Coordination Geometries and Electron Counts

In the following, we shall denote the number of electron pairs in the valence shell of the central atom by a digit in italics behind the formula. The mercury dihalides HgX2 (2) (X=C1, Br or I) and the Hg-Hg-bonded dimeric monohalides Hg2X2 (2) all have two bonding electrons pairs (and no nrmbonding electrons) in the valence shell of the merciuy atoms and all adopt linear structures. The structure of the ion [T1(CH3)2] (2) is also linear. 

The number of molecules with six IxMiding electron pairs in the valence shell was relatively large and included the neutral molecules SFg, SeFs, and TeFg and the 

Eighteen years later (in 1957), the VSEPR model was further developed and its predictive power enhanced by R. J. Gillespie and R. S. Nyholm [63]. They interpreted the less-than-tetrahedral valence angles listed in Table 8 as evidence that the repulsion between a lone pair and a bond pair is greater than the repulsion between two bmid pairs. Since a bond pair is extended further into space than a lone pair, it is reasonable to assume that it requires less space in the valence shell of the central atom. The larger than tetrahedral valence angles in trimethylamine and dimethyl ether are presumably determined by steric repulsion between the methyl groups. 

The central atom in CIF3 (5) is surroimded by three bond pairs and two lone pairs. If the repulsion between two lone pairs is greater than the repulsion between a lone pair and a bond pair, then both lone pairs should occupy equatorial positions, and this is again what is observed. The repulsion between two electron pairs thus appears to decrease in the order 

Lone pair — lone pair lone pair — bond pair bond pair — bond pair. 

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