INCREASED-VALENCE STRUCTURES AND SOME VALENCE-BOND REPRESENTATIONS FOR REACTION-MECHANISMS

The paper presents a summary and extension of some of the recently pub lished work on increased-valence theory. For four-electron three-centr e bonding units, increased-valence structures 2 and 4 may be derived f rom the Lewis structures 1 and 3 by means of the one-electron delocali zations that are indicated in the latter structures. [GRAPHICS] The th ree bonding electrons in each of the structures 2 and 4 occupy non-ort hogonal two-centre bonding molecular orbitals, and the A atom contribu tes only one atomic orbital to the bonding scheme. it is demonstrated that the A-atom valence is able to exceed unity in each of these struc tures when the valence is calculated according to a Wiberg-type proced ure. Increased-valence structures are used to illustrate how electroni c reorganization may proceed for radical transfer, thermal decompositi on and cycloaddition reactions that involve the 1,3-dipolar molecule N 2O, and gas-phase S(N)2 reactions. A relationship is established betwe en increased-valence and approximate molecular orbital descriptions of hydrogen bonding. (C) 1997 Elsevier Science B.V. (C) 1997 Elsevier Sc ience B.V.