Reduction of [ReO(3){eta(3)-B(pz)(4)}] (1) with PPh(3) 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-[ReO(2){eta(2)-B(pz)(4)}(L)(2)] (L = py (3), 4-Mepy (4), 4-NMe(2)py (5), 1-MeImz (6)) and trans-[ReO(2){eta(2)-B(pz)(4)}(P&arcraise;P)] (P&arcraise;P = dmpe (7), dppe (8)). In the presence of pyridine or dimethylphosphinoethane, the analogous [ReO(3){eta(3)-HB(pz)(3)}] is also reduced by PPh(3) to trans-[ReO(2){eta(2)-HB(pz)(3)}(py)(2)] (9) and trans-[ReO(2){eta(2)-HB(pz)(3)}(dmpe)] (10), respectively. In contrast, the reaction of [ReO(2)(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)(&mgr;-O)B(pz)(3)}(pz)(pzH)(2)] (11) and [ReO{(eta(2)-N,O)(&mgr;-O)B(pz)(3)}Cl(py)(2)] (12). Complexes 3-12 were characterized by different techniques, namely, IR, (1)H/(31)P{(1)H} 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 CH(2)Cl(2) (crystal data: triclinic space group P&onemacr;, with cell parameters a = 10.907(2) Å, b = 11.113(1) Å, c = 16.922(2) Å, alpha = 97.91(1) degrees, beta = 102.37(1) degrees, gamma = 94.21(1) degrees, V = 1973(1) Å(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) Å, b = 11.850(1) Å, c = 11.434(1) Å, alpha = beta = gamma = 90 degrees, V = 2496.1(4) Å(3), Z = 4). Compound 11 crystallizes from dichloromethane/n-hexane in the monoclinic space group P2(1)/n, with cell parameters a = 10.890(1) Å, b = 15.162(1) Å, c = 14.137(2) Å, beta = 102.07(1) degrees, V = 2282.6(4) Å(3), Z = 4.