The dioxo-Mo(VI) complexes LMoO(2)X [L = hydrotris(3,5-dimethylpyrazol-1-yl)borate (L(a)), hydrotris(3-isopropylpyrazol-1-yl)borate (L(b)), hydrotris(3,5-dimethyl-1,2,4-triazol-1-yl)borate (L(c)); X = Cl, Br, NCS, OMe, Oft, OPh, SPri, SPh, SCH(2)Ph] have been synthesized and characterized by spectroscopic and structural methods. The infrared spectra of the complexes exhibit nu(MoO2) bands at 940-920 and 910-890 cm(-1), and the H-1 NMR spectra are indicative of molecular C-s symmetry in solution. The X-ray crystal structures of three complexes are reported. L(a)MoO(2)(SPh) : monoclinic space group P2(1)/c, a = 18.265(6) Angstrom, b = 8.110(3) Angstrom, c = 18.299(3) Angstrom, beta = 117.06(2)degrees, V = 2414(1) Angstrom(3) with Z = 4. L(b)MoO2(OMe) : monoclinic space group C2/c, a = 30.365(4) Angstrom, beta = 8.373(1) Angstrom, c = 19.646(2) Angstrom, beta = 113.28(1)degrees, V = 4588(1) Angstrom(3) with Z = 8. L(c)MoO(2)(SPh) : orthorhombic space group P2(1)2(1)2(1), a = 7.9302(13) Angstrom, b = 16.627(2) Angstrom, c = 17.543(2) Angstrom, V = 2313.1(9) Angstrom(3) with Z = 4. The structures were refined by full-matrix least-squares procedures to R values of 0.043, 0.027, and 0.039, respectively. The mononuclear complexes feature facially tridentate N-donor ligands, mutually cis oxo and X ligands, and distorted octahedral geometries, The alkoxy and thiolate complexes undergo a reversible, one-electron reduction to form the corresponding dioxo-Mo(V) anions [LMo(V)O(2)X](-). : monoclinic space group P2(1)/c, a = 18.265(6) Angstrom, b = 8.110(3) Angstrom, c = 18.299(3) Angstrom, beta = 117.06(2)degrees, V = 2414(1) Angstrom(3) with Z = 4. L(b)MoO2(OMe) : monoclinic space group C2/c, a = 30.365(4) Angstrom, beta = 8.373(1) Angstrom, c = 19.646(2) Angstrom, beta = 113.28(1)degrees, V = 4588(1) Angstrom(3) with Z = 8. L(c)MoO(2)(SPh) : orthorhombic space group P2(1)2(1)2(1), a = 7.9302(13) Angstrom, b = 16.627(2) Angstrom, c = 17.543(2) Angstrom, V = 2313.1(9) Angstrom(3) with Z = 4. The structures were refined by full-matrix least-squares procedures to R values of 0.043, 0.027, and 0.039, respectively. The mononuclear complexes feature facially tridentate N-donor ligands, mutually cis oxo and X ligands, and distorted octahedral geometries, The alkoxy and thiolate complexes undergo a reversible, one-electron reduction to form the corresponding dioxo-Mo(V) anions [LMo(V)O(2)X](-).