Vibrational spectroscopy at high pressures. 54. Decacarbonyldimanganese and decacarbonyldirhenium

Raman spectra of Mn2(CO)10 and Re2(CO)10 have been studied to 27 and 98 kbar respectively, and the infrared (IR) spectrum of Re2(CO)10 to 30 kbar at ambient temperature. Both materials undergo a first-order structural phase transition at high pressure (Mn at approximately 8 kbar, Re at approximately 5 kbar) which is accompanied by dramatic changes in all regions of their vibrational spectra. These data are interpreted in terms of a change of molecular geometry from D4d to D4h. In the new high-pressure phases the M to carbonyl backbonding is increased significantly for the axial but not the equatorial carbonyls. The molecular torsion is implicated in the phase change but is not thought to drive it.