BERYLLOCENE AND RELATED SLIP-SANDWICH STRUCTURES

The structures of main group metal cyclopentadienyl complexes range from centrally bonded ferrocene analogues to peripherally bonded cyclopentadiene analogues. It is proposed that the wide variety of structures arises from two opposing forces. One is the delocalisation energy of the cyclopentadienyl ligand which favours central bonding. The other is the metal-carbon sigma-bond energy which favours peripheral bonding. Unusual structures such as the slip-sandwich geometry of beryllocene result when neither of these forces is dominant. This continuum model rationalises the available structural data and provides an explanation for some of the spectroscopic features of main group metal cyclopentadienyl compounds.