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 [MoIVOL4Cl]+ (L = CNBut, PMe3, 1/2Me2PCH2CH2PMe2) 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 Mo⋮O bond. The idealized point symmetries of the [MoIVOL4Cl]+ ions were established by X-ray crystallographic studies of the salts [MoO(CNBut)4Cl][BPh4] (C4v), [MoO(dmpe)2Cl]Cl·5H2O (C2v), and [MoO(PMe3)4Cl][PF6] (C2v); the lower symmetries of the phosphine derivatives are the result of the steric properties of the phosphine ligands. The Mo⋮O 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 dxy → dxz, dyz (n → π*(MoO)) ligand-field transition exhibit a large dependence on the geometry of the equatorial ligands. Specifically, the e...