Lone Pair Functionality in Divalent Lead Compounds

Abstract

The role of the lone pair of electrons of Pb(II) in determining the coordination geometry is analyzed from crystallographic studies and ab initio molecular orbital optimizations. Of particular interest are factors that contribute to the disposition of ligands around the lead with geometries that are (1) holodirected, in which the bonds to ligand atoms are distributed throughout the surface of an encompassing globe, and (2) hemidirect ed, in which the bonds to ligand atoms are directed throughout only part of an encompassing globe, i.e., there is an identifiable void in the distribution of bonds to the ligands. The preferred coordination numbers for lead were found to be 4 for Pb(IV) and 4 and 6 for Pb(II). All Pb(IV) structures in the CSD have a holodirected coordination geometry. Pb(II) compounds are hemidirected for low coordination numbers (2−5) and holodirected for high coordination numbers (9, 10), but for intermediate coordination numbers (6−8), examples of either type of stereochemistry are found. Ab initio molecular orbital studies of gas-phase Pb(II) complexes show that a hemidirected geometry is favored if the ligand coordination number is low, the ligands are hard, and there are attractive interactions between the ligands. In such complexes, the lone pair orbital has p character and fewer electrons are transferred from the ligands to the bonding orbitals of Pb(II), resulting in bonds that are more ionic. A holodirected geometry is favored when the coordination number is high and the ligands are soft and bulky or show strong interligand repulsion. The lone pair orbital has little or no p character when the geometry is holodirected, and the bonds are more covalent than in the hemidirected structures. The energy cost of converting a hemidirected to a constrained holodirected structure is of the order 8−12 kcal/mol in the absence of strong interligand interactions.

Gas-phase interactions between lead(II) ions and thiouracil nucleobases: A combined experimental and theoretical study

“…Finally, structures corresponding to tautomeric forms appear sometimes particularly stable (e.g., 2SU-4t) but never correspond to the global minimum. This is in contrast with recent findings reported for other singly charged ions, such as protonated uracil [42,43] and thiouracils [9] interacts with the sulfur atom, in agreement with previous results [44]. The charge transfer process is also more important compared with uracil (0.17-0.18 e-).…”

Preparation of a Novel Nano-scale Lead (II) Zig-Zag Metal–Organic Coordination Polymer with Ultrasonic Assistance: Synthesis, Crystal Structure, Thermal Properties, and NBO Analysis of [Pb(μ-2-pinh)N3 H2O]n

“…6, the lead atom is seven-coordinated, and a relatively large void is apparent on the same side of the N3O2 plane as the weakly bound N3 atom. The gap or hole in the coordination geometry around the metal ions is likely occupied by a stereochemcially active lone pair of electrons on lead(II) [12], so the coordination geometry is based on a Ψ-dodecahedron. This possibility is supported by the observed shortening of the Pb-N bonds on the side of the Pb 2+ ion opposite to the putative lone pair, i.e.…”

“…This stabilized 6s pair sometimes becomes an "inert pair," which explains why inorganic lead forms compounds in a lower oxidation state (less by two) than expected from its group number [12,13]. This may also influence the stereochemistry of lead(II)-organic coordination compounds.…”

“…In the absence of hybridization, the pair exhibits only s character, which makes it "stereochemically inactive." Thus, the complex does not show a gap or void in the bond distribution (holodirected coordination; see Scheme 1) [12].…”

Crystal structure of 2,2′-diphenyl-4,4-bithiazole,C18H12N2S2

“…Evidence of chain flexibility in solution comes from thermochromic and solvatochromic studies. In our aim to correlate stereoactive of valence shell lone electron pairs (6s 2 ) and overcrowding [2] in the coordination environment of lead(fl) [3][4], bismuth(ni) [5] and thallium® [6] complexes, we prepared the 2,2'-diphenyl-4,4'-bithiazole ligand.…”