Reduction of [ReO3{eta(3)-B(pz)(4)}] (1) with PPh3 in the presence of mono- or bidentate sigma-donor ligands (pyridines, imidazoles, or diphosphines) is a very convenient method for the synthesis of the neutral dioxorhenium(V) complexes : trans-[ReO2{eta(2)-B(pz)(4)}(L)(2)] (L = py (3), 4-Mepy (4), 4-NMe(2)py (5), 1-MeImz (6)) and trans-[ReO2{eta(2)-B(pz)(4)}(P?P)] (P?P = dmpe (7); dppe (8)). In the presence of pyridine or dimethylphosphinoethane, the analogous [ReO3{eta(3)-HB(pz)(3)}] is also reduced by PPh3 to trans-[ReO2{eta(2)-HB(pz)(3)}(py)(2)] (9) and trans-[ReO2{eta(2)-HB(pz)(3)}(dmpe)] (10), respectively. In contrast; the reaction of [ReO2(py)(4)]Cl with K[B(pz)(4)] leads to a mixture of species from which were identified the neutral mono-oxo complexes [ReO(eta(2)-N,O)(mu-O)B(pz)(3)}(pz)(pzH)(2)] (11) and [ReO{(eta(2)-N,O)(mu-O)B(pz)(3)}Cl(py)(2)] (12); Complexes 3-12 were characterized by different techniques, namely, IR, H-1/P-31{H-1} NMR spectroscopies and X-ray crystallographic analysis (5, 10, and 11). Compound 5 crystallizes from dichloromethane/n-hexane as orange crystals containing 3 molecules of solvated CH2Cl2 (crystal data : triclinic space group P (1) over bar, with cell parameters a = 10.907(2) Angstrom, b = 11.113(1) Angstrom, c = 16.922(2) Angstrom, alpha = 97.91(1)degrees, beta = 102.37(1)degrees, gamma = 94.21(1)degrees, V = 1973(1) Angstrom(3), Z = 2). Compound 10 crystallizes from dichloromethane/n-hexane as yellowish crystals containing one molecule of pzH (crystal data : orthorhombic space group Pnma, with cell parameters a = 18.422(2) Angstrom, b = 11.850(1) Angstrom, c = 11.434(1) Angstrom, alpha = beta = gamma = 90 degrees, V = 2496.1(4) Angstrom(3), Z = 4). Compound II crystallizes from dichloromethane/n-hexane in the monoclinic space group P2(1)/n, with cell parameters a = 10.890(1) Angstrom, b = 15.162(1) Angstrom, c = 14.137(2) Angstrom, beta = 102.07(1)degrees, V = 2282.6(4) Angstrom(3), Z = 4.