Stereochemistry and molecular rearrangements of some six-, seven-, and eight-coordinate chelates of early transition metals

This article is a review of the stereochemistry and molecular rearrangements of selected six-, seven-, and eight-coordinate chelates of early transition metals: primarily, metal beta-diketonates, dithiocarbamates, and monothiocarbamates. Dihalobis(beta-diketonato)metal complexes M(dik)(2)X(2) (M=Ti, Zr, or Hf) generally have an octahedral cis configuration, but Ti(acac)(2)I-2 and V(dik)(2)X(2) exist in solution as an equilibrium mixture of cis and trans isomers. Nuclear magnetic resonance (NMR) line-shape studies show that the dialkoxy complexes cis-Ti(dik)(2)(OR)(2) rearrange by an intramolecular twisting mechanism whereas the closely related O,O'-dimethyl dithiophosphates cis-Ti(Me(2)dtp)(2)(OR)(2) undergo metal-ligand bond rupture. The seven-coordinate complexes Zr(acac)(3)Cl, Ti(S(2)CNMe(2))(3)Cl, Ti(SOCNEt(2))Cl, and CpM(S(2)CNMe(2))(3) (M=Ti Zr, or Hf) have a pentagonal bipyramidal structure with the Cl or Cp ligand in an axial position. Eight-coordinate structures and stereoisomers are described for several T(IV), Zr(IV), Nb(V), and Ta(V) tetrakis chelates with beta-diketonate, dithiocarbamate, or monothiocarbamate ligands. The factors that determine the choice of coordination polyhedron, ligand wrapping pattern, and donor atom site occupancies are discussed. The seven- and eight-coordinate complexes are of special interest because they are among the first examples of higher coordination complexes that become stereochemically rigid on the NMR time scale.