Vibrational spectra for the prototype transition-metal alkyl complex pentacarbonylmethylmanganese have been obtained by incoherent inelastic neutron scattering (0-3200 cm-1, 0-400 meV) and by infrared (400-4800 cm-1) and Raman (0-4000 cm-1) spectroscopy. This study provides a comprehensive comparison of these complementary techniques. The resulting data have been used to assign all but a few of the fundamental vibrational modes in the complex. Significant findings in this regard are anomalously low methyl C-H stretching frequencies (2983 and 2910 cm-1), a significantly revised assignment for the manganese-methyl Stretch (416 cm-1), and a tentative assignment for the methyl torsion frequency (174 cm-1). A preliminary single-crystal X-ray diffraction study of the complex was also undertaken (Pcmn, a = 6.366 (2) Å, b = 11.151 (3) Å, c = 11.955 (3) Å, Z = 4), but a near Statistical disorder of the methyl group over all six of the octahedral coordination sites about the manganese precluded derivation of any significant bonding parameters. Prospects for the application of IINS to molecular spectroscopy and to the study of heterogeneous catalytic processes are discussed.