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

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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 dz2 orbitals. The σ–σ* energy separation suggests greater metal–metal interaction for the tantalum complex.

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