Application of Transition Metals

Application of Transition Metals The term transition metal (sometimes also called a transition element) has two possible meanings: In the past it referred to any element in the d-block of the periodic table, which includes groups 3 to 12 on the periodic table. All elements in the d-block are metals (In actuality, the f-block is also included in the form of the lanthanide and actinide series). It also states that a transition metal is an element whose atom has an incomplete d sub-shell, or which can give rise to cations with an incomplete d sub-shell. Group 12 elements are not transition metals in this definition. Introduction to application of transition metals: The use of transition metals in the synthesis was taken up slowly by organic chemists. This is at first surprising because the industrial use of transition metals has a much long history hydroformylation using cobalt began in the 1930s. The Mond process using nickel tetra carbonyl was developed in the 19 century. Industry was willing to accept and uses processes that it could not understand black box reactions as long as they were profitable. Academics were handicapped by the desire to understand the chemistry. This was impossible until the ideas about chemical bonding and the necessary instrumentation matured in the years in the Second World War. Even with in this place, the impact of transition metals on the organic synthesis came late possibly because of the many fantastic main group reagents appeared. Application of Transition Metals: The application of transition metals is as follows: 1. Transition metals are applied in the organic reactions. Transition metals complex under goes a series of reactions that are generally unlike those main group compounds. The most fundamental is the simple coordination and dissociation of ligands. Dissociation may also be achieved by destruction of a ligand. This is often done by the oxidation of co and co2 using an amide oxide. 2. Transition metals are applied in the synthesis of metal hydride. M=C=O + OH- ====> M-H + CO2 Here metal carbonyl group reacts with hydroxide to give metal hydride and carbon dioxide. Hydrides such as, sodium borohydride, lithium aluminium hydride, diisobutylaluminium hydride (DIBAL) and super hydride, are commonly used as reducing agents in chemical synthesis. The hydride adds to an electrophilic center, typically unsaturated carbon. Hydrides such as sodium hydride and potassium hydride are used as strong bases in organic synthesis. The hydride reacts with the weak Bronsted acid releasing H2. Hydrides such as calcium hydride are used as desiccants, i.e. drying agents, to remove trace water from organic solvents. The hydride reacts with water forming hydrogen and hydroxide salt. The dry solvent can then be distilled or vac transferred from the solvent pot. Hydrides are of important in storage battery technologies such as Nickel-metal hydride battery. Various metal hydrides have been examined for use as a means of hydrogen storage for fuel cell-powered electric cars and other purposed aspects of a hydrogen economy. Hydride intermediates are key to understanding a variety of homogeneous and heterogeneous catalytic cycles as well as enzymatic activity. Hydroformylation catalysts and hydrogenase both involve hydride intermediates. The energy carrier NADH reacts as a hydride donor or hydride equivalent. 3. Transition metal used in the complexes in fluorescence cell imaging. Transition metal complexes have often been proposed as useful fluorophores for cell imaging due to their attractive photo physical attributes, but until very recently their actual applications have been scarce and largely limited to ruthenium complexes in DNA and oxygen sensing. 4. Transition metal used as Catalysts. Some transition metals are good catalysts. For example: most automobiles have an emissions-control device called a catalytic converter. This device contains a screen of platinum or palladium along with rhodium, a metal. The presence of the transition metals, along with the heat of combustion generated by an automobile engine causes an exhaust coming from an internal combustion engine to be broken down from partially burned hydrocarbon compounds into less harmful compounds such as water vapour and carbon dioxide. Catalytic applications of transition metals in organic synthesis:- OXIDATION REACTIONS: The epoxidation, dihydroxylation and aminohydroxylation reactions of alkenes, especially their asymmetric variants, continue to attract considerable attention. The basic principles were covered in the previous review. The use of fluorous solvents has now been demonstrated formany transition metal catalysed reactions. One advantage that they offer for catalyticepoxidation is the fact that molecularoxygen has a high solubility in fluorous solvents. The combination of O2 with pivalaldehyde and manganese catalysts hasbeen shown to be effective for epoxidation of alkenes in aracemic and enantioselective sense. The fluorous soluble ligand afforded a manganese complex which was insoluble incommon organic solvents, but soluble in the fluorous phase.Indene was converted into indene oxide with high enantioselectivity,although other substrates afforded low selectivity