Bonding in clusters. Part 10. The nature of the BH groups in closo-borane anions. A study by vibrational spectroscopy, electronic spectroscopy for chemical analysis, and ab initio computation

Abstract

The i.r. and Raman spectra of Cs₂B_nH_n compounds have been studied, concentrating on the BH stretch vibrational bands which have very much the same energy independent of cluster site or size. Electronic spectroscopy for chemical analysis of the same compounds shows that the charge localised on the boron atoms is essentially independent of the details of cluster bonding. These observations allow the assignment of sp² hybridisation to all BH groups in the full set of anions and suggest that the correct starting point for a TSH description of borane clusters is one where each BH fragment makes a constant isolobal contribution to the cage, regardless of cage size. This assignment is consistent with the results of ab initio calculations on the closo sequence of anions.