Specific Cation Catalysis in the Thiocyanate Photoanation of Acidopentacyanocobaltate(III) Complexes

Abstract

The thermal anation of Co(CN)sBr3by 2 M thiocyanate ion at 22 °C in 0.1 M NaOH proceeds by two pathways. Direct anation occurs with a pseudo-first-order rate constant of about 8 x 10-6 s-1, while anation of Co(CN)5(H20)2-in equilibrium with Co(CN)j(C>H)3-formed by the concurrent base hydrolysis of Co(CN)5Br3_ occurs with a pseudo-first-order rate constant of 2.15 x 10-6 s-1. The thiocyanate anation product obtained shows an overall S:N-bonded linkage isomer ratio of 4.5 ± 0.2. Measurements of the product S:N linkage isomer ratio and of the ratio of aquation to anation were obtained for photosubstitution of CoCCNjsNa3-at 5 °C and pH 13 in 2 M aqueous thiocyanate with various counterions. For the group of singly charged ions Li+, Na+, K+, and NH4"* 1", the aquation:anation ratio ranged from 9.9 ± 1.4 for Li+ to 2.5 ± 0.5 for NH4"1" while the range of S:N product ratios was 12.7 ± 1.1 to 10.6 ± 2.6 over the same group. These results reveal a linear relationship between ln(aquation/anation) and 1/hydrated radius, while the S:N ratio is essentially constant. Compared with Li+, Ca2+ showed the increase in anation efficiency expected on the basis of its higher charge. A constant total quantum yield of 0.073 ± 0.005 was found for photosubstitution of Co(CN)5l3-45in NaClOVNaSCN mixtures of total concentration 4 M, showing that the anation and aquation are competitive processes. The cation apparently assists anation by helping overcome the Coulomb repulsion between the entering thiocyanate and a coordinatively unsaturated intermediate of like charge. The higher S:N ratio for the photoprocess is not a result of the cation assistance but is an intrinsic property of the intermediate, consistent with involvement of the triplet state fiveindependent aquation to anation product ratio. These results were attributed to the participation in the photoreaction of a

Salt and Specific Cation Effects in the Quenching of Triplet State Tetrakis(μ-pyrophosphite-P,P‘)diplatinate(II) by Acidopentacyanocobaltate(III) Anions

“…In the course of earlier work from this laboratory, we observed , specific cation effects on the ratio of photoaquation to photoanation by thiocyanate in some acidopentacyanocobaltate(III) complexes. Our ability to study these, and the general salt effects, was severely restricted by the precision of the product analysis.…”

Interplay of thermochemistry and Structural Chemistry, the journal (volume 19, 2008) and the discipline

“…We wonder what is the energy difference between the various S-and N-linkage isomers? Is the difference small as found for mononuclear Cu [274] and Ru complexes [275]?…”