Three-coordinate atom

In the case of trisulfane the protonation may occur either at a terminal or at the central sulfur atom. Both ions were found to be of Ci symmetry. On the basis of their Gibbs energies the ion [H2S-S-SH]+ is by 9 kJ mol more stable than the tautomer [HS-S(H)-SH]+ at 298 K. While the SS bonds in the latter ion are of almost identical length (205.4 and 205.7 pm), they are rather different in the former ion (214.9 and 200.9 pm) with the longer one originating from the three-coordinate atom [70]. These two ions have not been observed yet. 

Figures 16.8.2(h), (i), and (j) show the structures of Se2J, Se2+, and Se, respectively. They all consist of seven- or eight-membered rings connected by short chains. Each homopolyatomic cation has two three-coordinate atoms that formally carry the positive charges. Se2g has a bicyclo[2.2.4]decane geometry. The Se-Se bond distances vary between 225 and 240 pm, and the Se-Se-Se angles range from 97° to 106°. Se2+ and Se2+ comprise a pair of seven-membered rings connected by a three- and four-atom chain, respectively.

The six-membered ring has a chairlike conformation and the Se—Se bond lengths vary between 229.2 and 237.0 pm. The longest bonds are those originating from the three-coordinated atoms, as has been observed for a number of related sulfur and selenium compounds, e.g., Se102+, S7I+, or S80. 

Ill) microfacets ( 3.9). Small ensembles of three-coordinated atoms with a p3Tamidal structure resembling a reconstructed (111) polar surface can in principle form and have been indeed been proposed as the centers where oxygen exchange reactions occur. 

Addition and elimination are simply different aspects of the same transformation followed either in the forward or backward directions. In the process the coordination number of the reaction center is increased or decreased by one. The transition state of both reactions is characterized by a partial bond between the reactants. In Chapter 6 several examples of addition/elimination reactions have been discussed, primarily those involving carbonyl carbon and other three-coordinate atoms as reaction centers. The inorganic analogs given there illustrate the relationship between the organic transformations and the more general concept of addition and elimination of Lewis bases to and from Lewis acids. 

Only one type of Si(sp ) three-coordinated atom was obtained for the Si( 111) plane with the same procedure so that we did not considered it. A Si(lll) 7x7 supastructure was obsoved experimentally in [40] but cannot be modeled with the smaller SC considered herein. 

Still stronger is the differentiation of distances in the structure of cluster CS11O3 (Fig. 4.7). There are three classes of cesium atoms which are coordinated to one, two, and three oxygen atoms respectively. The intermetallic distances in this cluster are between 3.71 A for three-coordinated atoms and 4.27 A for the peripheral one-coordinated ones. 

Polydentate Diethylenetriamine (dien) H2N-(CH2)2-NH-(CH2)2-NH2 t Three coordinating atoms... 

The (111) surface of TMCs is a polar surface, and the surface atom loses three coordinated atoms in the vacuum side. As mentioned in Sec. I, all the TMC(lll) surfaces [except for VC(lll)] are covered with metal layers. The polar surface is usually unstable because of its large electrostatic potential at the surface, which usually induces surface reconstructions or facetings. However, no reconstruction of TMC( 111) has been found except for the VC( 111) surface (8,9), suggesting... 

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