ISOLATION OF AN EXTREMELY LABILE DIHYDROGEN COMPLEX, CR(CO)3(PPR(I)3)2(H2), CONTAINING THE SHORTEST LIGATED H-H BOND

First-row members of the group 6 series of transition metal dihydrogen complexes Cr(CO)3(PR3)2(eta2-H-2) (R = Cy, Pr(i)) have been synthesiz ed and examined by X-ray crystallography and NMR, infrared, and inelas tic neutron scattering spectroscopies. The complexes completely dissoc iate H-2 insolution to give Cr(CO)3(PR3)2, which contains an agostic C -H interaction. An X-ray structure of Cr(CO)3(PPri3)2(H2) showed a sid e-bonded dihydrogen ligand with H-H = 0.67(5) angstrom, and proton NMR of the HD isotopomer showed a J(HD) value of 35 Hz. Solid-state H-1 N MR spectroscopy of the PCy3 complex indicated that the H-H separation is 0.85(1) angstrom, which is believed to be a more precise and accura te measure of this key parameter. The barrier to rotation of the eta2- H-2 has been recalculated to be 1.17(10) kcal/mol, as compared with 1. 32(10) and 1.9(1) kcal/mol for the Mo and W analogues, based on H-H di stances from solid-state NMR. Correlations of properties of the group 6 species indicate that the H-2 --> M sigma-bonding strength appears t o increase as Mo < Cr approximately W but that dpi --> H-2 sigma back -bonding increases down the group. It might be generally concluded tha t back-bonding is more of a factor than sigma-bonding in influencing s tability, H-H distance, and possibly overall bond strengths. Crystallo graphic data: space group P2(1)/c, a = 13.457(3) 3) angstrom, b = 11.4 93(3) angstrom, c = 16.639(4) angstrom, beta = 102.25(2)degrees, Z = 4 , R = 3.32%.