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

Abstract

The ¹(δ → δ*) resonance-Raman and infrared spectra of [NEt₄]₄[Mo₂(¹³CN)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 ¹²C compound in the 300–415 cm⁻¹ region shift to lower wavenumbers (as, of course, do the ν(CN) bands at ca. 2100 cm⁻¹) and change in relative intensity upon ¹³C substitution. Normal-coordinate calculations on the two isotopomers of the [Mo₂(CN)₈]⁴⁻ ion reveal that the three modes responsible for the resonance-enhanced Raman bands are strongly mixed and of ν(MoMo), ν(MoC), and λ_op(MoCN) parentage, and that their intensities roughly scale with the amount of ν(MoMo) character in the modes.