Pentacoordination and the VSEPR model

The VSEPR model may also be used to rationalize the observation that the axial bonds in tbp molecules are longer than the equatorial The axial bond electron pair is repelled by three (equatorial) bond pairs over a valence angle of 90°, while the equatorial bond electron pair is repelled by two (axial) bond pairs over the same angle The additional repulsion leads to greater elongation of the axial bonds. 

Problem 15.2 Suggest a rationalization for the observation that the apical bonds in the square pyramidal pentaphenyl bismuth is shorter than the basal. 

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