THE AL-O BOND INTERACTION IN 4-COORDINATE ALUMINUM ARYLOXIDE COMPOUNDS

What is a normal Al-O bond? The nature of this common chemical bond is discussed with respect to the observed structural and spectroscopic parameters for monomeric aluminum aryloxide compounds, e.g. AlR(x)(OAr)(3-x)(L)(n) (x = 0, 1, 2 and n = 0, 1). Experimental evidence for the presence of short Al-O bond distances, the low reactivity of residual aluminum-alkyl groups, the electron rich nature of the aluminum center, and the weakening of Al-X bonding by the aryloxide ligand are presented. Based upon the experimental observations and theoretical studies, models for the Al-O bond are presented and discussed: steric effects, highly ionic bonding, d pi-p pi interactions and pi-bonding via donation into Al-X delta* orbitals.