Stereoselectivity of the secondary isotope effect in the aquation of [Co(NH3)5Cl]2+

A careful kinetic study of the acid hydrolysis of Co(NH(3))(5)Cl](2+) and the selectively deuterated species trans-Co(NH(3))(4)(ND(3))Cl](2+), trans-Co(ND(3))(4)(NH(3))Cl](2+), and Co(ND(3))(5)Cl](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(NH(3))(5)Cl](2+) involves an aberrant Co-Cl vibration component.