Ab initio theoretical study of three-centre bonding on the basis of bond index

Various methods of derivation of a multi-centre bond index and its physical significance are briefly discussed. The usefulness of the bond index model for the study of three-centre (3c) bonding is illustrated by taking a numb er of examples chosen from earlier calculations. The origin of the sign of 3c bond indices (I-ABC) predicted for three-centre two-electron (3c-2e) and three-centre four-electron (3c-4e) bonds is investigated using simple mode ls with different bonding topologies. Some examples are given highlighting the role of I-ABC in elucidating the nature of bonding in controversial mol ecules. The bond index and the effective pair population models are compare d and shown to be equivalent for an RHF wave function and even-centre bond indices. It is concluded that despite some inherent limitations such as the basis set sensitivity the bond index model provides a quite satisfactory d escription of 3c bonding. (C) 2000 Elsevier Science B.V. All rights reserve d.