Polynuclear complexes

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.

Cluster compoundscan

have complicated molecular structures. These include the relatively simple iron complex Fe₂(CO)„ (far left) and the compound of rho­dium Rh₆(CO),₆ Heftl. 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.

Organic chemistry

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

Date: 2015-12-11; view: 1378