Some metals form compounds in which two or more atoms of the same metallic element are bonded together. The most common examples are complicated structures, much like
traditional metal complexes. However, these structures contain two or more central metal atoms and surrounding ligands. The ligands are often carbonyl or nitrosyl groups. Carbonyl groups are compounds of carbon and oxygen. Nitrosyl groups are compounds of nitrogen and oxygen. With more than two metal atoms in the complex, cluster compounds are formed. Their structures are quite complicated.
There are other types of complex inorganic compounds that resemble the polymers found in organic chemistry. Polymers are chainlike molecules formed by linking many smaller molecules. For example, some types resemble icelike crystals. Others have fibrous, needlelike crystals. Still others may consist of long chains and rings of atoms.
have complicated molecular structures. These include the relatively simple iron complex Fe2(CO)„ (far left) and the compound of rhodium Rh6(CO),6Heftl. The rhodium compound has 16 carbon monoxide ligands, 12 bonded directly to the six rhodium atoms (disposed at the corners of an octahedron) and four others triply-bonded to three rhodium atoms.
Human blood—supplies of which are vital for transfusions in modern medicine-contains hemoglobin. This is a complex compound whose molecules consist of proteins coordinated to a central iron atom. Similar complexes are essential to many other biological systems.
Plasticsare extremely versatile synthetic organic chemicals. Since their development in the early part of the century—originating from the discovery of Bake-lite in 1909—they have transformed modern life to an almost inconceivable extent. It is probably true to say that there is not a single moment in a person's life when he or she is not using plastics or relying on them indirectly.
There are more substances in which carbon is the major element than those of all the other elements together. The basic reason is the unique ability of carbon to form chemical bonds with both itself and a very large number of other elements. Most important is the ability of carbon atoms to link into long chains or complex cyclic structures. Cyclic structures are able to repeat a pattern of carbon combinations many times. This ability is of profound importance to us. Without it, life on earth would probably never have arisen. All the fundamental groups of substances found in living organisms, such as proteins and carbohydrates, are based on carbon-atom chemistry.
At one time, it was believed that compounds found in living organisms could not be made in the laboratory. Scientists thought that they could be obtained only from matter that was living or had been alive. This idea was shown to be wrong more than 150 years ago. Since that time, the branch of chemistry concerned with carbon compounds has also included the proper study of inorganic carbon compounds. Nevertheless, this branch is still called organic chemistry.
Organic chemistry now embraces not only
substances produced by living organisms but also an immense range of synthetic chemicals. These include many of industrial importance. Nearly all plastics and synthetic fibers in everyday use are organic chemicals. So also are such diverse materials as dry-cleaning fluids, artificial sweeteners, pesticides, and many pharmaceuticals.
Many of today's industrially important organic chemicals are produced from fossil fuels—mainly oil and natural gas. These are the remains of once-living matter. In that respect, much of industrial organic chemistry retains its truly organic origins. Such is the sophistication of modern chemistry that if supplies of these raw materials become exhausted, then the same products can still be produced from other carbon sources, such as carbon monoxide gas.
Carbon's combining power
Inits outer shell, a carbon atom has four electrons. The shell is exactly half-filled. A single carbon atom can thus form four two-electron bonds. This means that a carbon atom is able to attach itself to up to four other atoms. By sharing one electron from each of four other atoms, the carbon atom "fills up" the four empty spots in its outer shell. The carbon atom can also share four electrons from only one other atom or any number of atoms up to four.
Not only is a carbon atom able to form bonds with other carbon atoms, it can also bond readily with hydrogen atoms and with atoms of the elements to its right in the periodic table: nitrogen and oxygen. A very large number of organic substances are made up of the elements carbon, hydrogen, oxygen, and/or nitrogen. Other elements often found in organic compounds include sulfur, phosphorus, and the halogens. The halogens include fluorine, chlorine, bromine, and iodine. Chlorine is particularly active with carbon.
In the laboratory in the past few decades, chemists have managed to combine carbon atoms with atoms of most other elements. A whole new branch of chemistry—called or-ganometallic chemistry—has opened up. This branch studies the behavior of substances involving carbon-metal bonds. Many of these have found important uses in industry as catalysts, substances that cause chemical changes without themselves being changed. Studying carbon-metal bonds has also proved valuable in extending scientific understanding of the nature of chemical bonding.
Another significant aspect of carbon-based chemistry is the ability of carbon atoms to link together by more than a single bond. Two carbon atoms can be joined by two or even three pairs of electrons, forming double or triple carbon-carbon bonds. Carbon atoms joined by multiple bonds are very reactive. This means that they readily enter into chemical activity with other atoms. Conversely, the greater the