Structure and Bonding in Metals

The closest packing model for metallic crystals assumes that metal atoms are uniform, hard spheres. 

Metals are characterized by high thermal and electrical conductivity, malleability, and ductility. As we will see, these properties can be traced to the nondirectional covalent bonding found in metallic crystals. 

A metallic crystal can be pictured as containing spherical atoms packed together and bonded to each other equally in all directions. We can model such a structure by packing uniform, hard spheres in a manner that most efficiently uses the available space. Such an arrangement is called closest packing (see Fig. 16.13). The spheres are packed in layers in which each sphere is surrounded by six others. In the second layer the spheres do not lie direotlv over those in the first layer. Instead, each one occupies an indentation (or dimple) formed by three spheres in the first layer. In the third layer the spheres can occupy the dimples of the second layer in two possible ways. They can occupy positions so that each sphere in the third layer lies directly over a sphere in the first layer (the aba arrangement), or they can occupy positions 

The closest packing arrangement of uniform spheres. In each layer a given sphere is surrounded by six others, (a) aba packing The second layer is like the first, but it is displaced so that each sphere in the second layer occupies a dimple in the first layer. The spheres in the third layer occupy dimples in the second layer so that the spheres in the third layer lie directly over those in the first layer (aba), (b) abc packing The spheres in the third layer occupy dimples in the second layer so that no spheres in the third layer lie above any in the first layer (abc). The fourth layer is like the first. 

The aba arrangement has the hexagonal unit cell shown in Fig. 16.14, and the resulting structure is called the hexagonal closest packed (hep) structure. The abc arrangement has a face-centered cubic unit cell, as shown in Fig. 16.15 on page 779, and the resulting structure is called the cubic closest packed (ccp) structure. Note that in the hep structure the spheres in every other layer occupy the same vertical position (ababab. . . ), while in the ccp structure the spheres in every fourth layer occupy the same vertical position (abcabca. . . ). A characteristic of both structures is that each sphere has 12 equivalent nearest 

SO that no sphere in the third layer lies over one in the first layer (the abc 

Components That Occupy the Lattice Points Metal atoms Nonmetal atoms Group 8A atoms Discrete molecules Ions 

Bonding Delocalized covalent Directional covalent (leading to giant molecules) London dispersion forces DIpole-dIpole and/or London dispersion forces Ionic 

The internal forces in a solid determine the properties of the solid. 

PowerLecture Comparison of a Molecular Compound and an Ionic Compound 

Copper is an excellent conductor of electricity, whereas argon and diamond are both insulators. Copper can be easily changed in shape it is both malleable (can be formed into thin sheets) and ductile (can be pulled into a wire). Diamond, on the other hand, is the hardest natural substance known. We will explore the structure and bonding of atomic solids in the next two sections. 

---

Ernst RD (1984) Structure and Bonding in Metal-Pentadienyl and Related Compounds. 57 1-53... 

The subjects of structure and bonding in metal isocyanide complexes have been discussed before 90, 156) and will not be treated extensively here. A brief discussion of this subject is presented in Section II of course, special emphasis is given to the more recent information which has appeared. Several areas of current study in the field of transition metal-isocyanide complexes have become particularly important and are discussed in this review in Section III. These include the additions of protonic compounds to coordinated isocyanides, probably the subject most actively being studied at this time insertion reactions into metal-carbon bonded species nucleophilic reactions with metal isocyanide complexes and the metal-catalyzed a-addition reactions. Concurrent with these new developments, there has been a general expansion of descriptive chemistry of isocyanide-metal complexes, and further study of the physical properties of selected species. These developments are summarized in Section IV. 

The most familiar metals are elemental substances such as iron, tin, aluminium etc. However, many compounds are metallic. As well as intermetallic compounds such as AgCd and NaTl, and a huge number of non-stoichiometric alloys, many oxides, sulphides, halides etc. have metallic properties. For details of structure and bonding in metallic substances, see Section 7.5. 

Structure and Bonding in Metal-Pentadienyl and Related Compounds... 

The general patterns of structures and bonding in metal poly-olefin complexes are similar to those in olefin complexes. Linear, branched and cyclic polyolefin complexes are known. The bonding is delocalized over the carbon atoms and the metal(s). These species also play important roles in organic syntheses. 

---