Three-centre -bonding interactions

About 300 (of the 1500 N—H- 0=C bonds considered) are three-centre bonds77. Three-centre bond (as shown in 15) is considered a better definition than bifurcated bond . The three-centre bond is a situation where the proton interacts with two hydrogen bond acceptor atoms both bonds are shorter than the sum of the van der Waals radii of the atoms involved. The two hydrogen bond acceptors may be different (Y Z in 15). 

Five-electron three-centre bonding units involve the distribution of five electrons amongst three overlapping AOs that are located on three atomic centres. The 7t-electrons of triatomic systems with 19 valence-shell electrons provide examples of these types of bonding units when only valence-shell pjt AOs are used to accommodate the electrons. Another example is provided by the indirect interactions of two nitrogen atoms via a hydrogen atom in medium-ring bicyclic compounds [23]. We consider here symmetrical systems, for which there are three canonical Lewis structures, XXVII-XXIX,... 

Nitroaniline molecules assemble through a bifurcated hydrogen bond (or a three-centre bond) as reported in 43, which is a symmetric bond, while 44 represents an asymmetric bond. 45 represents a monocoordinate hydrogen bond. These interactions induce the formation of an infinite polar chain. 

The bonding in Me Al has conventionally been discussed in terms of three-centre molecular orbitals. If the valence orbitals of A1 are sp hybridized, two of the hybrids can form the terminal A1—CHj bonds. These are classical electron pair bonds and account for two of the three valence electrons of each Al. The remaining sp hybrids are directed in the Al C plane and are well placed to interact with the Csp orbitals of the bridging methyl groups. Each of these three centre bonding m.o.s contains two electrons, one from Al and one from a bridging methyl group (Fig. 3.2(a)). 

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... 

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... 

As shown in Section II.E, such through-bond interactions can be explained on the basis of a simple three-centre model involving the two interacting jr-MOs and the intervening cr-MO. Of course, such models can also be applied to the changes in Ecr upon ionization, but all that can be learned from them in this context is that B = (jra H jrb increases. If... 

This corresponds to an isotope effect of approximately 3.5% per deuterium. In comparison, the secondary /3-deuterium KIEs in SN1 reactions are all normal and range from 5% to 15% per deuterium. Because the normal KIEs in SN1 reactions result from the weakening of the C,—L bond by a hyperconjugative interaction with the incipient carbocation in the transition state, the authors concluded that hyperconjugative interactions are present also in the transition state for the insertion reaction. The normal secondary /3-deuterium KIE observed for the insertion reaction is consistent with the dipolar three-centre transition state structure [15] proposed by Seyferth et al. (1970a,b) because the partial positive charge on the a-carbon is stabilized by hyperconjugation. 

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