ChemInform Abstract: The Linkage Isomerism of Thiocyanate Bonded to Cobalt(III).

Abstract

Abstract The structural differences between Co(III)-SCN and Co(III)-NCS bonding molecules are briefly reviewed as are attempts by Werner and his associates at the turn of the century to prepare (unsuccessfully) Co-SCN complexes. Work in the author's laboratory over a period of about 20 years is then reviewed. The thermal isomerisation of crystalline trans -[CO(NH 3 ) 4 ( 15 NH 3 )SCN](N 14 CS) 2 to give 40% trans -CoNCS 2+ , and 30% of each of cis -CoS 14 CN 2+ and cis -CoN 14 CS 2+ is discussed in terms of “frontside” intramolecular rearrangement to give the trans product, and “backside” attack of one specific lattice S 14 CN − to give both the cis 14 C-labelled products. In aqueous solution the thermal rearrangement of [Co(NH 3 ) 5 SCN] 2+ to give CoNCS 2+ also results in some 9% CoOH 2 3+ , and when carried out in the presence of S 14 CN − results in small amounts of CoS 14 CN 2+ and CoN 14 CS 2+ also being produced. The formation of these products is discussed in terms of reactive ion-pairs, with the Co 3+ ·SCN − ·S 14 CN − aq transition state incorporating small amounts of S 14 CN − and N 14 CS − in addition to OH 2 and NCS − . The Hg 2+ , Ag + and OH − catalysed reactions are also reviewed. Larger amounts of CoOH 2 3+ are produced in these reactions, but solvated anions (NO 3 − , ClO 4 − , CF 3 SO 3 − , N 3 − ) are also incorporated in addition to isomerised CoNCS 2+ . A transition state (or intermediate) containing pre-associated entering groups is required. There seems to be little discrimination between entering groups with stereochemical positioning and/or orientation of electron lone pairs being the most important factors in gaining entry. Very little activation for entry seems necessary, and the five-coordinate intermediate has no useful lifetime. Competition for entry only occurs between groups occupying 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.