Crystal structures of two cobalt complexes with tetrahedrally distorted trigonal bipyramidal co-ordination and semiempirical molecular orbital study of the distortion

Abstract

The crystal and molecular structures of two high-spin cobalt(II) complexes with the ligand tris(2-diphenylphosphinoethyl)amine (np) have been determined by X-ray diffraction. Crystals of [Co(np)Br][PF₆],EtOH, (I), are triclinic, space group P, with a= 10.51, b= 18.10, c= 13.09 Å, α= 110.78°, β= 94.95°, γ= 98.68°, Z= 2. Crystals of [Co(np) l][BPh₄], MePh. (II), are triclinic, space group P, with a= 1 6.555(1), b= 18.780(1), c= 10.168 (1)Å, α= 84.83(1)°, β= 88.14(1)°, γ= 95.48(1)°, Z= 2. For complex (I) 2 1 1 9 reflections were collected by photographic methods, and 4510 by counter methods for (II). The structures were solved by heavy-atom techniques and final R values are 0.098 (I) and 0.115 (II). The co-ordination is trigonal bipyramidal with large tetrahedral distortion in both complexes and is similar to that previously found in the corresponding chloro-derivative. Complex (II) is the only high-spin iodo-derivative in this series. The unusually large temperature factor of the halogen atom in both structures is attributed to a pseudo-Jahn–Teller effect. Semiempirical MO calculations have been performed on simplified models, reproducing complexes of this series and of related series, in order to understand which factors determine the geometry of co-ordination in this series of complexes.