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 multidentate chelating systems such as picolylaminediacetic acid (L1; N,N',O,O'), nitrilotriacetic acid (L2; N,O,O',O''), iminodiacetic acid (L3; N,O,O'), and bis(2-pyridyl)methane (L4; N,N'). The products of the substitution reactions were isolated and characterized by means of IR, NMR, MS, and X-ray structure analysis. NMR and crystallographic analyses confirmed the formation of single structural isomers in all cases with a ligand-to-metal ratio of 1:1. With ligands L1 and L2 and precursor 1 the tridentately coordinated complexes [Re(L1)(CO)3] (7) and [Re(L2)(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(L2)(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 L1 and L2 confirmed the loss of one CO during the reaction. The product of the reaction of 2 with L3 was identified as the neutral complex [Re(L3)(CO)2(NO)] (5), again, with the nitrosyl coordinated trans to the carboxylate. With 1, ligand L3 formed the anionic complex [Re(L3)(CO)3]- (9). Finally the reactions with L4 yielded the complexes [ReBr(L4)(CO)2(NO)]Br (6) and [ReBr(L4)(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) A, b = 8.0573(3) A, c = 24.7210(11) A, beta = 107.117(5) degrees, and Z = 4; 5, triclinic P1, with a = 6.9091(5) A, b = 9.8828(7) A, c = 14.2834(10) A, 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) A, b = 10.0949(8) A, c = 12.5346(10) A, 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) A, b = 13.3015(13) A, c = 9.0112(9) A, beta = 107.195(2) degrees, and Z = 7.