Vapour-phase Raman spectra of chromium, molybdenum, and tungsten hexacarbonyls

Abstract

Vapour-phase Raman spectra of the hexacarbonyls [M(CO)₆](M = Cr, MO, or W) have been recorded for the first time, and four of the six Raman-active fundamentals in the case of [Mo(CO)₆] and [W(CO)₆], and five of the six in the case of [Cr(CO)₆]; have been located. The values obtained compare well with those obtained from analyses of the wavenumbers of combination bands in the infrared. The half-band widths of the ν₁(a_1g), ν₂(a_1g), ν₃(e_g), and ν₁₁(t_2g) fundamentals are 5–6, 9–13, 18–20, and 16–18 cm^–1 respectively at the sample temperatures of 369 (Cr), 378 (MO), and 377 K(W). Rotational contours (OP. RS) to the non-totally symmetric bands are obscured by the presence of underlying hot bands. Vapour–solution wavenumber shifts are positive for the ν₁(a_1g) and ν₃(e_g) bands (both of which are CO-stretching modes), but negative for the ν₂(a_1g) and ν₁₁(t_2g) bands (MC-stretching and CMC-bending modes respectively). The relative values of the CO bond-polarisability derivatives, α⊥′/α_∥′, from vapour-phase measurements, lie in the –0.26₅ to –0.24 region.