A SCALE OF RELATIVE LEWIS ACIDITIES FROM PROTON MAGNETIC RESONANCE DATA

A scale of relative Lewis acidities has been set up for a number of inorganic halides and substituted inorganic halides based on the ratios of changes in the pmr chemical shifts of the αaL protons of ethers on complex formation. The sterically most favorable ether, tetrahydrofuran, was chosen as the reference base. Rapid exchange of Lewis acids among excess Lewis base sites occurs with all but the strongest acid-strongest base system studied. The pmr data on the complexes of boron fluoride and boron chloride indicate that back bonding persists in the complexes and that the bonding is dominantly electrostatic in character. When steric factors are not differentiating in either acid or base, relative Lewis acidities are unaffected by changes in the reference ether base. Steric inaccessibility in either acid or base causes pronounced changes in relative Lewis acidities toward ethers. For the chlorides of group IIIB elements, acidities increase with the electronegativity of the element. For the halides of group IVB elements steric rather than electronic factors appear to be controlling. © 1968, American Chemical Society. All rights reserved.