Negative ion Photoelectron spectroscopy of organometallic radicals: MCO and HMCO complexes of the first transition series

Photoelectron spectroscopy of coordinatively unsaturated organometallic anions can provide a means to probe the ground and low lying excited states of the corresponding neutral radicals. We report results for the early 3d transition metal monocarbonyls VCO and CrCO, and for the late metal complexes FeCO, CoCO and NiCO. Each spectrum displays a transition to the ground state of the neutral complex, and to an excited state whose spin multiplicity differs by two from that of the ground state. For a given complex, these states share nominally the same electron configuration but differ in the spin coupling of the metal 4s electron. There is a reversal in the state ordering as one proceeds across the transition series, from a high spin ground state for VCO (⁶σ⁺) and CrCO (⁷σ⁺) to a low spin ground state for FeCO (³σ^-), CoCO (²Δ) and NiCO (¹σ⁺). The measured state splittings and vibrational frequencies provide insight into the factors that determine the ordering and bonding properties of these states. Recent results for the linear H-M-CO isomers of Fe and Co are also reported.