BONDING IN THE ALUMINUM CAGE COMPOUNDS [AL(ETA-5-C5R5)]4 AND AL4X4, X = H, F, CL

We report extended basis set SCF treatments of the recently synthesized first stable Al(I) compound (AlC5R5)4, R = CH3 experimentally, R = H in the calculations. For comparison we consider the series AlX, X = H, F, Cl, C5H5, and its tetramers. Bonding in the tetramers, showing an Al4 tetrahedron, is much weaker than in corresponding boron compounds and is especially weak in (AlC5H5)4 due to pronounced pi-backbonding in the monomer which interferes with bonding in the Al4 tetrahedron.