The reactions of hydrochloric and triflic acids with the five-coordinate nitrosyl complexes trans-Re(CO)(2)(NO)(PR3)2 (2a, R = Ph; 2b, R = Cy) have been investigated. Reaction of anhydrous HCl with 2 results in a formal protonation of the nitrosyl ligand and addition of chloride to the metal, giving the neutral nitroxyl complex cis,trans-ReCl(CO)(2)(NH=O)(PR3)(2) (3a, R = Ph; 3b, R = Cy). Reaction of Bronsted bases with 3a or 3b results in clean conversion of 3 to 2 when the base is appropriately strong (pK(b) approximate to 7). Addition of HOSO2CF3 to solutions of 2a results in protonation at the metal and formation of the cationic rhenium hydride [trans, trans-ReH(CO)(2)(NO)(PPh3)(2)(+)][SO3CF3-] (4) in 74% yield; the deuteride [trans, trans-Re(H-2)(CO)(2)(NO)(PPh3)(2)(+)][SO3CF3-] (4-d) was analogously prepared from (HOSO2CF3)-H-2. 4 crystallized from CH2Cl2/Et2O solution in the orthorhombic space group Pnma, with a = 17.2201(2) Angstrom, b = 23.6119(3) Angstrom, c = 9.2380(2) Angstrom, and Z = 4. The least-squares refinement converged to R(F) = 0.039 and R(wF(2)) = 0.063 for the 4330 unique data with I > 2 sigma (I). The structure of 4 shows that the hydride (Re-H = 1.74 A Angstrom occupies the position trans to the linear nitrosyl ligand (Re-N-O = 178.1(4)degrees) in the pseudooctahedral complex cation. Complex 4 does not react with chloride to give 3a. DFT calculations carried out on free nitroxyl and its model complexes [Re(CO)(5)(NH=O)(+)] (5), [mer,trans-Re(CO)(3)(NH=O)(PH3)(2)(+)] (6), and cis,trans-ReCl(CO)(2)(NH=O)(PH3)(2) (7) indicate that coordinated nitroxyl acts as both a a-donor and T-acceptor ligand, consistent with the observed trend for v(NO) in free HN=O (1563 cm(-1)), [mer,trans-Re(CO)(3)(NH=O)(PPh3)(2)(+)] (1, 1391 cm(-1)), 3a (1376 cm(-1)), and 3b (1335 cm(-1)).