The fluorous phase, containing the active catalyst,could be recycled. Manganese salen complexes have also now been successfullyimmobilised within polymer supports, and still provide high Whilst the enantiomerically pure manganese salen complexes are still often the most enantio selective available for epoxidation of unfunctionalised alkenes, alternative systems are often reported. For example, End and Pfaltz have used rutheniumbis (oxazoline) complexes to provide up to 69% ee in the epoxidation of stilbene. The use of methyltrioxorhenium as a catalyst for epoxidationcontinues to attract attention. Herrmann and co-workershave shown that a combination of methyltrioxorhenium withpyrazole affords a highly efficient catalyst for the epoxidation of alkenes. Styrene was converted cleanly into styrene oxide with this catalytic combination. Reduction reactions The reduction of various functional groups can often be achieved using transition metal catalysts and a suitable reducing agent: often molecular hydrogen, silanes, boranes orhydrides. Amongst all of the possibilities, metal-catalysed hydrogenation has been the most widely studied, especially asan asymmetric process.Some recently reported examples of rhodium-catalyse dasymmetric hydrogenation of alkenes include the conversion ofthe enamide into the derivatised amino alcohols and the regioselective hydrogenation of dienyl acetate into the allyl acetate both using the Me-DuPhos ligand .Reports of new ligands for asymmetric hydrogenation of alkenes continue to appear, often providing highly selective examples.Ruthenium catalysed hydrogenation of alkenes is also popular,and an interesting example has been provided by Bruneau,Dixneuf and co-workers. The achiral substrate is hydrogenatedwith an enantiomerically pure ruthenium complex into compound , which behaves as propionic acid attachedt o a chiral auxiliary. The achiral auxiliary in the substrate is converted into an enantiomerically enriched one prior to a subsequent auxiliary controlled functionalisation. Lewis acid catalysed reactions:- Lewis acids are able to catalyse a wide range of reactions. Theaddition of cyanide to aldehydes is one such reaction and hasbeen studied by many groups. Recently, North, Belokon andco-workers have used a titanium (salen) complex to catalyse the addition of trimethylsilylcyanide to benzaldehyde withlow catalyst loadings. Less work has been reported on theenantioselective addition of cyanide to imines, although it providesa useful route to ÃŽÂ ±-amino acids (Strecker synthesis). However, there have been several reports of the enantio selective variant of this reaction by aluminium catalysts,non-metallic catalysts, and with the zirconium catalysts, reported here. The imine is converted into the ÃŽÂ ±-aminonitrile with good yield and enantio selectivity Scandium triflate is a good catalyst for the allylation of aldehydes with allylsilanes and stannanes. Aggarwal and Vennallhave detailed the allylation of aldehydes followed by in situ acylation.36 Benzaldehyde allylsilane and acetic anhydride undergo coupling to provide the homoallylic acetate withscandium triflate as the catalyst Kobayashi and co-workers have shown that a three component system comprising of benzaldehyde an amine,such as aniline and allylstannane affo rds the homoallylicamine The reaction works more quickly in the presence of sodium dodecylsulfate SDS, which provides amicellar system .The allylation of isolated imines with enantiomerically pure palladium complexes has been achieved with up to 82%enantiomeric excess. Catalytic coupling reactions:- The formation of C-C bonds, as well as C-X bonds can becatalysed by many transition metals, although palladium complexesseem to have a greater scope than other metals. The useof catalytic coupling reactions to provide biaryls has recently been reviewed. 5. REAL LIFE APPLICATIONS: The fact that the transition elements are all metals means that they are lustrous or shiny in appearance, and malleable, meaning that they can be molded into different shapes without breaking. They are excellent conductors of heat and electricity, and tend to form positive ions by losing electrons. Generally speaking, metals are hard, though a few of the transition metals-as well as members of other metal families-are so soft they can be cut with a knife. Like almost all metals, they tend to have fairly high melting points, and extremely high boiling points. Many of the transition metals, particularly those on periods 4, 5, and 6, form useful alloys-mixtures containing more than one metal-with one another, and with other elements. Because of their differences in electron configuration, however, they do not always combine in the same ways, even within an element. Iron, for instance, sometimes releases two electrons in chemical bonding, and at other times three. ABUNDANCE OF THE TRANSITION METALS: Iron is the fourth most abundant element on Earth, accounting for 4.71% of the elemental mass in the planets crust. Titanium ranks 10th, with 0.58%, and manganese 13th, with 0.09%. Several other transition metals are comparatively abundant: even gold is much more abundant than many other elements on the periodic table. However, given the fact that only 18 elements account