Solid-state metal carbonyls. Part 2.—Single-crystal infrared and Raman spectra of manganese and rhenium pentacarbonyl bromides and dimanganese and dirhenium decacarbonyls

Abstract

Single-crystal i.r. reflectance and transmittance spectra have been obtained for M(CO)₅Br and M₂(CO)₁₀, M = Mn, Re. Together with some additional Raman data for the bromides, these data have led to revised assignments for all four materials which show a good degree of internal consistency. In particular, we report the first detailed investigation of the far-i.r. spectra of these compounds. Almost all the predicted lattice modes were found, as well as the internal modes.

The i.r.-reflectance spectra were especially useful in showing that the v₁₅, Ev(CO) mode of the M(CO)₅Br compounds suffers a site-group splitting of 30 and 40 cm^–1 for Mn and Re, respectively. The A₁(2) mode in each molecule drops by nearly 20 cm^–1 from solution to solid but the other v(CO) modes are not much affected.

For the v(M—C), δ(MCO) region the assignment deduced for Mn(CO)₅Br differs from that of D. K. Ottesen, H. B. Gray, L. H. Jones and M. Goldblatt, Inorg. Chem., 1973, 12, 1051, in having v₁₀, B₁ and v₁₃, B₂ interchanged. Several modifications to extant assignment of the dimeric carbonyls were made on the basis of the new i.r. evidence.

Comparison of the Mn(CO)₅Br and Mn₂(CO)₁₀ assignments clearly confirms the relative changes in σ- and π-bond contributions on substituting Br for Mn(CO)₅, but differentiates between the (small) axial and (large) equatorial effects.