Electrons lone pairs, steric demands

A substantial divergence from the behaviour described up to now occurs with the 2,6-(V,(V-tetramethyl anilines (Table 7). The effect of the para substituent is no longer systematic, and no good correlation with substituent electronic properties is displayed. Evidently, the extent of nitrogen lone-pair delocalization in these compounds reflects a balance between electronic demand for, and steric inhibition of, delocalization that varies with the individual compounds. Nitrogen chemical shifts are a sensitive probe for these effects. 

Hydroxy- and Alkoxysulfonium Ions. In addition to the well-documented formation of the S02-SbFs complex in a mixture of the two components,227 Minkwitz and co-workers228 have recently detected the formation of the fluorodihydroxysulfonium cation 83 [Eq. (4.61)]. Crystal structure analysis of the isolated salt showed bond distances typical of S—O single bonds (1.537 and 1.522 A) and S—F single bonds (1.547 A). The steric demand of the lone electron pair of sulfur results in the compression of bond angles from the ideal tetrahedral arrangement... 

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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