The nature of the normal modes of [Mo2(CN)8]4−

The (1)(delta -->delta*) resonance-Raman and infrared spectra of [NEt4](4)[Mo-2((CN)-C-13)(8)] have been determined and are compared with those previously reported for the natural-abundance isotopomer. The three totally symmetric resonance-enhanced Raman fundamentals of the C-12 compound in the 300-415 cm(-1) region shift to lower wavenumbers (as, of course, do the nu(CN) bands at ca. 2100 cm(-1)) and change in relative intensity upon C-13 substitution. Normal-coordinate calculations on the two isotopomers of the [Mo-2(CN)(8)](4-) ion reveal that the three modes responsible for the resonance-enhanced Raman bands are strongly mixed and of nu(MoMo), nu(MoC), and lambda(op)(MoCN) parentage, and that their intensities roughly scale with the amount of nu(MoMo) character in the modes.