High resolution infrared absorption spectra of nine van der Waals complexes M(CO)(6) . Rg (M=Cr, Mo, W; Rg=Ar, Kr, Xe) formed in a supersonic jet expansion have been recorded near the 5 mu m carbonyl stretching fundamental bands of the hexacarbonyl monomers. In each case a single red-shifted perpendicular band was observed. It is shown that the spectral results are only consistent with a C-3 upsilon symmetric top structure for each dimer; no effects due to internal motions are seen in the spectra. The M-Rg separations deduced from analysing the spectra are slightly larger than the separations calculated from the van der Waals radii. Red-shifts of the band origin are partly explained by a simple vibrational dipole-induced dipole model.