Three-centre two-electron interactions

We have already described several examples of bonding pictures that involve the delocalization of electrons. In cases such as BF3 and SFg, this leads to fractional bond orders. We now consider two linear XY2 species in which there is only one occupied MO with Y—X—Y bonding character. This leads to the formation of a three-centre two-electron (3c-2e) bonding interaction. 

In a 3c-2e bonding interaction, two electrons occupy a bonding MO which is delocalized over three atomic centres. 

The second example of a linear triatomic with a 3c-2e bonding interaction is Xep2 (T ooh)- The bonding is commonly 

The [HF2] ion (see Fig. 10.9) has symmetry and the z axis coincides with the axis. The bonding in [HF2] can be described in terms of the interactions of the H li orbital (cTg symmetry) with the LGOs of an F—F fragment. If we assume a relatively large s—p separation for fluorine, then sets of LGOs can be constructed as follows  

2p orbitals with CTg symmetry becomes a non-bonding MO in XeF2. There are 22 valence electrons in XeF2 and aU MOs except one (the (t MO) are occupied. The partial MO diagram in Fig. 5.30 shows only those MOs derived from P2 orbitals on Xe and F. There is only one MO that has 

How many nodal planes does each of the dg and dg MOs shown at the top of Fig. 5.29 possess Where do these lie in relation to the H and F nuclei From your answers, confirm that the dg MO contains delocalized F—H—F bonding character, and that the dg MO has H—F antibonding character. 

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An agostic M—H—C interaction is a three-centre two-electron interaction between a metal centre, M, and a C—H bond in a ligand attached to M (e.g. structure 23.37). 

Xe—F bonding character and therefore the bonding in Xep2 can be described in terms of a 3c-2e interaction. Three-centre two-electron interactions are not restricted to triatomic molecules, as we illustrate in the next section with a bonding analysis of B2Hg. 

It is the only example of a free, persistent phosphirenylium ion, and also, only one stable transition-metal complex of this species was published [78,79]. Quantum chemical calculations [80,81] indicated that in the halogeno-phosphirenes the P-X bonds already possesses a high ionic character and can be described as interactions between phosphirenylium and halide ions. The aromatic character of the phosphirenylium ion was shown to be based on a three-centre two-electron bond of 7i-type and the resonance energy was assessed by calculation to 38 kcal/mol. Before the generation of 32, substituted phosphirenylium ions were... 

A common interpretation of the interaction of chalcogens with nucleophiles considers donation of electron density from a lone pair on the donor atom into the o- (E-X) orbital (Figure 15.1). As the degree of covalency increases, a hypervalent three-centre four-electron bond is formed. Real systems fall somewhere between secondary interactions and hypervalent (three centre - four electron) bonds. The two extremes can be distinguished by the correlation of X-E and E D distances.In the hypervalent case both bond distances decrease simultaneously, whereas in the secondary bond the distances are anticorrelated. This concept has been applied in a study of selenoquinones 15.17 (R = Ph, Me) with short Se 0 contacts,for... 

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