The thermodynamics of metalmetal bond dissociation in the [(C5Me5)Cr(CO)3]2 dimer was studied by variable-temperature room NMR study in toluene-d8. Dissociation constants and proton isotropic shifts at various temperatures were obtained. Application of the van ′ Hoff equation gives the following thermodynamic parameters: ΔH = 18.3 (±0.6) kcal mol-1; ΔS = 54.8 (±2.2) cal mol-1 K-1 (ΔG = 1.9 kcal mol-1 at 300 K). These parameters were compared with those of the [(C5H5)Cr(CO)3]2 dimer. The most likely spin delocalization mechanism for the methyl protons is believed to be a combination of σ and π spin delocalization. The ESR spectrum in toluene (g0 = 2.004±0.001) or at below 273 K is consistent with a non-degenerate ground state of the monomer.