The occurrence of bond-stretch isomers is now experimentally established via X-ray crystal structure determination. These are molecules, also called distortion isomers, whose only structural difference is a dramatic difference in the length of one (usually M—O) or several bonds. In this paper we provide two electronic mechanisms by which this may occur. One involves a real electronic crossing of filled and empty orbitals (a first-order Jahn—Teller effect) and the other a second-order Jahn—Teller distortion of the type important in other bond localization problems (allyl anion, benzene, etc.). The electronic conditions for optimal observation of each process are described for d 1 and d 2 transition-metal complexes. The ideas are extended to d 9 Cu 11 systems. © 1988, American Chemical Society. All rights reserved.