Solvent and countercation effects on ambidentate bonding patterns in thiocyanato- and selenocyanatopentacyanocobaltate(III) complexes

Abstract

The complexes [Co(NH 3) 5NCS] 2+ and [Co(CN) 5 SCN] 3− have frequently been cited as classic examples of symbiotic bonding in terms of the Hard/Soft Acid/Base Principle. The results of infrared and electronic spectral studies in our laboratory have revealed that this comparison is appropriate only in protic solvents, for the [Co(CN) 5SCN] 3− complex isomerizes to its N-bonded isomer in CH 2Cl 2, acetone, and nitrobenzene. In DMF, an equilibrium is established involving the N-bonded isomer and a species resulting from the dissociation of the SCN- group. Dissociation is essentially complete in CH 3CN and DMSO. S-bonding prevails in the protic solvent ethylene glycol, emphasizing the importance of H-bonding in stabilizing the S-bonded isomer. We have succeeded in preparing the corresponding [Co(CN) 5SeCN] 3− complex via an inner-sphere redox reaction between [Co(NH 3) 5 NCSe] 2+ and [Co(CN) 5] 3− and find that it follows the same bonding pattern as a function of solvent as its thiocyanate analog. Infrared data indicate that the [Co (CN) 5XCN] 3− complexes isomerize via a dissociative mechanism, possibly involving the formation of an ion pair, in CH 2Cl 2, acetone, nitrobenzene, and DMF, the thiocyanate complex isomerizing at a faster rate. The rates of dissociation of the four [Co(CN) 5CNX] 3− linkage isomers in CH3CN and DMSO vary in the order: CoSCN > CoSeCN > CoNCS ∼ CoNCSe. The size of the countercation can determine the bonding mode adopted by the complexes in the solid state. S-bonding prevails when K +, Cs +, and CH 3NH 3 + are present, a mixture (25% N-bonded) is obtained with (CH 3) 2NH 2 + and (CH 3) 4N +, like the previously reported (n-C 4H 9) 4N +, yields only the N-bonded isomer.