In more general terms, the formation of bridged complexes 6-8 presumably reflects the fact that fluoride uptake at calcium is accompanied by immediate 'capping' by a second equivalent of neutral complex 4, thereby leading to the formation of a product with a 2:1 Ca:F ratio. We attribute this to the low local concentration of the fluoride anion, due to the poor solubility of KF (in particular) in benzene compared to complex 4.