The infrared and laser Raman spectra of the M2C193" (M = Cr, W) complex ions are reported, and all observed fundamentals in the spectra of Cs3Cr2Clg, ( )30 2 9, (Bu4N)3Cr2Cls, K3W2Clg, Cs3W2Clg, and (Bu4N)3W2Clg are assigned on the basis of exact or approximate D3¡, symmetry. Vibrational normal coordinate analyses for the complex ions in Cs3Cr2Clg and K3W2Clg are reported which yield excellent agreement between calculated and assigned spectra and which are based on both reasonable spectroscopic assumptions and a general symmetrized valence force field. Contributions of the M-Clb-M and Clb-M-Clb deformations of the tri-^-chloro-dimetalate unit to the molecular force field of the Cr2Clg3~ion are assessed assuming no direct Cr-Cr interaction and are then transferred exactly to the molecular force field of the W2Clgs ~ion. The direct W-W interaction is then determined by fitting the calculated to the observed spectra. The tungsten-tungsten stretching force constant, £w-w, is calculated to be 1.15 ± 0.1 mdyn/'A. The vibrational eigenvectors and potential energy distribution show that this force field element contributes significantly to several of the Ai' vibrational modes, and they are analyzed to develop the basis for treating metal cluster species of the MsXs and M6Xi2 types.
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