The electronic absorption spectra of the complexes [(OC),Co-‘M”Co-‘(CO),]
with M = Zn, Cd, and Hg display an intense long-wavelength band which is
assigned to the allowed Σu+ -> Σg+ transition from a non-bonding σ to an
antibonding σ orbital of the Co-M-Co moiety. This transition is shifted to higher
energies and gains an increasing charge transfer (CT) contribution in changing M in
the order M = Hg, Cd, and Zn. In a limiting description the σn -> σ* transition can
be regarded as a metal to metal (MM) CT transition from Co-I to MII. MMCT
excitation induces a photoredox reaction: [(OC),Co-I-MII-Co-I(CO)4] ->MO +
]Co02(CO)8]. The quantum yields are φ = 0.45 (λirr = 333 nm) for M = Hg and
φ = 0.03 ( λirr = 313 nm) for Cd.