First-row members of the group 6 series of transition metal dihydrogen complexes Cr(CO){sub 3}(PR{sub 3}){sub 2}({eta}{sup 2}-H{sub 2})(R = Cy, Pr{sup i}) have been synthesized and examined by X-ray crystallography and NMR, infrared, and inelastic neutron scattering spectroscopies. The complexes completely dissociate H{sub 2} in solution to give Cr(CO){sub 3}(PR{sub 3}){sub 2}, which contains an agostic C-H interaction. An X-ray structure of Cr(CO){sub 3}(PPr{sup i}{sub 3}){sub 2}(H{sub 2}) 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 NMR spectroscopy of the PCy{sub 3} 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 {eta}{sup 2}-H{sub 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{sub 2} {yields} M {sigma}-bonding strength appears to increase as Mo < Cr {approximately} W but that d{pi} {yields} H{sub 2} {sigma}* back-bonding increases down the group. It mightmore » 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{sub 1}/c, a = 13.457-(3) {angstrom}, b = 11.493(3) {angstrom}, c = 16.639(4) {angstrom}, {beta} = 102.25(2){sup o}, Z = 4, R = 3.32%.« less