Compounds of hypervalent Group 16 elements model considerations

8 Compounds of hypervalent Group 16 elements model considerations 

A tetravalent Group 16 atom is presumably surrounded by five electron pairs, viz. four bond pairs and one non-bonding electron pair. According to the VSEPR model the five electron pairs should arrange themselves in a trigonal bipyramidal manner. Since the nonbonding electron pair requires more space around the central atom than the bond pairs, it is expected to occupy an equatorial position, and deform the molecule in such a way that the bond electron pairs are pushed away. This is indeed the kind of distortion that is observed. The difference between axial and equatorial bond distances may be explained in the same manner as for PF5 and the other compounds of hypervalent Group 15 elements. 

A hexavalent Group 16 atom is presumably surrounded by six bond electron pairs. The octahedral coordination geometry is in accord with the VSEPR model. The observation that the bond distances in the hexavalent compounds are 8-10 pm shorter than the axial bonds in 

We now turn our attention to molecular orbital descriptions. The compounds of tetravalent elements may be described in terms of two 2c, 2e bonds to the equatorial ligands. Such a description is in agreement with the observation that the equatorial bond distances in Te(CH3)4 is equal to the bond distances in Te(CH3)2. The non-bonding electron pair occupies a third hybrid atomic orbital in the equatorial plane. The axial bonds, like the axial bonds in the pentavalent Group 15 element compounds, be described in terms of a 3c, 4c bonding system. The observation that the axial bonds are longer than the equatorial is, of course, in accord with the molecular orbital description of the bonding. 

The observed variation of Te-C bond distances in the series Te(CH3)2, Te(CH3)4 and Te(CH3)6 is entirely consistent with the MO description of the bonding. The observed variation of S-F bond distances in the series SF2, SF4, and SFe, on the othCT hand, suggests that inductive effects leads to progressive shortening of the bonds with increasing number of attached F atoms. 

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