Cyanide Lability and Linkage Isomerism of Hexacyanochromate(III) Induced by the Co(II) Ion

Reactions between the [MIII(CN)6]3− (M = Fe and Co) anions and the mononuclear complex [CoII(dppe)2(H2O)][BF4]2 result in the formation of two isostructural trinuclear clusters {[CoII(dppe)2]2[MIII(CN)6]}(BF4). Surprisingly, reactions of [Co(dppe)2(H2O)](BF4)2 and [Co(triphos)(CH3CN)2](BF4)2 with [Cr(CN)6]3− yield the mononuclear complexes [Co(dppe)2(CN)](BF4) and [Co(triphos)(CN)2], respectively. In the former case, an unusual pentanuclear intermediate complex {[CoII3(dppe)4(MeCN)][CrIII(CN)6]2} was isolated. The reaction was probed by solution IR spectroscopy, which revealed a gradual conversion of the ν(CN) stretches of the starting materials to those of the CN-bridged intermediate and eventually to the single ν(CN) stretch of the final mononuclear product. The loss of carbon-bound CN− ligands from [Cr(CN)6]3− occurs on a sufficiently slow time-scale for observation of varying degrees of cyanide linkage isomerism in the trigonal bipyramidal complex {[Co(tmphen)2]3[Cr(CN)6]2}; the study was aided by the use of different Co(II) starting materials. Results obtained by a combination of X-ray crystallography, infrared spectroscopy, and magnetometry provide unequivocal evidence that the presence of certain Lewis acids (e.g., Co(II) in this work and Fe(II) ions and BPh3 in previously reported studies) promote the process of cyanide linkage isomerism, which, in the case of Co(II) species, leads to facile labilization of cyanide ligands from the [Cr(CN)6]3− anion.