Two complexes, 3 ] 0 and [Cr(2-hnic) 3 ] 0 (where 3-hpic = hydroxypicolinic acid and 2-hnic = 2-hydroxynicotinic acid anions), were prepared and characterized in solution. The 3-hpic ligand forms a 5-membered chelate ring via pyridine nitrogen and carboxlate oxygen atoms, whereas the 2-hnic ligand forms a 6-membered chelate ring via carboxylate and phenolate oxygen atoms. The kinetics of the acid-catalyzed aquation were studied spectrophotometrically in the 0.1-1.0 M HClO 4 range, at I ¼ 1:0 M . The rate equations for the first aquation step -the chelate-ring opening -was determined and a mechanism was proposed. In the case of [Cr(3-hpic) 3 ] 0 , the reversible chelate-ring opening at the CrAN bond precedes much slower than the second aquation step -a one-end bonded ligand liberation. The equation rate is of the form: k obs ¼ k 1 þ k À1 =Q 1 [H þ ], where k 1 and k À1 are the rate constants for the forward and the reverse processes in the unprotonated substrate and Q 1 is the protonation constant of the non-bonded pyridine nitrogen atom. In the case of [Cr(2-hnic) 3 ] 0 , the chelate-ring opening at the CrAO (phenolate) bond is the rate-determining step. The observed pseudo-first order rate constant increases as [H þ ] increases:where k 0 and k H are the rate constants of the spontaneous and acid catalyzed processes and Q H is the protonation constant of the coordinated phenolate oxygen atom. The results lead to the conclusion that an aquation mechanism depends on the coordination mode of the ligand.
Chromium(III)-isonicotinate complexes, cis-½CrðC 2 O 4 Þ 2 (N-inic)ðH 2 OÞ À and ½CrðC 2 O 4 ÞðH 2 OÞ 3 -OH-CrðC 2 O 4 Þ 2 (O-inic) À (N-inic = N-bonded and O-inic = O-bonded isonicotinic acid) were obtained and characterized in solution. Kinetics of acid-catalyzed isonicotinate ligand liberation were studied spectrophotometrically in the 0.1-1.0 M HClO 4 range, at I=1.0 M. The dependencies of the pseudo-first order rate constant on [H + ] were established: k obs ¼ k 0 þ k H Q H ½H þ and k obs ¼ k H Q H ½H þ for the N-inic and O-inic complex, respectively, where k 0 and k H are the rate constants of the spontaneous and the acid-catalyzed reaction paths, and Q H is the protonation constant of the carboxylic group in isonicotinic ligand. The obtained results indicate that N-bonded isonicotinic acid liberation occurs mainly via a spontaneous reaction path and is much slower than O-bonded inic liberation. The mechanisms for these processes are proposed.
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