THE LINKAGE ISOMERISM OF THIOCYANATE BONDED TO COBALT(III)

The structural differences between Co(III)-SCN and Co(III)-NCS bonding molecules are briefly reviewed as are attempts by Werner and his asso ciates at the turn of the century to prepare (unsuccessfully) Co-SCN c omplexes. Work in the author's laboratory over a period of about 20 ye ars is then reviewed. The thermal isomerisation of crystalline trans-[ CO(NH3)(4)((NH3)-N-15)SCN]((NCS)-C-14)(2) to give 40% trans-CoNCS2+ an d 30% of each of cis-(CoSCN2+)-C-14 and cis-(CoNCS2+)-C-14 is discusse d in terms of ''frontside'' intramolecular rearrangement to give the t rans product, and ''backside'' attack of one specific lattice (SCN-)-C -14 to give both the cis C-14-labelled products. In aqueous solution t he thermal rearrangement of [Co(NH3)(5)SCN](2+) to give CoNCS2+ also r esults in some 9% CoOH23+, and when carried out in the presence of (SC N-)-C-14 results in small amounts of (CoSCN2+)-C-14 and (CoNCS2+)-C-14 also being produced. The formation of these products is discussed in terms of reactive ion-pairs, with the Co3+.SCN-.(SCNaq-)-C-14 transiti on state incorporating small amounts of (SCN-)-C-14 and (NCS-)-C-14 in addition to OH2 and NCS-. The Hg2+, Ag+ and OH- catalysed reactions a re also reviewed. Larger amounts of CoOH23+ are produced in these reac tions, but solvated anions (NO3-, ClO4-, CF3SO3-, N-3(-)) are also inc orporated in addition to isomerised CoNCS2+. A transition state (or in termediate) containing pre-associated entering groups is required. The re seems to be little discrimination between entering groups with ster eochemical positioning and/or orientation of electron lone pairs being the most important factors in gaining entry. Very little activation f or entry seems necessary, and the five-coordinate intermediate has no useful lifetime. Competition for entry only occurs between groups occu pying adjacent stereochemical locations in the pre-associated reactant . A new mechanistic classification is suggested for Substitution at a metal centre. This emphasises the timing of the breaking and making of ligand-metal bonds, and recognises the necessity for pre-association and organization of entering groups.