Seven-coordinate d(0) and d(10) ions - Computational and experimental studies on tris(tropolonato)metal(III)-TOPO adducts

The stabilities of molecular adducts between tris(tropolonato)metal(III) ch elates of some tripositive ions of Groups 3 and 13 and trioctylphosphane ox ide (TOPO) were studied using both theoretical and experimental methods. De nsity-functional calculations were used to optimise the structure and chara cterise tris(tropolonates) of scandium and yttrium, a series of model chela tes of gallium, indium, thallium, scandium, and yttrium, as well as the 1:1 adducts of these chelates with trimethylphosphane oxide, The calculated en ergy of adduct fori-nation decreased in the order: Y > Sc > Tl > In. Stabil ity constants of the 1:1 (and some 1:2) TOPO adducts in solution, determine d by synergistic solvent extraction of metal tropolonates, decreased in the same order. Both methods show that gallium does not increase its coordinat ion number (CN) above six. The chelates of d(10) ions (CN 6) and their addu cts (CN 7) are less stable than the corresponding d(0) species, because the former are hypervalent compounds with no higher-lying metal nd orbitals pa rticipating in bonding, while the latter have easily accessible vacant (n - 1)d orbitals. A significant distortion of the chelate structure makes the adducts of smaller ions less stable. Examination of bonding molecular orbit als in the adducts shows an unusual origin of the metal-TOPO bond, which ma y be due to a contribution from pi -bonding. Differences in some characteri stics of indium and thallium. compounds can be explained by relativistic pr operties of thallium.