SUBSTITUENT EFFECTS IN CLUSTER SPECIES .2. PHOTOELECTRON-SPECTRA OF 2-SUBSTITUTED AND 2,4-SUBSTITUTED 1,6-DICARBA-CLOSO-HEXABORANE(6)

The He(I) photoelectron spectra of 2-X-1,6-C2B4H5 and 2,4-X2-1,6-C2B4H4 where X = CI, Br, and I are reported. The syntheses and characterizations of the new disubstituted derivatives are described. A simple three-parameter model is used to describe the effects of halogen substitution on the photoelectron spectrum of the closo carborane. A π-type interaction between the surface orbitals of the cage and the filled halogen p orbitals constitutes the major effect. The parameters generated permit the relative electronic charge at the 2 position of 1,6-C2B4H6 to be compared to that at the 1 and 2 positions of B5H9. In addition the relative extent of interaction of X with the six-atom cage is compared to that for a six-atom aromatic ring system. This work demonstrates that the surface (π endo) orbitals of a cluster play an important role in determining the effect of substitution on cluster properties. © 1979, American Chemical Society. All rights reserved.