Thermodynamic and Kinetic Study of Oxidative Addition/Reductive Elimination of H2 and D2 to FulvaleneCr2(CO)6: Evidence for Relatively Strong Metal−Metal Bonds in Fulvalenedimetals

Hydrogen adds reversibly to the chromium−chromium bond of FvCr 2 (CO) 6 yielding FvCr 2 (CO) 6 (H) 2 (Fv = fulvalene). Equilibrium data for this process have been measured in toluene and deuteriobenzene in the temperature range 50−80 °C and pressures up to 72 atm H 2 [Δ H ° = − 1.0(0.4) kcal/mol, Δ S ° = − 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 FvCr 2 (CO) 6 (H) 2 + [η 5 -C 5 H 5 Cr(CO) 3 ] 2 → FvCr 2 (CO) 6 + 2 η 5 -C 5 H 5 Cr(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 FvCr 2 (CO) 6 (H) 2 and η 5 -C 5 H 5 Cr(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 [η 5 -C 5 H 5 Cr(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 + [η 5 -C 5 H 5 Cr(CO) 3 ] 2 → FvCr 2 (CO) 6 + 2 [η 5 -C 5 H 5 Cr(CO) 3 ] - Na +, Δ 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 °C shows that it obeys first-order kinetics: [ k - 1 45.5 = 1.0 × 10 - 5 s - 1, Δ H ⧧ = 27.8(2.0) kcal/mol, Δ S ⧧ = 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 [Δ H ⧧ = 26.8(2.5) kcal/mol, Δ S ⧧ = −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 12 CO in FvCr 2 ( 12 CO) 6 by 13 CO occurs 1−2 orders of magnitude faster than does hydrogenation [ k 1 43.7 = 5.6 × 10 - 6 s - 1 atm - 1, Δ H ⧧ = 16.7(1.5) kcal/mol, Δ S ⧧ = − 30.0(4.0) cal/mol deg] but is much slower than in analogous complexes [η 5 -C 5 R 5 Cr(CO) 3 ] 2 (R = H, Me). The crystal structure of [FvCr 2 (CO) 6 ] 2 - (Na + ) 2 is reported.