The nu(CO) vibrational spectra of planar transition metal carbonyl clusters.

The nu(CO) vibrational spectra of planar transition cluster carbonyls containing M(CO)(4) groups are studied. It is possible to anticipate qualitatively, both for the infrared and Raman, the band intensity changes associated with increasing metallic nature of the cluster. These enable a unification of the band patterns shown by the species reported. As for (idealized) spherical clusters, the spherical harmonic model (SHM), suitably modified, becomes of more general applicability as cluster size increases, although for smaller species the tensor harmonic model (THM) makes a contribution.

Vibrational spectra of bridging carbonyl groups in transition metal carbonyl clusters.

The spherical harmonic model (SHM), previously used for the analysis of the terminal nu(CO) vibrations of transition metal carbonyl clusters, is applied to the corresponding bridging CO modes. The model is applicable, although the spectra show a greater sensitivity to the molecular geometry than is the case for their terminal counterparts. The reasons for this sensitivity are discussed. When both micro(2) and micro(3) CO groups are present in a molecule, a spectral distinction may not be apparent.

Application of the Spherical Harmonic Model to the Study of the Terminal nu(CO) Spectra of Transition Metal Carbonyl Clusters.

The application of the spherical harmonic model to the interpretation of the terminal nu(CO) spectra of transition metal carbonyl clusters is explored. Unless there is strong spectral evidence to the contrary (when the tensor harmonic model is applicable), the coupling between CO vibrators at each metal atom is to be ignored when these vibrators are symmetry-related. The overwhelming majority of carbonyl clusters conform to the spherical harmonic model, either in its simplest form-in which only a single infrared band is observed in the solution infrared spectrum-or in its more elaborate form. In the latter, bands of lower intensity are observed on the low-frequency side of the intense band. The greater the separation from the intense band, the weaker the additional band, indicating an intensity stealing mechanism. The observations have been interpreted in terms of a "cluster selection rule" analogous to the "surface selection rule" of metal surface spectroscopy and the implications of this rule are discussed.