Cobalt(III) complexes of tripod amines. Kinetics of aquation of dichloro[ N -(2-aminoethyl)- N , N - bis - (3-aminopropyl)amine]cobalt(III) ion

A new series of six-coordinate cobalt(III) complexes containing tripod tetradentate-N donors (N4), [Co(abap)Cl2]ClO4 · H2O, [Co(abap)(H2O)Cl]Cl(ClO4) and [Co(N4)X2]ClO4 (N4 = abap, X = N3, NO2; N4 = TPA, X = Cl, NO2) where abap = N-(2-aminoethyl)-N,N-bis(3-aminopropyl)amine and TPA = tris(2-pyridylmethyl)amine were synthesized and characterized by elemental analyses, UV–Vis, and IR. The kinetics of the primary aquation of the complex ion [Co(abap)Cl2]+ to [Co(abap)(H2O)Cl]2+ was followed spectrophotometrically in 50/50 (v/v) CH3CN/aqueous HClO4 (16–37°C and I = 0.50 M NaClO4). Rate constants (k 1) evaluated under pseudo first-order conditions, were found to be independent of the [H3O+] over the range of 0.10–0.50 M HClO4 and of the initial complex concentration. At 25°C, the k 1 value was found to be 1.65 × 10−3 s−1 indicating that this process is, unexpectedly, slightly slower than the corresponding reaction in [Co(tren)Cl2]+ (k 1 = 2.96 × 10−3 s−1). The activation parameters were calculated ( = 61.9 ± 1.8 kJ mol−1 and = −91.0 ± 6.1 J K−1mol−1), and the results are discussed in relation to other closely related systems with a dissociative I d mechanism.