Diphosphine and diarsine complexes of chromium(III). Crystal and molecular structure of [NPrn4][Cr(cis-Ph2PCHCHPPh2)Cl4]

Abstract

cis-[Cr(L–L)X₄]^– ions (X = Cl, Br, or I; L–L = diphosphine or diarsine) have been isolated as tetra-alkylammonium salts by reaction of [NR₄]X, [Cr(thf)₃X₃](thf = tetrahydrofuran), and L–L. The reaction of [Cr(thf)₃X₃] with L–L′ in CH₂Cl₂ gave Cr(L–L′)_1,5X₃[L–L′=o-C₆H₄(AsMe₂)₂, o-C₆H₄(AsMe₂)(PMe₂), o-C₆H₄(NMe₂)(PMe₂), or o-C₆H₄(PMe₂)(SbMe₂)], which are formulated as trans-[Cr(L–L′)₂X₂]cis-[Cr(L–L′)X₄]. In the case of o-C₆H₄(PMe₂)₂ only, it is possible to isolate both Cr[o-C₆H₄(PMe₂)₂]_1,5X₃ and [Cr{o-C₆H₄(PMe₂)₂}₂X₂]X depending upon the Cr : L–L ratio. The ligand Me₂As(CH₂)₃AsMe₂ gave Cr(L–L)_1,5X₃(X = Cl or Br) complexes for which a ligand-bridged structure is proposed [X₃(L–L)Cr(L–L)Cr(L–L)X₃]. Weaker donors (Ph₂PCH₂CH₂PPh₂, cis-Ph₂PCHCHPPh₂, or Ph₂AsCH₂CH₂AsPh₂) react with [Cr(thf)₃X₃] to give [Cr(L–L)₂X₃] which contain one uni- and one bi-dentate diphosphine or diarsine, and in the presence of moisture [Cr(L–L)(H₂O)X₃] can be isolated. The complexes [Cr(L–L)₂X₂]Y [X = Cl or Br; Y = ClO₄^– or PF₆^–; L–L =o-C₆H₄(AsMe₂)₂ or o-C₆H₄(PMe₂)₂] have also been obtained The complexes have been characterised by analysis, conductance, i.r. and electronic spectroscopy, and the electronic spectra analysed (d³) to produce Dq, B′, and β₃₅. The title complex is triclinic, a= 17.534(3), b= 11.930(2), c= 11.108(3)Å, α= 116.30(2), β= 75.12(2), γ= 110.61(1)°, and Z= 2; space group P(no. 2). 1 757 Reflections were refined to R= 0.0656 (R′= 0.0658). The structure consists of tetrahedral NPrⁿ₄⁺ cations, and pseudo-octahedral [Cr(Ph₂PCHCHPPh₂)Cl₄]^– anions. In the anion Cr–P = 2.485(4) and 2.511(4)Å; Cr–Cl = 2.331 (4), 2.31 9(4), 2.31 8(4), and 2.31 6(4)Å; P–Cr–P = 81.2°. The diphosphine does not exert any trans influence, consistent with weak binding to the hard Cr^III ion.