for 99.51% of Earths crust, the percentages for elements outside of the top 18 tend to very small. In the human body, iron is the 12th most abundant element, constituting 0.004% of the bodys mass. Zinc follows it, at 13th place, accounting for 0.003%. Again, these percentages may not seem particularly high, but in view of the fact that three elements-oxygen, carbon, and hydrogen-account for 93% of human elemental body mass, there is not much room for the other 10 most common elements in the body. Transition metals such as copper are present in trace quantities within the body as well. (industrial effulent) DIVIDING THE TRANSITION METALS INTO GROUPS. There is no easy way to group the transition metals, though certain of these elements are traditionally categorized together. These do not constitute families as such, but they do provide useful ways to break down the otherwise rather daunting 40-element lineup of the transition metals. In two cases, there is at least a relation between group number on the periodic table and the categories loosely assigned to a collection of transition metals. Thus the coinage metals-copper, silver, and gold-all occupy Group 9 on the periodic table. These have traditionally been associated with one another because their resistance to oxidation, combined with their malleability and beauty, has made them useful materials for fashioning coins. Likewise the members of the zinc group-zinc, cadmium, and mercury-occupy Group 10 on the periodic table. These, too, have often been associated as a miniature unit due to common properties. Members of the platinum group-platinum, iridium, osmium, palladium, rhodium, and ruthenium-occupy a rectangle on the table, corresponding to periods 5 and 6, and groups 6 through 8. What actually makes them a group, however, is the fact that they tend to appear together in nature. Iron, nickel, and cobalt, found alongside one another on Period 4, may be grouped together because they are all magnetic to some degree or another. This is far from the only notable characteristic about such metals, but provides a convenient means of further dividing the transition metals into smaller sections. To the left of iron on the periodic table is a rectangle corresponding to periods 4 through 6, groups 4 through 7. These 11 elements-titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, manganese, and rhenium-are referred to here as alloy metals. This is not a traditional designation, but it is nonetheless useful for describing these metals, most of which form important alloys with iron and other elements. One element was left out of the rectangle described in the preceding paragraph. This is technetium, which apparently does not occur in nature. It is lumped in with a final category, rare and artificial elements. It should be stressed that there is nothing hard and fast about these categories. The alloy metals are not the only ones that form alloys; nickel is used in coins, though it is not called a coinage metal; and platinum could be listed with gold and silver as precious metals. Nonetheless, the categories used here seem to provide the most workable means of approaching the many transition metals. GOLD. Gold almost needs no introduction: virtually everyone knows of its value, and history is full of stories about people who killed or died for this precious metal. Part of its value springs from its rarity in comparison to, say iron: gold is present on Earths crust at a level of about 5 parts per billion (ppb). Yet as noted earlier, it is more abundant than some metals. Furthermore, due to the fact that it is highly unreactive (reactivity refers to the tendency for bonds between atoms or molecules to be made or broken in such a way that materials are transformed), it tends to be easily separated from other elements. This helps to explain the fact that gold may well have been the first element ever discovered. No ancient metallurgist needed a laboratory in which to separate gold; indeed, because it so often keeps to itself, it is called a noble metal-meaning, in this context, set apart. Another characteristic of gold that made it valuable was its great malleability. In fact, gold is the most malleable of all metals: A single troy ounce (31.1 g) can be hammered into a sheet just 0.00025 in (0.00064 cm) thick, covering 68 ft  2  (6.3 m  2  ). Gold is one of the few metals that is not silver, gray, or white, and its beautifully distinctive color caught the eyes of metalsmiths and royalty from the beginning of civilization. Records from India dating back to 5000  B.C.  suggest a familiarity with gold, and jewelry found in Egyptian tombs indicates the use of sophisticated techniques among the goldsmiths of Egypt as early as 2600  B.C.  Likewise the Bible mentions gold in several passages. The Romans called it  aurum  (shining dawn), which explains its chemical symbol, Au. Gold is as popular as ever for jewelry and other decorative objects, of course, but for the most part, it is too soft to have many other commercial purposes. One of the few applications for gold, a good conductor of electricity, is in some electronic components. Also, the radioactive gold-198 isotope is sometimes implanted in tissues as a means of treating forms of cancer. SILVER. Like gold, silver has been a part of human life from earliest history. Usually it is considered less valuable, though some societies have actually placed a higher value on silver because it is harder and more durable than gold. In the seventh century  B.C.  , the Lydian civilization of Asia Minor (now Turkey) created the first coins using silver, and in the sixth century  B.C.  , the Chinese began making silver coins. Succeeding dynasties in China continued to mint these coins, round with square holes in them, until the early twentieth century. The Romans called silver  argentum,  and therefore today its chemical symbol is Ag. Its uses are much more varied than those of gold, both because of its durability and the fact that it is less expensive. Alloyed with copper, which adds strength to it, it makes sterling silver, used in coins, silverware, and jewelry. Silver nitrate compounds are used in silver plating, applied in mirrors and tableware. (Most mirrors today, however, use aluminum.) A large portion of the worlds silver supply is used by photographers for developing pictures. In addition, because it is an excellent conductor of heat and electricity, silver has applications in the electronics industry; however, its expense has led many manufacturers to use copper or aluminum instead. Silver is also present, along with zinc and cadmium, in cadmium batteries. Like gold, though to a much lesser extent, it is still an important jewelry-making component. COPPER. Most people think of pennies as containing copper, but in fact the penny is the only American coin that contains no copper alloys. Because the amount of copper necessary to make a penny today costs more than $0.01, a penny is actually made of zinc with a thin copper coating. Yet copper has long been a commonly used coinage metal, and long before that, humans used it for other purposes. Seven thousand years ago, the peoples of the Tigris-Euphrates river valleys, in what is now Iraq, were mining and using copper, and later civilizations combined copper with zinc to make bronze. Indeed, the history of prehistoric and ancient humans technological development is often divided according to the tools they made, the latter two of which came from transition metals: the Stone Age, the Bronze Age (c. 3300-1200  B.C.  ), and the Iron Age. Copper is also like its two close relatives in that it resists corrosion, and this makes it ideal for plumbing. Its use in making coins resulted from its anti-corrosive qualities, combined with its beauty: like gold, copper has a distinctive color. This aesthetic quality led to the use of copper in decorative applications as well: many old buildings used copper roofs, and the Statue of Liberty is covered in 300 thick copper plates. Why, then, is the famous statue not copper-colored? Because copper does eventually corrode when exposed to air for long periods of time. Over time, it develops a thin layer of black copper oxide, and as the years pass, carbon dioxide in the air leads to the formation of copper carbonate, which imparts a greenish color. The human body is about 0.0004% copper, though as noted, larger quantities of copper can be toxic. Copper is found in foods such as shell-fish, nuts, raisins, and dried beans. Whereas human blood has hemoglobin, a molecule with an iron atom at the center, the blood of lobsters and other large crustaceans contains hemocyanin, in which copper performs a similar function. ZINC. Together with copper, zinc appeared in another alloy that, like bronze, helped define the ancient world: brass. (The latter is mentioned in the Bible, for instance in the Book of Daniel,when King Nebuchadnezzar dreams of a statue containing brass and other substances, symbolizing various empires.) Used at least from the first millennium  B.C.  onward, brass appeared in coins and ornaments throughout Asia Minor. Though it is said that the Chinese purified zinc in about  A.D.  1000, the Swiss alchemist Paracelsus (1493-1541) is usually credited with first describing zinc as a metal. Bluish-white, with a lustrous sheen, zinc is found primarily in the ore sulfide sphalerite. The largest natural deposits of zinc are in Australia and the United States, and after mining, the metal is subjected to a purification and reduction process involving carbon. Zinc is used in galvanized steel, developed in the eighteenth century by Italian physicist Luigi Galvani (1737-1798). CADMIUM. In 1817, German chemist Friedrich Strohmeyer (1776-1835) was working as an inspector of pharmacies for the German state of Hanover. While making his rounds, he discovered that one pharmacy had a sample of zinc carbonate labeled as zinc oxide, and while inspecting the chemical in his laboratory, he discovered something unusual. If indeed it were zinc carbonate, it should turn into zinc oxide when heated, and since both compounds were white, there should be no difference in color. Instead, the mysterious compound turned a yellowish-orange. Strohmeyer continued to analyze the sample, and eventually realized that he had discovered a new element, which he named after the old Greek term for zinc carbonate,  kadmeia.  