The involvement of metal-to-CO charge transfer and ligand-field excited states in the spectroscopy and photochemistry of mixed-ligand metal carbonyls.: A theoretical and spectroscopic study of [W(CO)4(1,2-ethylenediamine)] and [W(CO)4(N,/N′-bis-alkyl-1,4-diazabutadiene)]

A new interpretation of the electronic spectroscopy, photochemistry, and photophysics of group 6 metal cis-tetracarbonyls [M(CO)(4)L-2] is proposed, that is based on an interplay between M-->L and M-->CO MLCT excited states. TD-DFT and resonance Raman spectroscopy show that the lowest allowed electronic transition of [W(CO)(4)(en)] (en=1,2-ethylenediamine) has a W(COeq)(2)-->COax charge-transfer character, whereby the electron density is transferred from the equatorial W(COeq)(2) moiety to pi* orbitals of the axial CO ligands, with a net decrease of electron density on the W atom. The lowest, emissive excited state of [W(CO)(4)(en)] was identified as a spin-triplet W(COeq)(2)-->COax CT excited state both computationally and by picosecond time-resolved IR spectroscopy. This state undergoes 1.5 ps vibrational relaxation/solvation and decays to the ground state with a similar to160 ps lifetime. The nu(CO) wavenumbers and IR intensity pattern calculated by DFT for the triplet W(COeq)(2)-->COax CT excited state match well the experimental time-resolved spectrum. For [W(CO)(4)(R-DAB)] (R-DAB=N,N'-bis-alkyl-1,4-diazabutadiene), the W(COeq)(2)-->COax CT transition follows in energy the W-->DAB MLCT transition, and the emissive W(COeq)(2)-->COax CT triplet state occurs just above the manifold of triplet W-->DAB MLCT states. No LF electronic transitions were calculated to occur in a relevant energetic range for either complex. Molecular orbitals of both complexes are highly delocalized. The 5d(W) character is distributed over many molecular orbitals, while neither of them contains a predominant metal-ligand sigma* 5d(W) component, contrary to predictions of the traditional ligand-field approach. The important spectroscopic, photochemical, and photophysical roles of M(COeq)(2)-->COax CT excited states and the limited validity of ligand field arguments can be generalized to other mixed-ligand carbonyl complexes.