Nuclear magnetic resonance and electronic spectral studies on the formation of π–olefin-tetracyanocobaltate(I) complexes

Abstract

Cyanocobaltate prepared at CN : Co <5 : 1 in water in the presence of alkali and under an atmosphere of hydrogen reacts readily with mono-olefins having electron-withdrawing substituents to give a dark red solution. Hydrogen-1 and ¹³C n.m.r. spectra of the solutions in D₂O clearly indicate that π-olefin complexes of cyanocobaltate are formed. The solutions also exhibit characteristic bands near 450 nm. The stoicheiometry of complex formation has been studied using PhCHC(CN)₂ and monitored by the absorbance of the complex at 440 nm. These results indicate that a new type of π-olefin–cobalt(I) complex, [Co(CN)₄(olefin)]^3–, is formed, where olefin is (NC)₂CC(CN)₂, PhCHC(CN)₂, cis- and trans-PhCHCHCN, E-PhCHC(CN)CO₂Et, E-PhCHC(CN)CO₂K, or cis- and trans-RO₂CCHCHCO₂R (R = Me, Et, or Na). The π complex of dialkyl maleate isomerises to that of dialkyl fumarate, probably by an electron-transfer mechanism.