Neutron Diffraction of Homopolyatomic Bismuth Ions in Liquid Bi5(AlCl4)3and ab Initio Study of the Structure and Bonding of the Isolated Bi53+Ion

Time-of-flight neutron diffraction measurements were carried out at 350 °C and 450 ± 5 °C for molten Bi 5 (AlCl 4 ) 3 . Contributions from AlCl 4 - were estimated using molten LiAlCl 4 data and yielded radial distribution functions RDF rem ( r ) that allowed for the determination of the D 3 h -Bi 5 3+ structure: d (Bi a −Bi e ) = 3.2 Å and d (Bi e −Bi e ) = 3.4 Å. The functions RDF rem (r) were simulated successfully by a model of intermediate range order similar to the environment in Bi 5 (AlCl 4 ) 3 crystals. The first sharp diffraction peak (FSDP) occurs at ca. 1.2 Å - 1 and suggests a distance between Bi 5 3+ and AlCl 4 - ions of about 5.5 Å in close agreement with values for the solid (5.6−6.2 Å). The free D 3 h -Bi 5 3+ ion was studied with RHF, MP2, and QCISD(T) methods. Effective core potentials were used in conjunction with polarized split-valence LANL1DZ basis sets. At the QCISD(T)/LANL1DZ+PP level, distances of d (Bi a −Bi e ) = 3.073 Å and d (Bi e −Bi e ) = 3.331 Å were determined. The Bi a −Bi e distances consistently are shorter by about Δ(Bi−Bi) = 0.26 Å in the free ion, by ca. 0.3 Å in Bi 5 (AlCl 4 ) 3 crystals, and by ca. 0.2 5 Å in the liquid. Natural population analysis shows a larger charge on Bi a (+0.74) than on Bi e (+0.51). Natural electron configuration analyses show intact 6s-type Bi lone pairs. The lowest-lying cluster MOs are a 1 ‘ (radial), e‘‘ (mixed), a 2 ‘‘ (∥), and e‘(⊥), and they are illustrated via contour plots of partial electron density functions. The molecular graph of Bi 5 3+ shows compelling evidence for strong bonding along all edges.