n --> pi*(MoO)) ligand-field transition exhibit a large dependence on the geometry of the equatorial ligands. Specifically, the electronic spectrum of [MoO(CNBut)(4)Cl](+) exhibits a single (1)[n --> pi*(xz,yz)] band, whereas the spectra of both [MoO(dmpe)(2)Cl](+) and [MoO(PMe3)(4)Cl](+) reveal separate (1)[n --> pi*(xz)] and (1)[n --> pi*(yz)] bands. A general theoretical model of the n --> pi* state energies of structurally distorted d(2) M(equivalent toE)L4X chromophores is developed in order to interpret the electronic spectra of the phosphine derivatives. Analysis of the n --> pi* transition energies using this model indicates that the d(xz) and d(yz) pi*(Moequivalent toO) orbitals are nonclegenerate for the C-2nu-symmetry ions and the n --> pi*(xz) and n --> pi*(yz) excited states are characterized by different two-electron terms. These effects lead to a significant redistribution of intensity between certain spin-allowed and spin-forbidden absorption bands. The applicability of this model to the excited states produced by delta --> pi and pi --> delta symmetry electronic transitions of other chromophores is discussed.