Sterically Inactive Lone Pairs

AX(,E Molecules and Sterically Weak and Inactive Lone Pairs... 

The effect of lone pairs can be difficult to predict. For examples of sterically active and inactive lone pairs in ions of formula AX " , see K. O. Christe and W. Wilson, Inorg. Chem., 1989, 28, 3275, and references therein. 

Reaction of Pb(ACAC)2 with two equivalents of HB(Arf)4 in CH2CI2 followed by HC(pz)3 or HC(3,5-Me2-pz)3 gives the complexes [L2Pb] 4 [B(Arf)4]2 (Arf=3,5-(CF3)2-C6H3). For the HC(pz)3 complex, the lead has a distorted octahedral geometry with a stereochemically active lone pair, while the complex with the more sterically encumbered HC(3,5-Me2-pz)3 has a trigonally distorted octahedral structure with a stereochemically inactive lone pair. ... 

Figure 12. Six-coordinate Pb(II) compounds can either be (a) hemidirected (with a stereo-chemically active lone pair) or (b) holodirected (with a stereochemically inactive lone pair). Here, in two six-coordinate compounds with similar ligand sets, the steric repulsions in the bulkier ligand (b) trigger the stereoinactivity of the lone pair. [Reprinted with permission from D. L. Reger, T. D. Wright, C. A. Little, J. J. S. Lambda, and M. D. Smith, Inorg. Chem., 40, 3810-3814 (2001). Copyright 2001 American Chemical Society.]...

Sterically demanding ligands and high coordination numbers can shift the lead geometry to a more spherically symmetric, holodirected structure, in which the lone pair of electrons is stereochemicaUy inactive. 

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