We calculated the harmonic force constants and vibrational frequencies of most molecules in the series of M(CO)(5)CX (M = Cr, Mo, W; X = O, S, Se) by local and gradient-corrected density functional theory including relativistic effects. The calculated vibrational frequencies agree well with the experimentally observed ones. In the case of W(CO)(6) we studied the contribution of relativistic effects to the reference geometry and the Hessian, These calculations have revealed that relativity has major impact on the geometry of the W complexes studied but has a less significant effect on the force constants comparing calculations at the same reference geometry. The stretching force constants of Cr(CO)(5)CX (X = O, S, Se) were compared with empirical force fields. Some of the calculated interaction constants disagree with the empirical ones, but they are consistent with what is expected based on qualitative molecular orbital models.