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 synthesized and examined by X-ray crystallography and NMR, infrared, and inelastic neutron scattering spectroscopies. The complexes completely dissociate 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 side-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 NMR 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 accurate 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 distances from solid-state NMR. Correlations of properties of the group 6 species indicate that the H-2 --> M sigma-bonding strength appears to increase as Mo < Cr approximately W but that dpi --> H-2 sigma* back-bonding increases down the group. It might be generally concluded that back-bonding is more of a factor than sigma-bonding in influencing stability, H-H distance, and possibly overall bond strengths. Crystallographic data : space group P2(1)/c, a = 13.457(3) 3) angstrom, b = 11.493(3) angstrom, c = 16.639(4) angstrom, beta = 102.25(2)degrees, Z = 4, R = 3.32%.