Bonding studies of compounds of boron and the group IV elements. Part III. First ionisation potentials of some simple boron compounds by electron impact and by a new empirical molecular orbital method

Abstract

A number of molecular ions BX2Y+ and BXYZ+(X, Y, and Z = F, Cl, Br, I, NMe2, OMe, OEt, OPrn, SMe, and Et), some from previously unknown neutral molecules, obtained by redistribution reactions, have been characterised mass spectrometrically. The first ionisation potentials of the parent ions from the simple boron compounds BX3, BX2Y, and BXYZ have been measured. (The data for the boron halides were in Part II.) Comparison of these data are made with a model based on symmetry arguments and on a simple inductive parameter, which enable appropriate correlation diagrams to be derived. These are smooth curves, which attest to the reliability of the data and allow predictions of the first ionisation potentials of other molecules to be made. The first ionisation potentials are a composite of the antibonding interaction between p-orbitals on two ligand atoms and the bonding interaction between pπ-orbitals on a ligand and the boron atom. It appears that for the boron trihalides and mixed trihalides the former is the dominant term, while for the other compounds the reverse is the case. Interaction between a given pπ-orbital on a ligand and the pπ-orbital on the boron atom decreases in the order B–N > B–O ≈ B–S > B–Hal. There appears to be some correlation between the first ionisation potentials and heats of redistribution reactions.