Stereoselectivity of the Secondary Isotope Effect in the Aquation of [Co(NH3)5Cl]2

• Published in: Inorganic chemistry 2002-12, Vol.41 (26), p.7077-7080
• Main Author: Jackson, W. G
• Format: Article
• Language: English
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/ic020253o

A careful kinetic study of the acid hydrolysis of [Co(NH3)5Cl]2+ and the selectively deuterated species trans-[Co(NH3)4(ND3)Cl]2+, trans-[Co(ND3)4(NH3)Cl]2+, and [Co(ND3)5Cl]2+ reveals that N-deuteration reduces the hydrolysis rate at both the cis and trans sites. On a per-D basis, the rate reduction is three times more effective for trans-Cl deuteration. This result is contrary to conventional wisdom but has never been tested previously. It appears to be the first example of significant stereoselectivity for the secondary isotope effect in a substitution reaction, in either inorganic or organic systems. An explanation is offered in terms of the effect of D-substitution on zero point energies, the different extents of coupling of NH(D) and Co−Cl vibrational modes according to their stereochemical relationship, and the view that the rate of acid hydrolysis of the [Co(NH3)5Cl]2+ involves an aberrant Co−Cl vibration component.