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)₄L₂] 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(CO_eq)₂ → CO_ax charge-transfer character, whereby the electron density is transferred from the equatorial W(CO_eq)₂ moiety to π* 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(CO_eq)₂ → CO_ax 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 ∼160 ps lifetime. The ν(CO) wavenumbers and IR intensity pattern calculated by DFT for the triplet W(CO_eq)₂ → CO_ax 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(CO_eq)₂ → CO_ax CT transition follows in energy the W → DAB MLCT transition, and the emissive W(CO_eq)₂ → CO_ax 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 σ* 5d(W) component, contrary to predictions of the traditional ligand-field approach. The important spectroscopic, photochemical, and photophysical roles of M(CO_eq)₂ → CO_ax CT excited states and the limited validity of ligand field arguments can be generalized to other mixed-ligand carbonyl complexes.