Collection of Czechoslovak Chemical Communications - digital archive 

Abstract

The forward- and back-donations of electrons between the CO ligand and central transition element atom lead to a system of cumulated multiple bonds of the type X=Y=Z. From a "vibrational" point of view this fact means a non-separability of vibrational motions of the three MCO nuclei. A coupling of the stretching vibrations ν₁(MC) and ν₃(CO) leads to analogous fundamental frequencies as in the case of the isolated stable molecules of O=C=O, S=C=O, Se=C=O, etc. The magnitude of the vibrational coupling between ν₁ and ν₃ is roughly proportional to the fundamental frequency of the degenerated deformation vibration ν_2ab of the MCO grouping, i.e. a ratio of the frequencies should be constant. With free molecules of CO₂, SCO, CS₂, N₂O and others, the ratio (ν₁ + ν₃)/ν_2ab of the fundamental frequencies is equal to 5.5 like with complex anions [(CO)PtX₃]^- regardless X = Cl, Br or I. For other transition metal carbonyls the ratio is within the range of 4.0-6.0. On the other hand, the H₃B-C=O molecule as a system with the essentially single B-C and the triple C=O bond is characterized by a markedly different value (9.0). In the latter, the local electronic state of CO is similar to the electronic ground state of the positive ion CO⁺ of the free molecule. In the transition metal carbonyls the chemical pseudoexcitation of the CO ligand leads to its local electronic states which are similar to electronic excited states lying between an excited valence state of the free neutral CO molecule (back-donation is equal to one electron) and an excited valence state of the free negative ion CO^- (back-donation is equal to two electrons.