The kinetics of the sequential hydrolysis reactions of the xxm-[CoCl2(dien)(py)]+ cation and the crystal structure of the ZnCl42-•2H2O salt

In acid solution, the title cation undergoes a sequence of four, increasingly slower, reaction steps to give, ultimately, Co2+ (ag). 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 HClO4, 25 degrees C) = 2.63 x 10(-4) s(-1) (Delta H-# = 107 kJ mol(-1), Delta S-# = + 44 J K-1 mol(-1)) is accompanied by mer --> fac-isomerisation to give ppf-[CoCl(dien)(py)(OH2)](2+). Subsequent loss of the chloro ligand from ppf-[CoCl(dien)(py)(OH2)](2+) k(H)(2) (I = 0.10 M, 0.05 M HClO4, 25 degrees C) = 1.56 x 10(-5) s(-1) (Delta H-# = 103, Delta S-# = + 9) gives ppf-[Co(dien)(py)(OH2)(2)](3+). The penultimate step is pyridine loss from ppf-[Co(dien)(py)(OH2)(2)](3+) k(py) (0.86 M HClO4, I = 1.0 M, 25 degrees C) = 3.50 x 10(-6) s(-1) (Delta H-# = 123, Delta S-# = + 43) to give f-[Co(dien)(OH2)(3)](3+). Spontaneous thermal reduction of f-[Co(dien)(OH2)(3)](3+) k(red) (H+-independent path, 25 degrees C) = 5 x 10(-12) s(-1) (Delta H-# = 145, Delta 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-CoCl2(dien)(py)](2)ZnCl4. 2H(2)O salt confirms the expected trans-dichloro configuration. (C) 1999 Elsevier Science S.A. All rights reserved.