The neutral rhodium(I) square-planar complexes [RhX(CO)(2)(L)] [X = Cl (3), I (4)] bearing a nitrogen-containing ligand L [diethylamine (a), triethylamine (b), imidazole (c), 1-methylimidazole (d), pyrazole (e), 1-methylpyrazole (f), 3,5-dimethylpyrazole (g)] are straightforwardly obtained from L and [Rh(mu-X)(CO)(2)](2) [X = Cl (1), I (2)] precursors. The synthesis is extended to the diethylsulfide ligand h for 3h and 4h. According to the CO stretching frequency of 3 and 4, the ranking of the electronic density on the rhodium center follows the order b > a approximate to d > c > g > f approximate to h > e. The X-ray molecular structures of 3a, 3d-3f, 4a, and 4d-4f were determined. Results from variable-temperature H-1 and C-13{H-1} NMR experiments suggest a fluxional associative ligand exchange for 4c-4h and a supplementary hydrogen-exchange process in 4e and 4g. The oxidative addition reaction of CH3I to complexes 4c-4g affords the neutral dimeric iodo-bridged acetylrhodium(III) complexes [RhI(mu-I)(COCH3)(CO)(L)](2) (6c-6g) in very good isolated yields, whereas 4a gives a mixture of neutral 6a and dianionic [RhI2(mu-I)(COCH3)(CO)][NHMeEt2](2) and 4h exclusively provides the analogue dianionic complex with [SMeEt2](+) as the counterion. X-ray molecular structures for 6d(2) and 6e reveal that the two apical CO ligands are in mutual cis positions, as are the two apical d and e ligands, whereas isomer 6d(1) is centrosymmetric. Further reactions of 6d and 6e with CO or ligand e gave quantitatively the monomeric complexes [RhI2(COCH3)(CO)(2)(d)] (7d) and [RhI2(COCH3)(CO)(e)(2)] (8e), respectively, as confirmed by their X-ray structures. The initial rate of CH3I oxidative addition to 4 as determined by IR monitoring is dependent on the nature of the nitrogen-containing ligand. For 4a and 4h, reaction rates similar to those of the well-known rhodium anionic [RhI2(CO)(2)](-) species are observed and are consistent with the formation of this intermediate species through methylation of the a and h ligands. The reaction rates are reduced significantly when using imidazole and pyrazole ligands and involve the direct oxidative addition of CH3I to the neutral complexes 4c-4g. Complexes 4c and 4d react around 5-10 times faster than 4e-4g mainly because of electronic effects. The lowest reactivity of 4f toward CH3I is attributed to the steric effect of the coordinated ligand, as supported by the X-ray structure.