The bis(dinitrogen)molybdenum complexes trans-Mo(N-2)(2){(ArCH2)(2)PC2H4P(CH2Ar)(2)}(2) (Ar = C6H5 (1a), C6H4- m-Me (1b), C6H4-o-Me (1c), C6H4-p-Me (1d), C6H4-o-F (1e), C6H4-m-F (1f) C6H4-p-F (1g), C6H4-m-OMe (1h), C6H4-p-OMe (ii)) are prepared by reduction of MoCl5 with magnesium under a dinitrogen atmosphere in the presence of the appropriate diphosphine ligand. Treatment of la and Ib with ethyl acetate in refluxing benzene under argon affords the formally 16-electron complexes Mo(CO){(ArCH2)(2)PC2H4P(CH2Ar)(2)}(2) (Ar = C6H5 (2a), C6H4-m-Me (2b)), which are shown by H-1 NMR spectroscopy to contain an agostic Mo H-C interaction between the molybdenum and an ortho aryl hydrogen atom. Complexes 2a and 2b react readily with small molecules H-2, N-2, and SO2 to give the adducts trans-Mo(L)(CO){(ArCH2)(2)PC2H4P(CH2Ar)(2)}(2) (L = eta(2)-H-2, Ar = C6H5 (3a), C6H4-m-Me (3b); L = N-2, Ar = C6H5 (4a), C6H4-m-Me (4b); L = SO2, Ar = C6H5 (5a), C6H4-m-Me (5b)). In contrast to the dihydride formulation in MoH2(CO){(RCH(2))(2)PC2H4P(CH(2)R)(2)}(2) (R = Me, Pr-i), the eta(2)-H-2 coordination in 3a and 3b is unambiguously established by IR, NMR, and neutron scattering spectroscopies and a single-crystal X-ray diffraction analysis of 3b. Thus, electronic control of eta(2)-dihydrogen versus dihydride coordination has been achieved in complexes of the type MoH2(CO){(RCH(2))(2)PC2H4P(CH(2)R)(2)}(2), an eta(2)-dihydrogen complex being formed when R is an electron-withdrawing aryl group, whereas a dihydride is formed when R is an electron-donating alkyl group.The spectroscopic data for 3a and 3b indicate that they exist exclusively as eta(2)-dihydrogen complexes with a normal H-H distance bath in solution and in the solid state, and so neither an elongated H-H bond nor an eta(2)-dihydrogen/dihydride tautomeric equilibrium has been observed although the pi-basicity of the metal center appears to be very close to the point where the H-H bond cleavage is expected to occur. This supports the suggestion that the reaction coordinate for H-2 cleavage in the molybdenum-diphosphine. system is rather flat and shows relatively little change in the H-H distance until relatively precipitous cleavage of the H-H bond. Crystallographic data for 3b: monoclinic, space group C2/c, a = 26.782(5) Angstrom, b = 9.365(2) Angstrom, c = 25.399(5) Angstrom, beta = 111.75(3)degrees, V = 5921(5) Angstrom(3), Z = 4, and R = 0.058. Crystallographic data for 4a: monoclinic, space group C2/c, a = 23.644(5) Angstrom, b = 13.558(3) Angstrom, c = 17.525(4) Angstrom, beta = 108.97(3)degrees, V = 5313 (4) Angstrom(3), Z = 4, and R = 0.034.