Theoretical evidence of persistent chirality in D₃ homoleptic hexacoordinate complexes with monodentate ligands

A theoretical study of the enantiomer interconversion pathway relevant to racemization reactions of hexacoordinate transition-metal complexes is presented based on density functional calculations. The potential energy surface for the trigonal twist pathway of the [Zr(SH)₆]^2- model compound has been explored. The optimum structure reproduces, to a very good approximation, the experimental geometry of the analogous compound in which the thiolato groups have C₆H₄-4-OMe substituents instead of H atoms. A barrier of about 19 kcal mol^-1 is estimated for the racemization of [Zr(SH)₆]_2- and exploratory calculations for [Zr(SC₆H₄-4-OMe)₆]^2- indicate that a larger barrier should be expected. For the chiral homoleptic organometallic complexes [ZrMe₆]^2- and [RhMe₆]^3- no significant racemization barrier is expected.