Solid-state metal carbonyls. Part 1.—Single-crystal infrared and Raman spectra of the group VI hexacarbonyls

Abstract

Single-crystal infrared reflectance spectra in polarised light (200–2500 cm^–1) at liquid-nitrogen temperature are reported for M(CO)₆(M = Cr, Mo, W). The reflectance accessory used is described. All the permitted factor-group components are identified and used to construct the first complete assignment for the v(CO) region of these solids. The general features of the mull spectra are accounted for. Both site- and factor-group splittings for the v₃(E_g) and v₆(T_1u) molecular modes are deduced: these effects are very slight for v₃ but considerable for v₆. The weak Raman band near 1966 cm^–1 in Mo(CO)₆ was shown to be of v₆ origin.

In the v(M—C), δ(MCO) region the reflectance spectra allowed identification of factor-group components but the splitting is small except for v₇(T_1u) of Cr(CO)₆.

Many new bands are found in the far-i.r. region, the lowest being at 31.5 cm^–1. All the permitted lattice modes are located. Components of the molecular modes v₉(T_1u), v₁₁(T_2g) and v₁₃(T_2u) are all present in the numbers allowed by site group rules. Combining i.r. and Raman data, they are located near 90, 110 and 65 cm^–1, respectively. Bands below 60 cm^–1 are attributed to translatory and libratory lattice modes.