A nonlocal, quasirelativistic density functional (DF) method, NL-SCF+QR, has been applied to the calculation of M-CO bond lengths and the first bond dissociation energy (FBDE) in the binary transition metal carbonyls M(CO)(4) (M = Ni, Pd, Pd), M(CO)(5) (M = Fe, Ru, Os), and M(CO)(6) (M = Cr, Mo, W). The calculated M-CO bond lengths are in good agreement with available experimental data with an error typically smaller than 0.01 Angstrom. The calculated FBDE’s are 29.9 (Ni), 12.3 (Pd), 15.7 (Pt), 45.7 (Fe), 33.0 (Ru), 34.7 (Os), 46.2 (Cr), 39.7 (Mo), and 43.7 ON) kcal/mol, respectively. These values compare well with the available experimental estimates of 25 (Ni), 42 (Fe), 28 (Ru), 31 (Os), 37 (Cr), 41 (Mo), and 46 (W), respectively. Calculations have also been carried out on the CO association energy, AE, corresponding to the following process : M(CO)(6) + CO (M = Cr, Mo, W) --> M(CO)(7) + AE. The calculated AE’s are 47.0 (Cr), 40.4 (Mo), and 35.8 (W) kcal/mol. These calculations underline that CO substitution in M(CO)(6) (M = Cr, Mo, W) can proceed by CO dissociation as well as CO association. The relativistic effects are found to contract M-CO bonds by between 0.07 and 0.16 Angstrom and strengthen the FBDE’s by 5-11 kcal/mol for third-row compounds. The relativistic stabilization of the FBDE’s among the 5d elements makes in general the M-CO bond of the 4d element the weakest within a triad. The way in which relativity enhances the M-CO bond is analyzed by an energy decomposition scheme based on the Extended Transition State (ETS) method.