High coordination numbers of metal ions in chelate complexes: molecular adducts of scandium tris-alpha-diketonates in solution

It has been shown that scandium(III) ion in strong chelate complexes with b identate ligands can increase its coordination number typically from six to seven by formation of inner-sphere adducts with trioctylphosphine oxide (T OPO) or water. Stability constants of tris(tropolonato)scandium(III) and tr is(ethylmaltolato)scandium(III) in water, log beta(3) = 21.6(1) and 23.3(1) , and the formation constants of their 1:1 adducts with TOPO in toluene: lo g beta(3,1) = 4.07(2) and 1.69(1), respectively, were determined by the sol vent extraction method. Tris(acetylacetonato)scandium(III), chelate of simi lar stability, does not form adducts with TOPO. Comparison of these data wi th those published elsewhere shows that formation of inner-sphere adducts b y metal chelates is governed by factors related to both stability and struc ture of the chelate. For complexes with octahedral or slightly distorted oc tahedral environment the stability of adducts increases with decreasing sta bility of the complex, in line with the generally accepted rule. For comple xes with octahedral environment considerably distorted toward trigonal pris matic, adduct formation is possible even if the complex is very strong. In this case, the ability to form adducts increases with decreasing ligand bit e. (C) 1999 Elsevier Science S.A. All rights reserved.