Electronic spectra of some pentacarbonylhalogenometal complexes and related species

Abstract

U.v. and visible absorption spectra of the complexes [M(CO)₅X](M = Mn, Re, Cr, MO, or W; X = Cl, Br, or I) are reported. These and spectra of [Mn(CO)₅X](X = H, Me, CF₃, and SiH₃) and other metal carbonyl halide complexes are assigned using a molecular-orbital bonding picture based on recent approximate calculations and photoelectron spectra of these species. The highest-filled orbital of the halogeno-complexes is considered to have part metal d_π and part halogen p_π character. The unique lowest-energy band, which gives rise to the colour of these complexes and is not present in the other metal pentacarbonyl species discussed, is assigned to a ¹A₁→¹E transition with electron transfer from this highest-filled orbital to a mainly σ* orbital of the metal–halogen bond. This is in contrast to earlier assignments which considered the acceptor orbital to have mainly π*(CO) character.