Crystal chemistry of the VA element trihalides: Lone pair, stereochemistry, and structural relationships

The crystal structures of NCl ₃, PCl ₃, PBr ₃, AsF ₃, and AsCl ₃ liquids at room temperature have been recently determined. The data for the last two are reported in the present paper. The structures of the following trihalides are discussed and classified in structural types: AsF ₃, SbF ₃, αBiF ₃, YF ₃ (NCl ₃, PCl ₃, PBr ₃, βSbCl ₃, βSbBr ₃, γBiF ₃, and βBiCl ₃), AsBr ₃ (AsCl ₃, AsBr ₃, "αSbCl ₃," and αSbBr ₃), PI ₃ and αAsI ₃ (αAsI ₃, SbI ₃, and BiI ₃). Structural relationships have been established between AsF ₃ and XeO ₃, P X₃E and PO X₃ ( X = Cl, Br), and AsBr ₃E has been compared with PuBr ₃ and VOCl ₃. A structural mechanism is proposed for the polymorphic transformation αSbBr ₃ /ai βSbBr ₃. The CrCl ₃ structure type is proposed for the high-temperature form of AsI ₃ and a mechanism is given for the phase transition αAsI ₃ /ai βAsI ₃. A proposal is made for a possible high-pressure transformation of the αAsI ₃ structure into a VF ₃ structure type. The stereochemistry is discussed in terms of coordination polyhedra around M∗; all these trihalides are characterized by a basic molecular unit M∗X ₃. The steric effects of the lone pair E carried by M∗ is especially emphasized and discussed; two characteristics of E are presented: the centroid of the electronic doublet located by E_c and the sphere of influence located by its center E_s. The correlative variations of M∗E _c and M∗E _s are studied according to the nature of M∗ (N(III), P(III) to Cl(V), Ga(I) to Br(V), In(I) to Xe(VI), and Tl(I) to Po(IV)).