Molecular structure tetrahedral arrangement

Figure 5.4 The molecular structure of basic beryllium acetate showing (a) the regular tetrahedral arrangement of 4 Be about the central oxygen and the octahedral arrangement of the 6 bridging acetate groups, and (b) the detailed dimensions of one of the six non-planar 6-membeted heterocycles. (The Be atoms are 24 pm above and below the plane of the acetate group.) The 2 oxygen atoms in each acetate group are equivalent. The central Be-O distances (166.6 pm) are very close to that in BeO itself (165 pm).

Figure 24 shows the molecular structure of the dianion [Fe4(CO)i3]2-. ld The iron atoms are disposed in a tetrahedral arrangement and all the carbonyl groups are terminally coordinated, except the one triply bridging the Fe3 base. 

An intermediate, often transient, appearing in a chemical or enzymatic reaction in which a carbon atom, which had been double-bonded (i.e., in a trigonal structure) in a particular molecular entity, has been transformed to a carbon center having a tetrahedral arrangement of substituents. Tetrahedral intermediates of proteases have been stabilized with cryoenzymological tech-niques ... 

At this stage in the literature, there is no method available by which one can directly determine the orientation of molecules of liquids at interfaces. Molecules are situated at interfaces (e.g., air-liquid, liquid-liquid, and solid-liquid) under asymmetric forces. Recent studies have been carried out to obtain information about molecular orientation from surface tension studies of fluids (Birdi, 1997). It has been concluded that interfacial water molecules, in the presence of charged amphiphiles, are in a tetrahedral arrangement similar to the structure of ice. Extensive studies of alkanes... 

The compound Ru4(jt-H)4(CO)12 is obtained as a yellow air-stable powder, which is soluble in most organic solvents, but insoluble in water. The IR spectrum contains v(CO) bands at 2081 (s), 2067 (vs), 2030 (m), 2024(s), and 2009(w)cm-1 (cyclohexane solution) the HNMR spectrum has a resonance at <5 — 17.98 (CDC13 solution). The molecular structure of Ru4(/i-H)4(CO)12 has been determined by X-ray diffraction the four hydrogen atoms bridge the edges of the tetrahedral Ru4 core in a D2d arrangement, while three CO ligands are terminally bonded to each ruthenium.8 The deuterated complex Ru4(/i-D)4(CO)12 can be prepared in the same way if D2 is used in place of H2.6... 

The molecular structure of S4N4 is unusual, having a tetrahedral arrangement of S atoms with the N atoms at the comers of a square. 

Now that we have obtained the electron-pair arrangement that gives the least repulsions, we can determine the positions of the atoms and thus the molecular structure of CH4. In methane each of the four electron pairs is shared between the carbon atom and a hydrogen atom. Thus the hydrogen atoms are placed as shown in Fig. 13.14, giving the molecule a tetrahedral structure with the carbon atom at the center. 

The molecular structure of methane. The tetrahedral arrangement of electron pairs produces a tetrahedral arrangement of hydrogen atoms. 

It is very important to recognize that the name of the molecular structure is always based on the positions of the atoms. The placement of the electron pairs determines the structure, but the name is based on the positions of the atoms. Thus it is incorrect to say that the NH3 molecule is tetrahedral. It has a tetrahedral arrangement of electron pairs but not a tetrahedral arrangement of atoms. The molecular structure of ammonia is a trigonal pyramid (one triangular side is different from the other three), rather than a tetrahedron. 

Carbon occurs in the allotropes (different forms) diamond, graphite, and the fullerenes. The fullerenes are molecular solids (see Section 16.6), but diamond and graphite are typically network solids. In diamond, the hardest naturally occurring substance, each carbon atom is surrounded by a tetrahedral arrangement of other carbon atoms, as shown in Fig. 16.26(a). This structure is stabilized by covalent bonds, which, in terms of the localized electron model, are formed by the overlap of sp3 hybridized atomic orbitals on each carbon atom. 

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