Four-center bonds

NN and CO multiple bonds are broken and 2 C-N and one C-C bond are formed in an unusual four center bond-breaking and bond-making sequence. Once again, Sm(II) is providing access to unusual chemistry. 

Only two-center bonds TWo-, three- and four-center bonds ... 

The situation is very different for the four-center bonds. They are much less common, with an average of, in general, below 1%. The amino acids are again an exceptional class of compounds, with about 15% of four-center bonds. 

Table 8.10. Geometry of the four-center bonds in the crystal structures of the amino acids (neutron diffraction)...

The proportion of three-center bonds in the pyrimidine, purine, and barbiturate crystal structures of about 30% is comparable to the number observed in the carbohydrate crystal structures (Thble 2.3). There is also a small proportion, about 1 %, of four-center bonds. A particular feature of the three-center bonds in these crystal structures is the occurrence of chelation, where both acceptor atoms are covalently bonded to the same atom(s), as in Fig. 15.7. 

Multicenter H-bondsn are known in a number of compounds, the most common being 3-center bonds of the type (2-IA) four-center bonds are rare. H-bonds of type (2-IB) are also known. 

Although the Mulliken population analysis is useful for the discussion of two-center bonding, it is not sufficient for the description of the three- or four-center bonds that may exist in metal clusters. In order to elucidate the multicenter bonds, we examine contour maps of the charge densities of Na4 and Mg4 as typical tetramers of AM and AE, respectively. We use the differential charge density, Ap defined by eq. (3)., to express the charge redistribution on the formation of chemical bonds. 

Rather less symmetrical tetrameric (LiEt)4 molecules have been found (by X-ray diffraction ) in crystalline ethyUithium, again held together by hypercoordinate carbon atoms forming four-center bonds to three neighboring metal atoms located 2.19-2.47 A distant. The Li—Li distances range from 2.42 to 2.63 A and the Li-C-Li angles range from 66° to 67°. 

Furthermore, the search for the global minimum of the metastable Al4 (it is not stable with respect to an electron detachment) cluster revealed that the planar square structure was indeed the lowest in energy. The AdNDP analysis shows that four canonical MOs of Al4 can be transformed to four lone pairs with one located on every aluminum atom. Three other canonical MOs stay as four-centered bonds. The HOMO is clearly a completely bonding jt-MO. Two electrons on that MO make this cluster jt-aromatic. The HOMO-1 is a completely bonding MO formed by p -radial AOs. Two electrons on that MO make this cluster -aromatic. The HOMO-2 is a completely bonding MO formed by p,-tangential AOs. Two electrons on that MO make this cluster Oj-aromatic. Thus, this is an example of a system with double (a,.-, and Jt-) aromaticity. 

As in B4CI4, we may assume each Li atom to direct one sp hybrid AO toward a point above the centre of each triangular face of the Li4 tetrahedron. One such hybrid from each corner meet at a point above the midpoint and combine with an sp hybrid AO of the C atom to form a four-center bonding orbital which contains two electrons. See Fig. 12.10. Since C is significantly more electronegative than Li, there is no doubt that in this 4c LCAO molecular orbital the coefficient of the sp orbital of the carbon atom will be significantly larger than those of the Li orbitals. In the ionic limit the coefficient will aproach unity while the coefficients of the three Li AOs will approach zero. 

The four-center bonding exists in methyllithium (Figure 1.2) and methylsodium. 

Si surface takes place and where our investigations of the Si dynamics start. Molecular orbitals mirroring the change in electron density are presented for the equilibrium geometries and the two Si/So Coins. The molecular orbital, which mainly constitutes the c-bond in CHD, contributes to the TT-system in hexatriene and changes in between to form a three-center bond at Colnmin involving three C-atoms, respectively a four-center bond at C 2-Coln formed by four C-atoms (Fig. 2). 

Important structural subunits of complexes 20 and 21 are shown in Figs. 30 and 31. In either case, an N2 molecule side-on bridges two diphenylnickel units, allowing a weak Ni— Ni bonding. Furthermore, N2 interacts in the end-on fashion with lithium atoms. In complex 20, the lone pair of one N atom interacts with just one Li ion, while that of the other N atom is involved in a two-electron four-center bond with a Li3 ring. In the case of 21, similar two-electron three-center bonds to Li at both ends of N2 are observed. 

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