Dynamic Stereochemistry of Tris-Chelate Complexes. I. Tris(dithiocarbamato) Complexes of Iron, Cobalt, and Rhodium

The temperature-dependent pmr spectra of tris(N,N-disubstituted dithiocarbamato)metal(III, IV) complexes, M^III(dtc)₃ and M_IV(dtc)₃BF₄, where M = Fe(III), Fe(IV), Co(III), and Rh(III) have been examined in noncoordinating solvents. Fe(II) complexes of the type Fe(dtc)₂(o-phenanthroline) have also been examined. All of the iron complexes are stereochemically nonrigid, and kinetic parameters were determined for intramolecular metal-centered rearrangement by nmr line broadening techniques. This rearrangement results in optical inversion and the trigonal twist mechanism has been proved to be the primary rearrangement pathway. This result derives directly from pmr environmental averaging patterns. Co(dtc)₃ is also stereochemically nonrigid but the mechanism which results in optical inversion could not be determined; however, the trigonal twist mechanism is considered the most probable by analogy with kinetic activation parameters. The Rh(dtc)₃ complex was rigid up to +200° in NO₂C₆D₅. The overall metal ion dependence on the rate of optical inversion via the trigonal twist mechanism is: Fe(II) (S = 2) > Fe(III) (S = ½ ⇄ S = 5/2) ~ Fe(IV) (S = 1) > Co(III) (S = 0) > Rh(III) (S = 0). Within the Fe(III) class, the rate depended on the position of the spin-state equilibrium, i.e., the more high spin complexes generally rearranged faster. Trends in the rate of optical inversion are considered in light of solid-state structural parameters and electronic configuration. In particular, a consideration of ligand field stabilization energies for trigonal prismatic and trigonal antiprismatic coordination is important.