The ligand substitution behavior of [ReBr3(CO)(3)](NEt4)(2) (1) and [ReBr3(CO)(2)(NO)]NEt4 (2) in aqueous media was compared. Ligand exchange reactions were performed with multiclentate chelating systems such as picolylaminediacetic acid (L-1; N,N',O,O'), nitrilotriacetic acid (L-2; N,O,O',O"), iminodiacetic acid (L-3; N,O,O'), and bis(2-pyridyl)methane (L-4; N,N'). The products of the substitution reactions were isolated and characterized by means of IR, NMR, MS, and X-ray structure analysis. NIVIR and crystallographic analyses confirmed the formation of single structural isomers in all cases with a ligand-to-metal ratio of 1:1. With ligands L-1 and L 2 and precursor 1 the tridentately coordinated complexes [Re(L-1)(CO)(3)] (7) and [Re(L-2)(CO)(3)](2-) (8) were formed. With precursor 2 the same ligands unexpectedly coordinated tetradentately after displacing a CO ligand, yielding complexes [Re(L1)(CO)(NO)] (3) and [Re(L 2) (CO)(NO)](-) (4). In both complexes NO was found to be coordinated trans to the carboxylate group. Time-dependent IR spectra of the reaction of 2 with ligand L-1 and L 2 confirmed the loss of one CO during the reaction. The product of the reaction of 2 with L 3 was identified as the neutral complex [Re(L-3)(CO)(2)NO)] (5), again, with the nitrosyl coordinated trans to the carboxylate. With 1, ligand L-3 formed the anionic complex [Re(L-3)(CO)(3)](-) (9). Finally the reactions with L-4 yielded the complexes [ReBr(L-4)(CO)(2)(NO)]Br (6) and [ReBr(L1)(CO)3] (10), in which bromide was found to be coordinated trans to the NO and CO, respectively. The X-ray structures of 3, 5-7, and 10 are discussed: 3, monoclinic P2(1)/n, with a = 14.6071(6) angstrom, b = 8.0573(3) angstrom, c = 24.7210(111) angstrom, beta = 107.117(5)degrees, and Z = 4; 5, triclinic P1, with a = 6.9091(5) angstrom, b = 9.8828(7) angstrom, c = 14.2834(10) angstrom, alpha = 89,246(9)degrees, beta = 89.420(9)degrees, gamma = 86.196(9)degrees, and Z = 4; 6, triclinic P1, with a = 9.8236(8) angstrom, b = 10.0949(8) angstrom, c 12.5346(10) angstrom, alpha = 108.679(9)degrees, beta = 111.992(9)degrees, gamma = 95.426(10)degrees, and Z = 2; 10, monoclinic P2(1)/c, with a 12.7491(12) angstrom, b = 13.3015(13) angstrom, c = 9.0112(g) angstrom, beta = 107.195(2)degrees, and Z = 7.