The stability, structure and reactivity of the outer sphere complex formed between the hexakis (d 4-methanol) cobalt(II) ion and pyridine in d 4-methanol

Abstract

Proton NMR spinlattice ( Tι) relaxation times and chemical shifts were used to characterize the outer sphere complex formed between the hexakis (d 4-methanol) cobalt(II) ion and pyridine in d 4-methanol. Analysis of the bulk α-hydrogen Tι data of pyridine resulted in an outer sphere complex stability constant ( K os) which varied from 0.51 ± 0.16 M −1 at −60.0 °C to 0.62 ± 0.17 M −1 at −40.0 °C. The extrapolated K os value at 25.0 °C (1.0 M −1) is almost an order of magnitude larger than predicted larger predicted by theory. The combination of Tι and shift data suggested a preferred orientation of pyridine in the outer sphere of the [Co(CD 3OD) 6] 2+ ion. Further quantitative structural information was obtained by the application of the Solomon-Bloembergen relationship to the system studied. Pyridine dissociation rate constant data were combined with solvent exchange and equilibrium data to obtain values for the interchange probability or statistical factor (f) which ranged from (7.0 ± 3.7) × 10 −3 at −60.0 °C to (9.5 ± 4.7) × 1- −3 at −40.0 °C. These values are more than two orders of magnitude smaller than normally assumed and represent the first direct measurement of an ion-neutral outer sphere complex's reactivity. Implications regarding the relationship between outer sphere complex stability and reactivity are discussed.