A review is given of the values of the CO bond-stretching wavenumbers, νCO (in cm−1, and the CO internuclear distances, r(CO) (in Å), in respectively the vibrational spectroscopic and crystallographic literatures of carbonyl-containing inorganic coordination and cluster compounds. Particular attention is given to the published results for the less common types of CO/metal bonding patterns.
For the linearly-bonded MCO (σ-CO) groups and the two-fold bridge-bonding M2CO(μ2-CO) groups, the very extensive literature is reduced by considering primarily CO ligands of these types of Pt-based compounds; for the less-common forms of CO ligand the coverage is more comprehensive. Where multiple CO ligands of the same type occur in a given compound the appropriately averaged νCO values are considered.
The relatively wide wavenumber-ranges found for νCO values of MCO and M2CO groups are analysed in terms of the effects of net charges or of electron-donating or electron-withdrawing CO-ligands on the same metal atoms. For the Pt compounds the approximate νCO wavenumber shifts, relative to the values for compounds with all-carbonyl ligands, are as follows: a unit negative charge, −190 cm−1; PR3 −50 (aryl ligands) to 70 cm−1 (alkyl ligands); cyclopentadienyl, ca. −50 cm−1; Cl or Br, ca. +50 cm−1. With isoelectronic sequences involving different metals, e.g. V(CO)−6 and Cr(CO)6, or Mo(CO)6 and Re(CO+6, the effect of a unit negative charge is ca. −140 cm−1, and of a unit positive charge is ca. +90 cm−1, relative to the values for the uncharged complexes.
In the absence of such net charges or strongly-perturbing ligands on the same metal atom, the average νCO values for the various types of CO ligands are estimated to be as follows: MCO (σ-CO), 2070-2050; M2CO(μ2-CO) 1870; M3CO(μ3-CO) 1800; ketocarbonyls Mk2CO (μk2-CO), ca. 1700 for Pt; isocarbonyls MCO⋯M′, 1730 and M3CO⋯M′, 1380; dihapto carbonyls, MCO/M′, 1700 and M3CO/M′, 1450; and MCO/2M′, i.e. CO σ-bonded to one metal atom and π-bonded to two others, 1380 cm−1. These values are shown to correspond well to those observed at low coverage from MCO, M2CO and M3CO species on flat metal surfaces, and the additional values are used to consider the structural origins of species which give low-wavenumber νCO values on metal surfaces, usually involving steps, or elements to the left of transition-metal series.
With few exceptions, some of which clearly relate to experimental errors, an expected smoothly curved relationship is found between the related νCO and r(CO) values, regardless of the chemical structures of the carbonyl-containing molecules. Typical pairs of value (Å/cm−1) are 1.15/2030; 1.20/1730; 1.25/1490; 1.30/1320; 1.35/1180, with probable errors of ±0.025 Å/+-100 cm−1
