Visualizing lone pairs in compounds containing heavier congeners of the carbon and nitrogen group elements

In this mini-review, I discuss some recent work on the stereochemistry and bonding of lone pairs of electrons in divalent compounds of the heavier carbon group elements (Sn-II, Pb-II) and in trivalent compounds of the heavier nitrogen group elements (Bi-III). Recently developed methods that permit the real-space visualization of bonding patterns on the basis of density functional calculations of electronic structure, reveal details of the nature of s electron lone pairs in compounds of the heavier main group elements - their stereochemistry and their inertness (or lack thereof). An examination of tetragonal P4/nmm SnO, alpha -PbO and BiOF, and cubic Fm (3) over barm PbS provides a segue into perovskite phases of technological significance, including ferroelectric PbTiO3 and antiferroelectric/piezoelectric PbZrO3, in both of which the lone pairs on Pb atoms play a pivotal role.