Metallic bonding, three-dimensional nature

Metallic and nonmetalbc elements can react with each other to form com-ponnds by transferring electrons from the metal atoms to the nonmetal atoms. The ions formed attract each other becanse of their opposite charges, and these attractions are called ionic bonds. However, in a sobd ionic compound, a single pair of ions does not bond together instead, an almost inconceivably huge number of both types of ions forms a lattice that extends in three dimensions. The three-dimensional nature of the sodium chloride structure (Figure 5.9) is typical of ionic solids. 

Electronic effects of substituents can affect organic molecules in diverse ways. Usually, loci near the substituent are most influenced, but often electron density is altered several bonds removed from the substituent. Aromatic organometallic tr-complexes provide fascinating systems for the study of substituent effects, for the three-dimensional nature of these complexes adds interesting complexities to the analysis of the distribution of electron density in such molecules. This chapter is intended to summarize homoannular, interannular, and metal-ring substituent effects in metallocene and similar systems with the idea of spurring additional research in the area. 

Abstract The last few years have seen a considerable increase in our understanding of catalysis by naturally occurring RNA molecules, called ribozymes. The biological functions of RNA molecules depend upon their adoption of appropriate three-dimensional structures. The structure of RNA has a very important electrostatic component, which results from the presence of charged phosphodiester bonds. Metal ions are usually required to stabilize the folded structures and/or catalysis. Some ribozymes utilize metal ions as catalysts while others use the metal ions to maintain appropriate three-dimensional structures. In the latter case, the correct folding of the RNA structures can perturb the pKa values of the nucleo-tide(s) within a catalytic pocket such that they act as general acid/base catalysts. The various types of ribozyme exploit different cleavage mechanisms, which depend upon the architecture of the individual ribozyme. 

The simple oxide and hydroxide minerals exhibit important characteristics common to all minerals. The relative numbers of various component atoms are given by chemical stoichiometry. These atoms may be arranged in three-dimensional space in different ways. Geothite and lepidocrocite, for example, share the same stoichiometry (FeOOH), but Fe-O and Fe-OH bond lengths and geometric relationships between octahedrally coordinated Fe atoms are different. As a consequence, the coordinative environment and accessibility of Fe atoms to incoming adsorbate molecules are different for the two minerals. In other situations, minerals comprised of different metals, such as hematite (FeiOj) and corundum (AI2O3), have similar three-dimensional structures. In situations where surface structure is more important than the nature of the incorporated metal, hematite and corundum should exhibit similar behavior. 

For polymeric chains, the act of coordination almost invariably requires a change in the shape of the chain as a consequence of satisfying the demands of the metal ion for a preferred stereochemistry and a set of donors with bond distances within a limited range. Thus, coordinate bond formation has consequences that clearly alter the local environment around the metal ion, but may also alter polymer chain conformation over an extended range. Since three-dimensional shape in biopolymers plays a role in function, natural complexation evolved by Nature usually plays a positive role, whereas unnatural complexation through the addition of foreign metal ions may be deleterious to function. 

Designing derivatives of metal complexes that can form productive complexes with substrates and can manifest high effective molarities toward peptide bonds is not easy. Nature has adopted macromolecular polypeptides as the backbones of enzymes to tune the positions of catalytic elements in enzyme-substrate complexes, and thus, to achieve high effective molarities of the catalytic groups. The idea of macromolecular systems for the catalyst-substrate complexes may be applied not only to a macromolecular catalyst and a small substrate as in enzymatic systems, but also to a small catalyst and a macromolecular substrate. If a protein is used as the substrate, even a small catalyst may form a productive complex by utilizing the three dimensional (3D) structure of the protein substrate (22). 

---