CHEMICAL BONDING OF THE BINARY INDIUM BROMIDES

In order to understand the chemical bonding of the binary indium bromi des, we have performed both classical and quantum mechanical studies o n all five crystallographically characterized phases (InBr, In5Br7, In 2Br3, InBr2, and InBr3). Using a bond length-bond strength Ansatz, the different oxidation states-of indium can be satisfactorily described by taking 266.7, 242.0, and 240.3 pm as standard bond distances r(0) f or In+-Br-, In2+-Br- and In3+- Br- interactions. On the basis of charg e-self-consistent semiempirical bandstructure calculations, it is argu ed that the reduced phases (InBr, In2Br3, and In5Br7) are ''soft'' and easy to perturb upon chemical reaction (in the spirit of Pearson's HS AB concept). Because of their electrophilicity, In2Br3 and In5Br7 may serve usefully as slightly acidic melts. Although coordination polyhed ra around In+ ions are highly irregular because of the influence of th e almost doubly filled indium 5s atomic orbital, the total In+-Br- bon ding interaction is similarly weak in all cases, and the crystal poten tial around Inf seems to be very soft. In none of the cases, however, has there been found a directed electron ''lone-pair'' effect for In+. While In+-Br- bonds are characterized by antibonding contributions at the frontier bands (out-of-phase combination between indium 5s and br omine 4p orbitals), true In-in interactions can be found in the case o f the In2Br62- species (In2+-In2+ single; bond) and in the structure o f InBr, here playing a stabilizing role for the unusual 7-fold coordin ation geometry. Judging from energetic considerations, the probability of In+-In+ partial bonds as centric defects inside an otherwise acent ric In2Br3 crystal structure is nonzero but small.