Three-center bonding interactions

FIGURE 41 Structure of Os3(CO)8(PPh2)Ph(PPhC6H4). Note the /(i3 bonding of the (PPhCsHa) ligand and the two-electron, three-center bond interaction with the CsHs ligand. 

The NBO picture of 3c/2e T-bonding may be developed along lines parallel to those for 3c/4e tu-bonding in Eqs. (3.194) et seq. Let us first consider the special case of a homopolar two-center bond aAc = 2-1/2(hA + hB) interacting with a vacant one-center orbital nB = hB to form a three-center bond tABc of the form... 

Figure 3.102 The residual ate NBO in H3BC2H4, showing the strong banana-bond shape of the NBO and the effective preparation for forming a second three-center bond (or agostic interaction) with an empty orbital approaching from the opposite face of the original ethylene moiety.

Although the generalization to localized three-center bonds inevitably involves certain complications, important simplifications of the localized Lewis-structure picture remain. In the case of 3c/4e co-bonding, the corrections can often be adequately described in terms of two-term resonance between alternative two-center bonding patterns. In the case of 3c/2e T-bonding, the resonance-theoretic description (although possible in principle) becomes unwieldy. However, in terms of the single best three-center localized Lewis-structure description (or the resonance mixture of symmetry-equivalent structures), one can still identify specific localized donor-acceptor interactions (e.g., of type) that dominate the delocaliza-... 

Agostic interactions, i. e., the three-center bonds related to structure 51 [26, 44-49], were noted earlier by Green and Brookhart and have been cited above in the methoxycarbonylation chemistry (Figure 1.9). These bonds are often characterized by low frequency (hydride-like) proton chemical shifts, and/or substantially reduced /( C, H) values. Often, it is necessary to cool the NMR sample in order to freeze the equilibrium. Complex 52 represents a nice example of an agostic C-H bond, with relevance to polymerization chemistry [47]. 

As can be seen from the structure, the dimerization proceeds via nitrogen-magnesium interactions. To our knowledge, dimerization via Mg—R—Mg two-electron three-center bonds in alkylmagnesium amides has neither previously been found in the solid state nor proved by X-ray techniques, although the existence of such bonds has been claimed (10). 

FIGURE 19.3 Three -center bonding molecular orbitals in diborane. Each of the two MOs (one shown darker than the other) is formed by the interaction of an sp3 hybrid orbital on each B atom and the Is orbital on one bridging H atom. Each MO contains one pair of electrons and accounts for the bonding in one B-H-B bridge. 

However, solvation and entropy (which could be very different between these two types of amines) must be taken into account when comparing the two methods (titration and infrared spectroscopy). The ionization constants of a series of silylmethylpiperidines and their carbon analogs were measured potentiometric ally in methanol. The data indicates a lower basicity for these compounds, compared to non-silylated analogs, which was explained in terms of inductive effects, three-center-bond formation and (p-d)cr interactions.26 From a study of the effect of solvation by acetone on the basicity of... 

In (VI), an end-on or side-on approach of the metal atom to the C—H bond is depicted. This weak interaction between the metal atom (presumably unsaturated) and the C—H bond could conceivably lead to a three-center-bonded intermediate (VII), which would in turn transform into the final insertion product (VIII). Al-... 

Whatever kind of representation one chooses, one has to keep in mind that both A and B symbolize the same fact, but in a slightly different way. The electrons in a three-center bond are located in molecular orbitals which are composed from contributions of all the involved atoms. Although prorating of electrons to individual two-center interactions is not... 

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