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

Abstract

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.

Secondary Deuterium Kinetic Isotope Effect for Aquation, Solvolysis, and Isomerization Reactions of trans-[Co(en)2(OSMe2)N3]2+, and the Resolution of a Mechanistic Anomaly

“…Absorbance−time traces at the cis -/ trans -[Co(en) 2 (OH 2 )(N 3 )] 2+ isosbestic (538 nm) fitted to a single exponential provided the more accurate rate data because this strategy reduced the analysis for a consecutive reaction to a simple A → B process. Runs were conducted simultaneously in a 7-cell thermostatic housing in the HP8453 diode array spectrophotometer, as described for the [Co(NH 3 ) 5 Cl] 2+ system, to minimize temperature fluctuations from run to run.…”

“…Our recent publication on the aquation of stereoselectively N-deuterated [Co(NH 3 ) 5 Cl] 2+ complexes is the first, as far as we are aware, on the stereoselectivity of the skie . Even for the widely studied organic systems there are few data.…”

26 Mechanisms of reactions in solution

“…The much-studied aquation of simple 6-coordinate Co() complexes such as [Co-(NH 3 ) 5 Cl] 2ϩ continues to attract mechanistic interest. 139 Jackson has shown experimentally that deuterium substitution in either trans or cis positions significantly reduces the rate of aquation and this is attributed to zero-point energy effects and coupling of modes of vibration. Aquation of a series of complexes, [Co(en)-(L) 2 (CO 3 )] ϩ , where L is imidazole or 1-methyl-or 2-methylimidazole follows the reaction path of initial protonation of CO 3 then ring-opening.…”

“…Rate constants for the substitution of coordinated water on La 3ϩ with acetohydroxamic acid have been determined from measurements of line widths in 139 La NMR spectra. 217 Ligand exchange.…”

Synthesis, Spectroscopic and X-ray Structural Study ofcis-diazidobis- (ethylenediamine)Cobalt(III) Thiocyanate

“…Cobalt(III) complexes continue to receive attention [1][2][3] owing to their important role in the development of inorganic chemistry. Cobalt is one of those trace metals that is present in the human body as a metal cofactor in the form of biologically important molecules such as the B 12 coenzyme and vitamin B 12 [4][5][6][7].…”