Why Are the Terminal ν(CO) Infrared Spectra of Metal Cluster Carbonyls So Often So Simple?

Two spherical tensor harmonic models for the spectral intensities of infrared and Raman bands of large, and particularly spherical, molecules are presented and applied to the terminal ν(CO) modes of homonuclear metal carbonyl clusters. The first is applicable when it appears that the coupling between CO groups on different metals effectively competes with that on one metal. It predicts that the closer the arrangement of carbonyl groups approaches spherical, the closer will the spectral pattern be to a single infrared band and two Raman bands, one on either side of the infrared band. When it appears that coupling between CO groups on one metal dominates that between CO groups on different metals, a different intensity pattern is expected, although simplifications compared to the results of a point-group analysis are again predicted.