Stretching bonds in main group element compounds - Borderlines between biradicals and closed-shell species

Biradicals are molecules consisting of two unpaired electrons in two nearly degenerate non-bonding molecular orbitals (NBMOs). They play a central role in bond breaking and formation processes and are usually very short-lived species under standard laboratory conditions. By replacing the carbon-based skeletons of archetypal organic biradicals by main group elements, intriguing mimics of otherwise only transient species are isolable, which can then be characterized by generally applied methods for stable molecules. Nonetheless, the price to pay for gaining stability is the reduction of the biradical character, which makes their designation as biradicaloids more appropriate. Recent advances in the syntheses and characterization of main group element biradicaloids as well as their chemical reactivity and theoretical investigations served as topic of this review article. (c) 2006 Elsevier B.V. All rights reserved.