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Although compounds that feature 3-centre 2-electron (3c-2e) bonds are well known, there has been no previous effort to classify the interactions according to the number of electrons that each atom contributes to the bond, in a manner analogous to the classification of 2-centre 2-electron (2c-2e) bonds as either normal covalent or dative covalent. This article provides an extension to the Covalent Bond Classification (CBC) method by categorizing 3c-2e interactions according to whether (i) the two electrons are provided by one or by two atoms and (ii) the central bridging atom provides two, one, or zero electrons. Class I 3c-2e bonds are defined as those in which two atoms each contribute one electron to the 3-centre orbital, while Class II 3c-2e bonds are defined as systems in which the pair of electrons are provided by a single atom. Class I and Class II 3c-2e interactions can be denoted by structure-bonding representations that employ the "half-arrow" notation, which also provides a convenient means to determine the electron count at a metal centre. In contrast to other methods of electron counting, this approach provides a means to predict metal-metal bond orders that are in accord with theory. For example, compounds that feature symmetrically bridging carbonyl ligands do not necessarily have to be described as "ketone derivatives" because carbon monoxide can also serve as an electron pair donor to two metal centres. This bonding description also provides a simple means to rationalize the theoretical predictions of the absence of M-M bonds in molecules such as Fe(2)(CO)(9) and [CpFe(CO)(2)](2), which are widely misrepresented in textbooks as possessing M-M bonds.

The orbital structure of bent metallocenes and how their geometry depends on the number of d electrons are described. Bonding by a metallocene unit is exemplified by reference to the known hydrides. The reactivity of metallocene derivatives is illustrated with particular emphasis on the differences between ansa-bridged and unbridged compounds; the reactions include ring opening polymerisation of ferrocenophanes, elimination from and addition to Group 6 metallocene derivatives and Ziegler-Natta polymerisation of olefins by Group 4 metallocenes.

This paper describes a simple orbital picture for understanding the optical transitions and the second-order nonlinear optical response of metallocene-based chromophores of the form metallocene−(π-bridge)−acceptor, and experimental studies to test this model. From a combination of UV photoelectron spectroscopy, cyclic voltammetry, and density functional calculations, it is deduced that the three highest occupied orbitals are little perturbed from the parent metallocenes, that the HOMO-3 is a π-orbital delocalized between the metallocene cyclopentadienyl ring and the unsaturated bridge, and that the LUMO is acceptor based. The lowest energy transition in the UV/visible/near-IR spectra of these compounds is assigned to a metal-to-acceptor transition, while the higher energy transition is attributed to a transition to the acceptor-based LUMO from the delocalized HOMO-3 orbital. The variations in oscillator strength can be rationalized by considering the low-energy transition as borrowing intensity from the high-energy transition. Stark spectroscopy confirms that large dipole moment changes are associated with both transitions, as expected from our assignment. These dipole moment changes indicate that, according to the perturbation theory-derived expression for the first hyperpolarizability, β, both transitions contribute significantly to the observed optical nonlinearity.