The pKa's at 298 K, μ = 0.10 (NaCl), and the temperature dependence (273-343 K) for the deprotonation of the pyrrole NH of several imidazoles coordinated to (NH3)5M3+ moieties (M = CoIII, RhIII, IrIII, RuIII) are reported. A greater importance of dn configuration over ion size is found. Data summarized for various systems are as follows (ligand, M (pK298, ΔHa° in kcal/mol, ΔSa° in eu)): imidazole = imH, CoIII (9.99, 14.0 ± 0.5, 1.3 ± 1.6), RhIII (9.97, 13.6 ± 0.3, 0.1 ± 1.3), IrIII (10.05, 13.4 ± 0.3, 1.2 ± 1.0), RuIII (8.9, 10.0 ± 0.8, 3.7 ± 1.2); 2-methylimidazole = 2-MeimH, CoIII (10.67, 17.8 ± 0.7, 11.2 ± 2.4); 2,4(5)-dimethylimidazole = 2,5-Me2imH, CoIII (11.04, 13.4 ± 0.5, 5.3 ± 1.6), RuIII (10.20, 13.2 ± 0.6, -2.1 ± 1.6). 1H NMR spectra of low-spin d6 complexes of imidazoles and ring-methylated imidazoles are discussed for CoIII, RhIII, IrIII, and RuIII. C-2 and remote ring, C-5, substituents are shifted downfield relative to the free imidazole ligand in the order H+ > CoIII > RhIII > IrIII. The C-4 position is influenced competitively by σ-withdrawal ring substituents and TIP effects for CoIII. Assignments of the remote isomer for (NH3)5M(2,5-Me2imH)3+ (M = CoIII, RuIII) are made from the 1H NMR spectra of the CoIII and RuII complexes. The RuIII complexes of 2,5-Me2imH and the imidazolate form (2,5-Me2im-) both exhibit LMCT spectra. The imidazolato form has three bands at 655, 377, and 272 nm, proposed for II1 → IId, II2 → IId, and n → IId transitions, where II1, II2, and n are the highest HOMO's of the imidazolato ring.