Theoretical Investigations Of Transition Metal Complexes (multiple Scattering, X-alpha, Bonds, Iron-sulfur Clusters).

Abstract

The X(alpha) scattered-wave method has been used to investigate the electronic structure of several transition metal complexes, including molybdenum(II) and tungsten(II) carboxylates, hydrido-bridged dimers of tantalum and rhenium, a model for an unsupported Ta(TBOND)Ta triple bond, tantalum(II) and niobium(II) monomers, and iron-sulfur prismane clusters. Calculations were performed for Mo(,2)(O(,2)CH)(,4) and W(,2)(O(,2)CH)(,4). We find that the overall electronic structures of the two species are quite similar. Calculated ionization potentials for the (delta) and (pi) metal-metal bonding orbitals compare well with the experimental values for Mo(,2)(O(,2)CH)(,4) and the Mo and W pivalate and trifluoroacetate congeners. The electronic absorption spectra of W(II) pivalate and W(II) trifluoroacetate are assigned; our calculations (and other experimental evidence) indicate that the (delta) (--->) (delta)* singlet (('1)A(,1g) (--->) ('1)A(,2u)) transition occurs at (TURNEQ)23000 cm('-1), but is obscured by other more intense bands. The very weak band centered at ca. 700 nm in each spectrum is assigned as the (delta) (--->) (delta)* triplet (('1)A(,1g) (--->) ('3)A(,2u)) transition. The Ta-H(,(mu)) bonding in TaCl(,2)(PH(,3))(,2) (,2)((mu)-H)(,4), TaCl(,2)(PH(,3))(,2) (,2)((mu)-H)(,2), and ReH(,2)(PH(,3))(,2) (,2)((mu)-H)(,4) was examined. The results compare well with a previous study of such species by Hoffman et al. The spectrum of TaCl(,2)(PMe(,3))(,2) (,2)((mu)-H)(,4) is assigned. Paramagnetic Ta(II) and Nb(II) chlorido-phosphine monomers were investigated. The highest occupied levels in these compounds are metal-localized d-orbitals. The spectra of MCl(,2)(PMe(,3))(,4) and MCl(,2)(dmpe)(,2) (M = Ta,Nb) are assigned. Finally, a new class of iron-sulfur clusters, prismanes, were examined for stability and reactivity. A comparative study was made of the electronic structures of Fe(,6)S(,6)Cl(,6)('-3) (which contains the basic Fe(,6)S(,6)('+3) prismane unit), H(,3)MoFe(,6)S(,6)Cl(,6)('-3) (a mono-capped prismane, and a possible model for the active center of the cofactor of nitrogenase), and Fe(,8)S(,6)Cl(,8)('-4) (a di-capped prismane whose structure resembles a repeating unit of the mineral pentlandite). We find that sulfur lone pairs in the uncapped prismane are responsible for binding the capping fragments. All sulfur levels decrease in energy monotonically in the series prismane (--->) mono-capped prismane (--->) di-capped prismane.