Vibrational Fine Structure in the Valence Ionizations of Transition-Metal Hexacarbonyls: New Experimental Indication of Metal-to-Carbonyl π Bonding

The first observations of metal-carbon vibrational structure in photoionization bands are reported. Attention is focused on the predominantly metal d ionizations of M(CO)₆ (M = Cr, Mo, and W), and the methods for obtaining high resolution and very high signal-to-noise He I ionization data are detailed. The ²T_2g ionization band of Cr(CO)₆ and the spin-orbit split ²E″ and ²U′ bands of W(CO)₆ show distinct vibrational progressions which correspond to the totally symmetric (a_1g) metal-carbon stretching mode in the positive ion states. The metal-carbon stretching frequencies are found to be significantly less in the positive ion states than in the ground states, indicating a reduction of metal-carbon bond order upon the loss of a t_2g electron. Evaluation of the vibrational progressions shows that the metal-carbon bond length increases on the order of 0.10 Å upon t_2g ionization in the case of W(CO)₆ and about 0.14 Å in the case of Cr(CO)₆. In addition, the beginning of a short progression in the a_1g carbon-oxygen stretching mode is observed in the Mo(CO)₆ spectrum and is clearly seen in the W(CO)₆ spectrum. All of these observations show that removal of an electron from the predominantly metal t_2g orbitals, which are strictly π symmetry with respect to the carbonyls, substantially weakens the metal-to-carbonyl bond.