Pentaphenyl bismuth

Pentaphenyl bismuth, BiPhs, is also approximately square pyramidal in the crystalline phase, the largest ZCBiC angles being 150° and 164°. The apical Bi-C bond distance, 221 pm, is considerably shorter than the mean basal bond distance of 232 pm. 

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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