The syntheses, structures, bonding, and reactivity of cyclopentadienylaluminum(III) derivatives

Cyclopentadienylaluminum compounds have been a curiosity due to the ring-sl ipped structures they exhibit and due to their highly fluxional nature, whi ch has prevented the freezing out of discrete solution structures on the NM R time scale. Lately, increased attention toward these compounds in the for m of molecular structure determinations, theoretical calculations, and Al-2 7-NMR spectroscopic measurements is providing a clearer picture of the bond ing between the aluminum and its cyclopentadienyl ring(s). A variety of rin g coordination geometries (eta(1), eta(2), eta(3), eta(5)) have been identi fied in molecular structures of these compounds, and although theoretical c alculations reveal very small energy differences between different haptotro pes, consistent with their highly fluxional behavior in solution, there is close agreement between the calculated ground state geometries for the comp ounds and the geometries they exhibit in the solid state and in solution. T heory and experiment reveal a delicate balance of covalent pi-interactions and ionic interactions between the aluminum and its cyclopentadienyl ring. Although the balance is easily tipped by steric effects, some structural tr ends are emerging. The solution Al-27-NMR chemical shifts of cyclopentadien ylaluminum compounds are surprisingly diagnostic of their geometries, exhib iting a strong correlation with coordination numbers that take into account the hapticity of the cyclopentadienyl ring. (C) 1999 Elsevier Science S.A. All rights reserved.