Skeletal shapes

However, the need for a better description of the formal molecular body, the need to account for molecular volume effects, the necessity to describe finer details of changes in electron distributions during conformational changes and chemical reactions, and the requirement of a more precise evaluation of molecular similarity are the factors which have motivated chemists to move beyond the stereochemical skeletal shape concept. 

Because of their completely deltahedral pseudospherical shapes, the dianions, the singly charged monocarbaborane anions [CB iH ], and the neutral carboranes of formulae C2B 2H are known as closo (closed cage) clusters. Their skeletal shapes are shown in the left-hand column of Figure 3.1. 

Figure 3.1. Deltahedral and deltahedral fragment skeletal shapes of typical closo, nido, and arachno boranes and carboranes.

Entries in the second column of Figure 3.1 and in Figures 3.3 and 3.4 show the deltahedral fragment skeletal shapes most commonly encountered among boranes and carboranes of formulae B H +4, CB iH +3, C2B 2H +2, and so on, all of which can be regarded as derived from hypothetical anions [B H ]4 by protonation and/or isoelectronic replacement of B by C. Their molecular formulae are sometimes written as CxB H +4, where x + y = n, the total number of skeletal atoms. The skeletal structures of these nido species. 

Figure 3.25. Some of the skeletal shapes found for large carboranes and metallacarboranes, showing connectivities of four or six in closo systems.

The solution to any problem with a stereochemical aspect requires access to molecular models. Of these, there are two main kinds. The first is the skeletal or framework model such as those devised by Dreiding or Kendrew. These indicate the centres of bonds that join atoms, and are useful to find conformations suitable for interaction between two molecules. The other type of model, space-filling (e.g. CPK, or Courtauld), shows both the shape of the molecule and the volume that it occupies. This kind is very useful for showing the overall shape, surface and volume of a molecule. With practice, a chemist can learn to see a conformational drawing as a three-dimensional skeletal shape, and eventually as a space-filling molecule. There are also the CCS models, which are fundamentally skeletal models that can be quickly converted to space-filling types and back again (Clarke, 1977). 

Bone Modelling The generation and shaping of new bone during the period of skeletal growth. 

C09-0114. In the lower atmosphere, NO2 participates in a series of reactions in air that is also contaminated with unbumed hydrocarbons. One product of these reactions is peroxyacetyl nitrate (PAN). The skeletal arrangement of the atoms in PAN appears at the right, (a) Complete the Lewis structure of this compound, (b) Determine the shape around each atom marked with an asterisk, (c) Give the approximate values of the bond angles indicated with arrows. 

This geometry is undoubtedly imposed by the skeletal stabilizing unit. Other eight membered phosphorus(III)-nitrogen rings, (MePNMe)i, (9) and (PrNO G NP) (11) are crown shaped containing all chemically equivalent phosphorus atoms. In XI, because the phosphorus atoms are of two sets and one set is in a highly protected environment, the possibility of selective, "cavitand," coordination at phosphorus sites exists. 

The formation of hard skeletal structures that give some life forms their shape is a consequence of calcium. Simply said, the shells of lower organisms are generally made up of brittle calcium carbonate and the interior skeletons of higher animals are made up of tough calcium phosphate. 

Smooth muscle cells are small and spindle shaped (thin and elongated see Table 12.1). Similar to skeletal muscle, the contractile apparatus in smooth... 

By combining the skeletal hybrid composition (4.44) with the bond angles of Table 4.3, we can recognize the idealized molecular shape(s) corresponding to each ML coordination. 

However, the important new feature of metal alkylidenes (4.51) is metal-carbon pi-bonding. As discussed in Section 2.8, pi bonds between transition metals and main-group elements are of d -p type, much stronger than corresponding p —pn bonds between heavier main-group elements. Compared with simple metal hydrides and alkyls, metal-carbon pi-bonding in metal alkylidenes affects the selection of metal d orbitals available for hybridization and skeletal bond formation, somewhat altering molecular shapes. 

It can be seen that application of the 18-electron rule to clusters necessitates this arbitrary assignment of the number of orbitals of a particular predominant character. The number of orbitals per metal used for cluster skeletal bonding is a consequence of N and E. It varies from two for the M(CO)4 unit in Os3(CO)12 to three for an M(CO)3 unit in Ir4(CO)12. Since these two metal moieties differ, this variation seems reasonable. However, the distribution for an M(CO)3 unit varies with cluster shape as in Ir4(CO)12 and [Os6(CO)18]". ... 

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