VIBRATIONAL STUDY OF THE EFFECTS OF HIGH EXTERNAL PRESSURES ON THE STRUCTURE AND PHASE-BEHAVIOR OF PENTACARBONYL(METHYL)MANGANESE(I) AND PENTACARBONYL(METHYL)RHENIUM(I)

Micro-Raman spectra of crystalline CH3M(CO)5 (M = Mn, Re) have been recorded in the CO stretching and low-energy regions at ambient temperature and selected pressures up to 40 kbar with the aid of a diamond-anvil cell. The IR spectrum of the manganese compound was similarly investigated in the CH stretching region at various pressures up to 18 kbar. The vibrational data indicate that both complexes undergo a phase transition (most probably second order): CH3Mn(CO)5, at approximately 9 kbar; CH3Re(CO)5, at approximately 22 kbar. The plots of pressure vs wavenumber for the CO stretching modes reveals that the pi-back-bonding interactions between the metals M and the CO groups (especially the axial ones trans to CH3) in the low-pressure phase of CH3Mn(CO)5 and in both phases of CH3Re(CO)5 are appreciably strengthened by increases in the applied external pressure. The vibrational modes in the low-pressure phases exhibit similar pressure dependences, in agreement with the isomorphous structures of the two compounds. There are significant differences, however, in the pressure dependences of the vibrational modes of the high-pressure phases. The extent of the pi-back-bonding is apparently steadily decreasing for CH3Mn(CO)5, while it is increasingly favored for CH3Re(CO)5.