Optical Spectra of Transition-Metal Carbonyls: Cr(CO)₆, Fe(CO)₅, and Ni(CO)₄

Near-UV gas-phase optical spectra and absolute absorption cross sections of the transition-metal carbonyls Cr(CO)₆, Fe(CO)₅, and Ni(CO)₄ are presented. They have been recorded using a fast-scanning spectral photometer to minimize heterogeneous decomposition during the measurement. The excited states in the experimental energy range were analyzed employing an INDO/S CI formalism. Calculations on the basis of single excitation CI for singlet-singlet transitions yield good agreement both for transition energies and intensities. From these theoretical models it is deduced that transition-metal carbonyl near-UV spectra are determined exclusively by metal-to-ligand charge-transfer transitions, whereas d → p and d → s excitations do not contribute significantly below 6.2 eV (above 200 nm). The lowest lying singlet excitations in Cr(CO)₆ and Fe(CO)₅ are predicted to be d → d. Although they are predicted to have little or no dipole allowed character, they should be observable because they are well separated from the first allowed bands. Indeed, they have been detected for Cr(CO)₆. The spectra of several substituted complexes (CO)₅CrR (R = CS, PF₃, CHOH) are also calculated and compared with available experimental data.