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Triple bonding is conventionally considered to be the limit for multiply bonded main group elements, despite higher metal-metal bond orders being frequently observed for transition metals and lanthanides/actinides. Here, using high-level theoretical methods, we show that C2 and its isoelectronic molecules CN+, BN and CB? (each having eight valence electrons) are bound by a quadruple bond. The bonding comprises not only one ?- and two ?-bonds, but also one weak 'inverted' bond, which can be characterized by the interaction of electrons in two outwardly pointing sp hybrid orbitals. A simple way of assessing the energy of the fourth bond is proposed and is found to be ~12-17 kcal mol?1 for the isoelectronic species studied, and thus stronger than a hydrogen bond. In contrast, the analogues of C2 that contain higher-row elements, such as Si2 and Ge2, exhibit only double bonding. [ABSTRACT FROM AUTHOR]

Reactivity principles based on orbital overlap and bonding/antibonding interactions are well established to describe the reactivity of organic species, and atomic structures are typically predicted by Hund's rules to have maximum single-electron occupancy of degenerate orbitals in the ground state. Here, we extend the role of exchange to transition states and discuss how, for reactions and kinetics of bioinorganic species, the analogue of Hund's rules is exchange-controlled reactivity. Pathways that increase the number of unpaired and spin-identical electrons on a metal centre will be favoured by exchange stabilization. Such exchange-enhanced reactivity endows transition states with a stereochemistry different from that observed in cases that are not exchange-enhanced, and is in good agreement with the reactivity observed for iron-based enzymes and synthetic analogues. We discuss the interplay between orbital- and exchange-controlled principles, and how this depends on the identity of the transition metal, its oxidation number and its coordination sphere. [ABSTRACT FROM AUTHOR]

The article reports that strong C-H bonds are routinely cleaved by iron active sites of enzymes as the enhancement of the reactivity of a biomimetic diiron complex is attained through conversion of the oxo bridge into a terminal ligand and the iron centre from low spin to high spin. The important reactivity features of highly reactive species was ranked and the factors that enhanced reacitivity towards C-H cleavage and oxo-transfer reaction to triphenyl phosphine was determined through variations between complexes. Ground states with four unpaired electrons that result in a quintet spin on each iron centre are possessed by natural non-haem enzymes, whether based on mono- or diiron centres.

A correction to the article "Exchange-enhanced reactivity in bond activation by metal-oxo enzymes and synthetic reagents" that was published in a 2011 issue is presented.