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.