Analytical Sciences

Abstract − Analytical Sciences, 17(1), 187 (2001).

Spectrophotometric Analysis of 5-Coordinate Cobalt(II) Species for Ligand Substitution of Hexakis(acetonitrile)cobalt(II) with Bulky 1,1,3,3-Tetramethylurea in Noncoordinating Nitromethane
Yasuhiro INADA,*  Nami HOTTA,** Hiroshi KUWABARA,** and Shigenobu FUNAHASHI**
*Research Center for Materials Science, Nagoya University, Nagoya 464-8602, Japan
**Laboratory of Analytical Chemistry, Graduate School of Science, Nagoya University, Nagoya 464-8602, Japan
The ligand substitution reaction of [Co(an)6]2+ (an = acetonitrile) with 1,1,3,3-tetramethylurea (TMU) in the noncoordinating solvent, nitromethane, was spectrophotometrically investigated by titration. The observed spectral changes were analyzed using a model with the four steps of ligand substitution. The component complexes involved in the substitution were found to be 6-coordinate [Co(an)6]2+ and [Co(an)5(tmu)]2+, 5-coordinate [Co(an)3(tmu)2]2+ and [Co(an)2(tmu)3]2+, and 4-coordinate [Co(tmu)4]2+. The logarithmic values of the stepwise equilibrium constant are 2.17 ± 0.26, 1.06 ± 0.15, 1.19 ±0.06, and -0.4 ± 0.4 at 25°C. The decrease in the coordination number of the Co(II) ion from 6 to 5 during the formation of [Co(an)3(tmu)2]2+ and from 5 to 4 during the formation of [Co(tmu)4]2+ is ascribed to the steric repulsion between the coordinating bulky TMU molecules.