Fischer's landmark discovery of transition-metal alkylidyne complexes two decades ago' initiated an extensive research effort that has resulted in the development of a large, diverse class of molecules that exhibit fascinating reaction chemistry.2 Much of the descriptive chemistry of these species hinges on the attribution of a formal bond order of 3 to the linkage between the alkylidyne ligand (CR) and the metal. This bonding description was initially based on the required valency of the ligated carbon atom1 and on X-ray crystallographic studies, the latter of which yielded the expected findings of a short M-C bond and a nearly linear M-C-R linkage.3 Direct evidence for the high multiplicity of the M-C bond order was ultimately provided by the sizable M-C force constant (k(W=C) = 7.00-7.40, k ( C e C ) = 5.18 mdyn A-l), which was determined, together with the M=C stretching frequency (v(M=C) = 1300-1400 cm-I), from an elegant series of vibrational spectroscopic experiments undertaken by Dao, Fischer, and co-workers on thearchetypal M(=CR)(C0)4X (M = Cr, Mo, W; R = Me, Ph; X = anionic ligand) complexes.4 Although v(M=CR) bands with similar frequencies have subsequently been observed for other alkylidyne compounds,2e~5 an acknowledged complication with the determination of v(M=C) and k(M=C) for M(CR)(CO)dX complexes arises from the vibrational complexity of their R groups; the M=C stretching coordinate is strongly coupled with other symmetry coordinates, especially the MC-C stretch and deformations of the R group, f University of Pittsburgh. t Wabash College. (1) Fischer, E. 0.; Kreis,G.; Kreiter, C. G.; Muller, J.; Huttner,G.; Lorenz, H. Dao,N. Q.; Fevrier, H.; Jouan, M.; Huy, N. H. T.; Fischer, E. 0.; Neugebauer, D. J. Organomet. Chem. 1982,241, C53-C56. (f) Dao, N. Q.; Fevrier, H.; Jouan, M.; Fischer, E. 0. Noun J. Chim. 1983,7,719-724. (9) Fischer, E. 0.; Friedrich, P.; Lindner, T. L.; Neugebauer, D.; Kreissl, F. R.; Uedelhoven, W.; Dao, N. Q.; Huttner, G. J . Organomet. Chem. 1983,247,239-246. (h) Dao, N. Q.; Fevrier, H.; Jouan, M.; Fischer, E. 0.; Rbll, W. J. Orgunomer. Chem. 1984,275,191-207. (i) Dao, N. Q.; Jouan, M.; Fonseca, G. P.; Huy, N. H. T.; Fischer, E. O.;J. Organomet. Chem. 1985,287,215-219. 6 ) Dao, N. Q.; Foulet-Fonseca, G. P.; Jouan, M.; Fischer, E. 0.; Fischer, H.; Schmid, J. C. R. Acad. Sci. Ser. 2 1988, 307, 245-250. (k) Foulet-Fonseca, G. P.; Jouan, M.; Dao, N. Q.; Fischer, H.; Schmid, J.; Fischer, E. 0. Spectrochim. Acta 1990,46A, 339-354. (1) Foulet-Fonseca, G. P.; Jouan, M.; Dao, N. Q.; Huy, N. H. T.; Fischer, E. 0.