The kinetics of the sequential hydrolysis reactions of the xxm-[CoCl 2(dien)(py)] + cation and the crystal structure of the ZnCl 4 2− ·2H 2O salt

Abstract

In acid solution, the title cation undergoes a sequence of four, increasingly slower, reaction steps to give, ultimately, Co 2+(aq). We have followed the first three substitution reactions, and the last is a previously studied reduction process. The first chloride release step, k H 1 ( I=0.10 M, 0.05 M HClO 4, 25°C)=2.63×10 −4 s −1 (Δ H #=107 kJ mol −1, Δ S #=+44 J K −1 mol −1) is accompanied by mer→ fac-isomerisation to give ppf-[CoCl(dien)(py)(OH 2)] 2+. Subsequent loss of the chloro ligand from ppf-[CoCl(dien)(py)(OH 2)] 2+ k H 2 ( I=0.10 M, 0.05 M HClO 4, 25°C)=1.56×10 −5 s −1 (Δ H #=103, Δ S #=+9) gives ppf-[Co(dien)(py)(OH 2) 2] 3+. The penultimate step is pyridine loss from ppf-[Co(dien)(py)(OH 2) 2] 3+ k py (0.86 M HClO 4, I=1.0 M, 25°C)=3.50×10 −6 s −1 (Δ H #=123, Δ S #=+43) to give f-[Co(dien)(OH 2) 3] 3+. Spontaneous thermal reduction of f-[Co(dien)(OH 2) 3] 3+ k red (H +-independent path, 25°C)=5×10 −12 s −1 (Δ H #=145, Δ S #=+25) is the final process [P. Wilariat, C.S. Garner, J. Inorg. Nucl. Chem. 33 (1971) 1833]. The crystal structure of the green [ xxm-CoCl 2(dien)(py)] 2ZnCl 4·2H 2O salt confirms the expected trans-dichloro configuration.