Metal complexes with macrocyclic ligands. XVI. Spectrophotometric and thermodynamic studies of solvents and unidentate ligands interaction with the pentacoordinate Co2+‐, Ni2+‐ and Cu2+‐complexes of 1,4,8,11‐tetramethyl‐1,4,8,11‐tetraazacyclotetradecane

The interaction of solvents and of unidentate ligands such as N , SCN−, OCN− and OH− with the Co2+‐, Ni2+ and Cu2+‐complexes of 1,4,8,11‐tetramethyl‐1,4,8,11‐tetraazacyclotetradecane (TMC) have been studied by Spectrophotometric and calorimetric techniques. The spectra in different solvents (Table 2) show that the Ni2+‐ and probably also the Cu2+‐complex with TMC exist as square planar or pentacoordinate species or as a mixture of both, depending on the donor properties of the solvent. The [Co(TMC)]2+‐complex is pentacoordinate in all the solvents studied. Ternary complexes [M(TMC)X]n+ are also formed by the unidentate ligands X = N , OCN−, OH−, F− and NH3 and their stability constants have been determined. Interesting is the high selectivity of [Ni(TMC)]2+ towards the addition of a further donor (Table 3). Only small ligands such as those listed above form stable adducts, whereas the larger ones such as imidazole or pyridine do not. This is a consequence of the special structure of the complex and of the trans‐I‐(RSRS)‐ conformation of the ligand in these complexes. Since the four methyl groups are all on the side of the macrocycle to which the additional unidentate ligand binds, steric interaction between the four methyl groups and the larger ligands prevents the formation of the adducts. The calorimetric measurements show that the stability of the complexes [M(TMC)X]n+ is due to both an enthalpic and entropic contribution which differ in their magnitude (Table 4). This indicates that several antagonistic factors are important in determining the overall stability.