Hydrogen adds reversibly to the chromium-chromium bond of FvCr(2)(CO)(6) yielding FVCr2(CO)(6)(H)(2) (Fv = fulvalene). Equilibrium data for this process have been measured in toluene and deuteriobenzene in the temperature range 50-80 degrees C and pressures up to 72 atm H-2 [Delta H degrees = - 1.0(0.4) kcal/mol, Delta S degrees = - 14.0(2.0) cal/mol deg]. These data are used to estimate the enthalpy [16.5 (2.0) kcal/mol] and entropy (11 cal/mol deg) for breaking the Cr-Cr bond in FvCr(2)(CO)(6). A theoretical calculation of the isodesmic process FVCr2(CO)(6)(H)(2) + [eta(5)-C5H5Cr(CO)(3)](2) --> FvCr(2)(CO)(6) + 2 eta(5)-C5H5Cr(CO)(3)H predicts it to have an enthalpy of -1.74 kcal/mol, very close to the experimental value of -2 kcal/mol. Thus, and assuming that the individual Cr-H bonds in FVCr2(CO)(6)(H)(2) and eta(5)-C5H5Cr(CO)(3)H are equal, these results lead to the conclusion that the metal-metal bond in the fulvalene system is 2 +/- 2 kcal/mol stronger than that in [eta(5)-C5H5Cr(CO)(3)](2), despite the greater Cr-Cr bond length in the former. This result is also in accord with the measured enthalpy of reaction [FvCr(2)(CO)(6)](2-)(Na+)(2) + [eta(5)-C5H5Cr(CO)(3)](2) --> FvCr(2)(CO)(6) + 2 [eta(5)-C5H5Cr(CO)(3)]Na--(+), Delta H = - 2.4(0.4) kcal/mol. Deuterium was found to add to FvCr(2)(CO)(6) with a normal equilibrium isotope effect: K-eq(H-2)/K-eq(D-2) 1.45(0.10). A kinetic study of reductive elimination of H-2 in the temperature range 45-75 degrees C shows that it obeys first-order kinetics: [k(-1)(45.5) = 1.0 x 10(-5) s(-1), Delta H-double dagger = 27.8(2.0) kcal/mol, Delta S-double dagger = 5.6(3.0) cal/mol deg]. Similarly, the oxidative addition reaction reveals first-order behavior in both FvCr(2)(CO)(6) and H-2 [Delta H-double dagger = 26.8(2.5) kcal/mol, Delta S-double dagger = -19.6 (6.0) cal/mol deg]. The obtained combined kinetic data are in reasonable agreement with the measured equilibrium constant. The rate of hydrogenation under 60 atm of H-2 is not retarded by addition of 15 atm of CO, arguing against a transition state involving CO loss. Studies of the rate of oxidative addition/reductive elimination of D-2 suggest a late transition state for the former which involves cleavage of the D-D (H-H) bond. Substitution of (CO.)-C-12 in FvCr(2)((CO)-C-12)(6) by (CO)-C-13 occurs 1-2 orders of magnitude faster than does hydrogenation [k(1)(43.7) = 5.6 x 10(-6) s(-1) atm(-1), Delta H-double dagger = 16.7(1.5) kcal/mol, Delta S-double dagger = - 30.0(4.0) cal/mol deg] but is much slower than in analogous complexes [eta(5)-C5R5Cr(CO)(3)](2) (R = H, Me). The crystal structure of [FVCr2(CO)(6)](2)-(Na+)(2) is reported.