Indeed, cadmium typically appears in nature along with zinc or zinc compounds. Silvery white and lustrous or shiny, cadmium is soft enough to be cut with a knife, but chemically it behaves much like zinc: hence the idea of a zinc group. MERCURY. One of only two elements-along with bromine-that appears in liquid form at room temperature, mercury is both toxic and highly useful. The Romans called it  hydragyrum  (liquid silver), from whence comes its chemical symbol, Hg. Today, however, it is known by the name of the Romans god Mercury, the nimble and speedy messenger of the gods. Mercury comes primarily from a red ore called cinnabar, and since it often appears in shiny globules that form outcroppings from the cinnabar, it was relatively easy to discover. Several things are distinctive about mercury, including its bright silvery color. But nothing distinguishes it as much as its physical properties-not only its liquidity, but the fact that it rolls rapidly, like the fleet-footed god after which it is named. Its surface tension (the quality that causes it to bead) is six times greater than that of water, and for this reason, mercury never wets the surfaces with which it comes in contact. Mercury, of course, is widely used in thermometers, an application for which it is extremely well-suited. In particular, it expands at a uniform rate when heated, and thus a mercury thermometer (unlike earlier instruments, which used water, wine, or alcohol) can be easily calibrated. (Note that due to the toxicity of the element, mercury thermometers in schools are being replaced by other types of thermometers.) At temperatures close to absolute zero, mercury loses its resistance to the flow of electric current, and therefore it presents a promising area of research with regard to superconductivity. IRON. In its purest form, iron is relatively soft and slightly magnetic, but when hardened, it becomes much more so. As with several of the elements discovered long ago, iron has a chemical symbol (Fe) reflecting an ancient name, the Latin  ferrum.  But long before the Romans ancestors arrived in Italy, the Hittites of Asia Minor were purifying iron ore by heating it with charcoal over a hot flame. The ways in which iron is used are almost too obvious (and too numerous) to mention. If iron and steel suddenly ceased to exist, there could be no skyscrapers, no wide-span bridges, no ocean liners or trains or heavy machinery or automobile frames. Furthermore, alloys of steel with other transition metals, such as tungsten and niobium, possess exceptionally great strength, and find application in everything from hand tools to nuclear reactors. Then, of course, there are magnets and electromagnets, which can only be made of iron and/or one of the other magnetic elements, cobalt and nickel. In the human body, iron is a key part of hemoglobin, the molecule in blood that transports oxygen from the lungs to the cells. If a person fails to get sufficient quantities of iron-present in foods such as red meat and spinach-the result is anemia, characterized by a loss of skin color, weakness, fainting, and heart palpitations. Plants, too, need iron, and without the appropriate amounts are likely to lose their color, weaken, and die. COBALT. Isolated in about 1735 by Swedish chemist Georg Brandt (1694-1768), cobalt was the first metal discovered since prehistoric, or at least ancient, times. The name comes from  Kobald,  German for underground gnome, and this reflects much about the early history of cobalt. In legend, the Kobalden were mischievous sprites who caused trouble for miners, and in real life, ores containing the element that came to be known as cobalt likewise caused trouble to men working in mines. Not only did these ores contain arsenic, which made miners ill, but because cobalt had no apparent value, it only interfered with their work of extracting other minerals. Yet cobalt had been in use by artisans long before Brandts isolated the element. The color of certain cobalt compounds is a brilliant, shocking blue, and this made it popular for the coloring of pottery, glass, and tile. The element, which makes up less than 0.002% of Earths crust, is found today primarily in ores extracted from mines in Canada, Zaire, and Morocco. One of the most important uses of cobalt is in a highly magnetic alloy known as alnico, which also contains iron, nickel, and aluminum. Combined with tungsten and chromium, cobalt makes stellite, a very hard alloy used in drill bits. Cobalt is also applied in jet engines and turbines. NICKEL. Moderately magnetic in its pure form, nickel had an early history much like that of cobalt. English workers mining copper were often dismayed to find a metal that looked like copper, but was not, and they called it Old Nicks copper-meaning that it was a trick played on them by Old Nick, or the devil. The Germans gave it a similar name:  Kupfernickel,  or imp copper. Though nickel was not identified as a separate metal by Swedish mineralogist Axel Fredrik Cronstedt (1722-1765) until the eighteenth century, alloys of copper, silver, and nickel had been used as coins even in ancient Egypt. Today, nickel is applied, not surprisingly, in the American five-cent piece-that is, the nickel-made from an alloy of nickel and copper. Its anti-corrosive nature also provides a number of other applications for nickel: