This work describes new synthetic routes to produce mixed carbonyl-nitrosyl complexes of technetium(I) and rhenium(I) in aqueous media. NaNO2, NOHSO4, and NO2(g) have been used to produce in situ nitrous acid as the primary source of NO+. Starting from the organometallic precursor fac-[MX3(CO)(3)](+), 1 (M = Tc-99, Re; X = Cl, Br), the formation of mixed dicarbonyl-mononitrosyl complexes was observed in aqueous hydrochloric and hydrobromic acid. Time-dependent analyses of the reactions by means of HATR-IR and Tc-99 NMR spectroscopy in solution revealed the almost quantitative substitution of one CO ligand by NO+ and, thus, the formation of complexes with facial arrangement of the three pi-acceptor ligands. In the case of technetium, the monomeric complex (NEt4)[TcCl3(CO)(2)NO] (3a) and the dimeric, chloride-bridged, neutral complex [TcCl(mu-Cl)(CO)(2)NO](2) (4a) were produced. In the case of rhenium, the monomeric species (NEt4)[ReBr2X(CO)(2)NO] (X = Br (3b), NO3 (5)) was solely isolated. The X-ray structure of complexes 4a and 5 are discussed. The crystallographic analyses revealed the coordination of the NO+ group trans to the terminal chloride (4a) or the bromide (5), respectively. Crystal data: complex 4a (C4Cl4N2O6Tc2), monoclinic, Cc, a = 18.82(3) Angstrom, b = 6.103(6) Angstrom, c = 12.15(2) Angstrom, alpha = 90degrees, beta = 105.8(2)degrees, gamma = 90degrees, V = 1343(3) Angstrom(3), Z = 4; complex 5 (C10H20N3O6Br2Re), orthorhombic, P2(1)2(1)2(1) a = 10.2054(5)degrees, b = 12.5317(7) Angstrom, c = 13.9781(7) Angstrom, V = 1787.67(16) Angstrom(3), Z = 4. The isolated complexes and their potential facial isomers have been further investigated by density functional theory (DFT) calculations. The energy differences of the isomers are relatively small; however, the calculated energies are consistent with the formation of the observed and isolated compounds. The calculated bond lengths and angles of complex 5 are in good agreement with the data determined by X-ray diffraction. Experiments on the no-carrier-added level starting from fac-[Tc-99m(H2O)(3)(CO)(3)](+) revealed the formation of the complex fac-[(99)mTcCl((HO)-O-2)(2)(CO)(NO)-N-2](+) in reasonable good yields. This aqueous-based, synthetic approach will enable the future evaluation of this novel, low-valent metal precursor for potential use in radiopharmacy.