Stereochemistry of seven-co-ordinate compounds bis(bidentate ligand)-tris(unidentate ligand)metal

Abstract

Repulsion-energy calculations have been used to examine the stereochemistry of compounds of the type [M(bidentate)₂(unidentate)₃]^x±, For bidentate ligands having small normalised bites (b < 1·1), it is predicted that the only stable stereochemistry, (I), will be the pentagonal bipyramid with the bidentate ligands spanning two of the equatorial edges, the symmetry being C_2ν. This prediction is in excellent agreement with known structures. As the normalised bite is increased, the pentagonal plane progressively distorts with loss of the mirror planes but retention of the two-fold axis, with the polyhedron approaching first a capped trigonal prism with the bidentate ligands spanning opposite triangular edges of the triangular prism and secondly a pentagonal bipyramid with the bidentate ligands spanning edges linking axial and equatorial sites. Compounds with stereochemistry (II) may be expected to occur with bidentate ligands having normalised bites above ca. 1·3. Stereochemistry (II) also has a two-fold axis, and again the polyhedron chosen to represent the structure best depends on the value of the normalised bite. The predictions are in agreement with experiment. A third minimum on the potential-energy surface becomes of sufficiently low energy to be considered as a possible stereochemistry for normalised bites in the region b ∼ 1·2–1·3. This stereochemistry, (III), contains a mirror plane and is well represented by a capped trigonal prism with the bidentate ligands again spanning two of the triangular edges of the prism.