Trimethylphosphine complexes of niobium(IV) and tantalum(IV). Crystal, molecular, and electronic structures of [M2Cl4(µ-Cl)4(PMe3)4](M = Nb or Ta)

Abstract

Reaction of [NbCl₄(NCMe)₃] or [NbCl₄(thf)₂](thf = tetrahydrofuran) with three equivalents of PMe₃ gives the seven-co-ordinate complex [NbCl₄(PMe₃)₃]. In refluxing benzene this complex converts to [Nb₂Cl₄(µ-Cl)₄(PMe₃)₄] which is obtained in good yield by reacting [NbCl₄(NCMe)₃] with two equivalents of PMe₃. [NbCl₄(thf)₂] reacts with one equivalent of PMe₃ in acetonitrile to give [{NbCl₄(PMe₃)}_n]. [Ta₂Cl₄(µ-Cl)₄(PMe₃)₄] was obtained by reacting dilithium t-butyl-phosphide and PMe₃ with TaCl₅. Reduction of TaCl₅ with Na–Hg amalgam in the presence of [NEt₄]Cl gave [NEt₄]₂[TaCl₆]. X-Ray structure determinations show the [M₂Cl₄(µ-Cl)₄(PMe₃)₄] complexes (M = Ta or Nb) are isostructural (space group Im3), containing four symmetrical chlorine bridges [Ta–Cl 2.544(2), Nb–Cl 2.545(1)Å] across two metal centres [Ta–Ta 2.830(1), Nb–Nb 2.836(1)Å] each of which have co-ordinated two terminal chlorines [Ta–Cl 2.497(2), Nb–Cl 2.506(1)Å] and two terminal phosphine ligands [Ta–P 2.677(3), Nb–P 2.675(1)Å]. Scattered wave Xα calculations on the electronic structure of the dinuclear complexes [M₂Cl₄(µ-Cl)₄(PH₃)₄](M = Nb or Ta) indicate the formation of a metal–metal single σ bond between the metal atom d_z² orbitals. The σ–σ* energy separation suggests greater metal–metal interaction for the tantalum complex.