Abstract
It is shown that the shifts in the frequency of the carbonyl stretching vibration observed in adduct formation between carbonyl compounds and different covalent metal halides are not reliable measures of the strengths of the halides as Lewis acids. Equilibrium constants (K) for 1:1 adduct formation between phenalen-1-one and seven covalent halides, in ether solution at 25°, have been determined. Solid adducts involving the same reactants have also been prepared. The observed sequence of acidity is GaCl3 > SnCl4 > ZnBr2∼ SnBr4∼ ZnCl2∼ PhSnCl3 > BF3, with K for GaCl3 about a thousand-fold (or more) greater than K for BF3, whereas toward nitrogen bases these two acids exhibit roughly equal acidities. This anomaly is explained. Comparison of carbonyl and aniline bases of equal basicity towards protons, reveals that the oxygen bases are relatively much more basic towards covalent metal halides. This fact is also rationalised.
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