The complexes [Mo(NNPh(2))(2)Cl-2(PPh(3))] (3), [M(NNMePh)(2)Cl-2(PPh(3))(2)] (4), [Mo(NNRPh)(2)Cl-2(PMePh(2))(2)](R Ph, 5; R = Me, 6), [Mo(NNRPh)(2)Cl-2(PMe(2)Ph)(2)] (R = Ph, 7; R = Me, 8), and [Mo(NNRPh)(2)Cl-2(dppe)] (R = Ph, 9; R = Me, 10) have been obtained in good yields by reaction of [Mo(NHNRPh)(NNRPh)(acac)Cl-2] (R = Me, 1; R = Ph, 2) with PMe(3-n)Ph(n) (n = 1-3) or Ph(2)PCH(2)CH(2)PPh(2) in acetonitrile. In refluxing toluene, 4 undergoes dissociation of a PPh(3) ligand to give [Mo(NNMePh)(2)Cl-2(PPh(3))] (11). Complexes 3, 4, and 11 react with an excess of acetylacetone in refluxing acetonitrile to give back 1 or 2. Complexes 3-11 have been studied by H-1, P-31 NMR, IR, and UV-visible spectroscopy and by electrochemistry. In addition, compounds 3 and 4.0.5CH(2)Cl(2) have been characterized by X-ray crystal analyses. Complex 3 is monoclinic, space group P2(1)/c, with a = 21.787(5) Angstrom, b = 17.638(1) Angstrom, c = 20.066(14) Angstrom, beta = 95.32(1)degrees, and Z = 8. The structure was solved using 3965 unique observed reflections; the refinements of 735 variables gave final R and R(w). values of 0.049 and 0.053, respectively. Compound 4.0.5CH(2)Cl(2) is triclinic, space group P (1) over bar, with a 14.384(12) Angstrom, b = 16.599(6) Angstrom, c = 23.919(11) Angstrom, a = 77.44(4)degrees; beta = 77.51(1)degrees, gamma = 64.39(2)degrees, and Z = 4. The structure was solved using 5456 unique observed reflections; the refinements of 901 variables converged to R and R(w) values of 0.070 and 0.083, respectively. Complex 3 displays a distorted trigonal bipyramidal geometry with the cis-hydrazido ligands in the equatorial plane, while 4 disp,lays a distorted octahedral geometry with cis-hydrazido and trans-phosphine ligands.