Bonding studies of boron and the Group 3–5 elements. Part XV. He(I) photoelectron spectra of monomeric Group 3 trihalide, trimethyl, and mixed halogenomethyl species

Abstract

The He(I) photoelectron (p.e.) spectra of monomeric Group 3 metal trihalide, trimethyl. and mixed halogeno-methyl species, MX₃, MeMX₂, Me₂MX, and MMe₃(X = Cl, Br, or I; M = B, Al, Ga, or ln), are analysed. There is close similarity between the p.e. spectra of the heavier Group 3 metal trihalides and those of their boron congeners and this forms the basis for assignments. Aspects of earlier interpretations of the spectra are reassessed in the light of the data now available for the various series. For BMe₃ the BC e′ orbital is split by a Jahn–Teller effect in the excited state; C–H non-bonding interactions decrease as B is replaced by a heavier metal in MMe₃ and the splitting is obscured in these molecules. For MX₃ the highest observed ionisation potential (i.p.) is the substantially central-atom a₁ which decreases with increasing mass of M. The next in energy is the é which shows an essentially constant and high spin–orbit splitting and thus has little central-atom M character. This is followed by the a₂″ which may give some measure of X→M π bonding (greatest for B). It is proposed that the baricentre of the first two i.p.s for each of these molecules provides a measure of their relative Lewis acidity: Al > B ∼ Ga > ln.