alloyed with steel, for instance, it makes a protective layer for other metals. PLATINUM. First identified by an Italian physician visiting the New World in the mid-sixteenth century, platinum-now recognized as a precious metal-was once considered a nuisance in the same way that nickel and cadmium were. Miners, annoyed with the fact that it got in the way when they were looking for gold, called it  platina,  or little silver. One of the reasons why platinum did not immediately catch the worlds fancy is because it is difficult to extract, and typically appears with the other metals of the platinum group: iridium, osmium, palladium, rhodium, and ruthenium. Only in 1803 did English physician and chemist William Hyde Wollaston (1766-1828) develop a means of extracting platinum, and when he did, he discovered that the metal could be hammered into all kinds of shapes. Platinum proved such a success that it made Wollaston financially independent, and he retired from his medical practice at age 34 to pursue scientific research. Today, platinum is used in everything from thermometers to parts for rocket engines, both of which take advantage of its ability to with stand high temperatures. 6. Application of transition metals complex formation in gas chromatography. we will be discusing applications of superselective liquid phases containing transition metal salts or complexes in gas chromatography Introduction: Metal complexation may be used for four purposes in gas chromatography: to help the separation of certain compounds present in the sample. In this case complexation is performed by using a stationary phase containing a metal; to utilize GC for the calculation of stability constants orother physico-chemical data; to analyse the metals themselves, by making organic volatile complexes and analysing them by GC; to increase sensitivity for inorganic and organic compounds by forming metal complexes and utilize e.g. an electron capture detector which has an increased sensitivity for such compounds. The present review discusses only the first two of these four application fields. The effect of the formation of eleetron-donor-acceptor complexes (EDA) [1-3] of transition metal cations with organic molecules containing n-bond(s) or free electron pairs (hi, O, S, halogens) may be used for the gas chromatographic separation of these molecules. The column packings containing the transition metals may be termed as superselectivepackings, because a slight difference in the structure of the separated compounds (e.g. cis- and transisomers) can give considerable difference in the retention time representing several minutes The reaction of complex formation should be rapid and reversible In the case of a 1:1 complex formation gas chromatography is convenient for the determination the stability constants of the newly formed adducts The formation of n-complexes with cations of the transition metals is particulary widely applied in gas chromatography. The termal stability of these complexes changes i~ a very broad temperature range depending on the metal and the ligand.complexes together with the temperatures of their chromatographic analysis. As seen chromatography permits as to examine the~ systems at temperatures higher than their thermal stability determined by static methods. The superselective packings can be divided into two group~ 1. Superselective liquid phases in which a salt or met~complex is melted or dissolved in a common liquid phase. 2. Superselective adsorbents in which a transition metal exists in various forms such as a salt or other co~pounds coated on the surface of a support, a porous i~ organic salt, a zeolite with the transition metal cation~ an inorganic oxide, or an inorganic or organometall~polymer. Steric Effect Substitution of bulky alkyl groups at a carbon double bond decrease the stability constants of n-complexes. The steric effect depends on the position of substitution in the following order: 2 > 4/> 3 > 5 >~ 6 [9, 78]. The small steric effect of the substituent in position 3 can be explained by considerable participation of electronic effect which, for alkyl groups has the opposite influence on stability constants than the steric effect. Electronic Effect The choice of the substituents at the double bond can increase or decrease the stability of the complex according to their electronic nature. For example, the substitution of D for H at the double bond increases the stability of the 7rcomplexes and for Rh 2* even bulk substituents increase the stability of complexes formed. This was called an inverse steric effect The electron-withdrawing effect of C1 on the electrons an aromatic ring causes a decrease in the stability constant of the n-complex of a transition metal with chlorobenzene as compared to the same complex with ethylbenzene Strain Effect:- Due to the large strain of the cyclobutene ring its ~r-complexes are less stable than those with five- and six-membered cycloolefms The Hg 2+ cation forms very strong complexes with olef~ and aromatic hydrocarbons. This is the reason why it applied for the selective retention of such compounds fr0~ hydrocarbon mixtures The stability constants of Hg ~+ complexes with molecules of organic compounds containing oxygen have been