Determination of the crystal structure of the mono-caesium derivative of the macrccyclic tetraphenol p-tert-butylcalix[4]arene shows the metal atom to be bound within the cup formed by the ligand; the metal is thereby in much closer proximity to the aromatic carbon atoms than to the phenolic oxygens, indicating that it is involved in polyhapto bonding to a delocalised anionic ligand.
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Harrowfield et al. (1991) studied this question.
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