Magnetic circular dichroism spectra of bis(η-cyclopentadienyl)-nickel and -cobalt

Abstract

Electronic and magnetic circular dichroism spectra are reported of cobaltocene and nickelocene, [M(η-C₅H₅)₂](M = Co and Ni), isolated in argon and nitrogen matrices. Most, but not all, of the cobaltocene spectra exhibit a very well developed vibrational fine structure which is attributed to a progression in an e_2g vibrational mode which becomes totally symmetric when the symmetry of the molecule is lowered by a Jahn–Teller distortion. No similar effect is clearly visible in the nickelocene spectra but it is suggested that the structure on the long-wavelength bands of both compounds should be attributed to vibronic effects rather than to individual d–d transitions. Reasons for the marked similarity between the room-temperature solution and matrix spectra of nickelocene are discussed.