Vibrational energy relaxation of degenerate CO stretches of four tungsten carbonyl complexes, W(CO)(6), W(CO)(5)(CS), W(CO)(5)(CH3CN), and W(CO)(5)(CD3CN), is observed in nine alkane solutions by subpicosecond time-resolved infrared (IR) pump-probe spectroscopy. Between 0 and 10 ps after the vibrational excitation, the bleaching signal of the ground-state IR absorption band shows anisotropy. Decay of the anisotropic component corresponds either to the rotational diffusion of the molecule or to the intramolecular vibrational energy transfer among the degenerate CO stretch modes. The time constant of the anisotropy decay, tau(aniso), shows distinct solvent dependence. By comparing the results for the T-1u CO stretch of W(CO)(6) and the A(1) CO stretch of W(CO)(5)(CS), the time constant of the rotational diffusion, tau(r), and the time constant of the intramolecular energy transfer among the three degenerate vibrational modes, tau(e), are determined as 12 and 8 ps, respectively. The tau(aniso) value increases as the number of carbon atoms in the alkane solvent increases. After 10 ps, the recovery of the bleaching becomes isotropic. The isotropic decay represents the vibrational population relaxation, from v=1 to v=0. In heptane, the time constant for the isotropic decay, tau(1), for W(CO)(5)(CS) and W(CO)(6) was 140 ps. The tau(1) for the two acetonitrile-substituted complexes, however, shows a smaller value of 80 ps. The vibrational energy relaxation of W(CO)(5)(CH3CN) and W(CO)(5)(CD3CN) is accelerated by the intramolecular energy redistribution from the CO ligand to the acetonitrile ligand. In the nine alkane solutions, the tau(1) value of W(CO)(6) ranges between 124 and 158 ps, showing the apparent V-shaped solvent dependence with its minimum in decane, while the tau(1) value shows little solvent dependence for W(CO)(5)(CH3CN) and W(CO)(5)(CD3CN). (C) 2007 American Institute of Physics.