INDO‐Type calculations on the ground state and various ionic states of transition metal tricarbonyls

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By means of the ΔSCF and transition operator (TO) methods based on a recently developed INDO extension to the first transition metal series, the first ionization potentials of benzene—chromium tricarbonyl ( I ), cyclopentadienyl manganese tricarbonyl ( II ), the iron—tricarbonyl complexes with trimethylenemethane ( III ), and cyclobutadiene ( IV ) have been calculated and compared with experimental data. It is shown that the electronic structure of I to IV can be rationalized by Hoffmann's fragment approach in both the ground state and the cationic hole states. Within the series I—IV there are remarkable energy differences in the ground state for MOs derived from the 1 a 1 and 1 e orbitals of the M(CO) 3 fragment. The observation that only one band is associated with the ionization events from MOs predominantly localized at the metal site is traced back to large relaxation effects. In the cationic hole states the split of the M(CO) 3 fragment orbitals 1 a 1 and 1 e is minute in all four compounds.