THE BARRIER TO ROTATION OF DIHYDROGEN IN CR(CO)3(PCY3)2(ETA-2-H2), A COMPLEX WITH AN UNUSUALLY HIGH DIFFERENCE IN SOLID-STATE AND SOLUTION-STATE STABILITIES

The third member in the group 6 series of dihydrogen complexes, Cr(CO)3(PCy3)2(eta(2)-H2), has been synthesized as a solid which completely dissociates H2 in solution. Inelastic neutron scattering showed the barrier to rotation of the eta(2)-H2 to be 1.3 kcal/mol, compared to 1.7 and 2.2 kcal/mol for the Mo and W analogues. Nu(s)(M-H2) and nu(as)(M-H2) occurred at 950 and 1540 cm-1, similar to that for M = W but higher than for M = Mo (nu(s) = 885 cm-1). These results indicate that the overall M-H2 bond strength is less for Mo than for either Cr or W, but that d-pi --> sigma* H2 back-bonding is lowest for Cr.