Another is ferrocene, Fe(C5H5)2, which has a more elaborate structure with the iron atom sandwiched between two C5H5 rings (see Figure 23). Some compounds with metal-carbon bonds are not regarded as organometallic, because the constituent carbon atom is not part of an organic group; two examples are metal carbides--such as Fe3C, a hard solid that is a component of cast iron--and metal cyanide compounds--such as the deep blue paint pigment Prussian blue, KFe2(CN)6.

Historical developments 

The first synthetic organometallic compound, K[PtCl3(C2H4)], was prepared by the Danish pharmacist William C. Zeise in 1827 and is often referred to as Zeise's salt. At that time Zeise had no way of determining the structure of his new compound, but it is now known that the structure contains an ethene molecule (H2C=CH2) attached through both carbon atoms to the central platinum (Pt) atom. The platinum atom also is bonded to three chlorine (Cl) atoms. The potassium ion, K+, is present to balance the charge.

The attachment of the ethene carbon atoms to the central platinum atom qualifies Zeise's salt as an organometallic compound. A development with a more immediate impact on the field of chemistry was the discovery in 1849 by the German-trained British chemist Edward C. Frankland of diethylzinc, H5C2-Zn-C2H5, which he showed is very useful in organic synthesis. Since then, an ever-increasing variety of organometallic compounds have been utilized in organic synthesis in both the laboratory and industry.

Another milestone in the development of the field was the discovery of tetracarbonylnickel (Figure 24) by the German-educated British industrial chemist Ludwig Mond and his assistants in 1890. More recently, the discovery of the remarkably stable compound ferrocene (shown above, Figure 23) occurred at a time (1951) when techniques for the determination of structures were becoming widely available. Since the 1950s, organometallic chemistry has become a very active field, marked by the discovery of new organometallic compounds along with their detailed structural and chemical characterization and their application as synthetic intermediates and catalysts in industrial processes. Two organometallics encountered in nature are vitamin B12 coenzyme (Figure 25), which contains a cobalt-carbon (Co-C) bond, and dimethylmercury, H3C-Hg-CH3, which is produced by bacteria to eliminate the toxic metal mercury. However, organometallic compounds are generally unusual in biological processes.

s- and p-block organometallic compounds 

It is common practice to consider the possibilities for the metal in main-group organometallic compounds to include the groups of the s block (i.e., groups 1 and 2) plus the heavier elements in groups 13 through 15. (Groups 13-18 constitute the p block; see Figure 22.) The elements at the borderline between the d block and p block--namely, zinc, cadmium, and mercury--will be discussed along with the p-block organometallics because of the similarity of their organometallic chemistry. In an internationally sanctioned system of nomenclature, the organic group is named first, followed by the metal, as in dimethylmercury. In writing the formula, this order is reversed, Hg(CH3)2. As indicated in Table 19, the organic groups, which are also called ligands, are named in the same way as for any organic compound. The number of carbon atoms on a group that are attached to the metal is indicated by the superscript in n. This convention is known as hapto nomenclature. A single point of attachment, 1, is usually not explicitly indicated, as in the above formula for dimethylmercury, a monohapto species. The compound with the common name ferrocene (Figure 23) has the systematic name bis(5-cyclopentadienyl)iron, where the number of cyclopentadienyl ligands (two) is indicated by the prefix bis and the number of sites of attachment (five) for each of these is indicated by 5. Ferrocene is thus called a pentahapto compound. To avoid complexity, Figure 23 does not contain lines showing the individual Fe-C bonds. The number of sites of attachment are similarly indicated in the formula Fe(5-C5H5)2.

The stability and reactivity of organometallic compounds 

These characteristics are associated with the nature of the organic ligands and the metal to which they are attached. In each of the main groups of the periodic table (groups 1, 2, and 13-15 of Figure 22), the thermal stability of a given type of organometallic compound generally decreases from the lightest to the heaviest element in a group. For example, in compounds containing group-1 metals, methyllithium (LiCH3) is much more stable than methylpotassium (KCH3), and, in those with group-14 metals, tetramethylsilicon, Si(CH3)4, is stable in the absence of air at 500 C (932 F), whereas tetramethyllead, Pb(CH3)4, rapidly decomposes at that temperature. This trend in stability is a consequence in part of the decrease in M-C bond strength on going down within a group.

The trend does not hold for the d-block elements (groups 3-12 of Figure 22), where M-C bond strengths and stability often increase going down a group.

The reactivities of organometallic compounds with water and air vary widely. The highly active main-group metals such as Li, Na, Mg, and Al form highly air- and water-sensitive organometallic compounds. For example, Al2(CH3)6 undergoes immediate and violent reaction with water to liberate methane (CH4) gas, and it bursts immediately into flame on contact with air. For the elements toward the right of the periodic table (groups 14 and 15), the organometallic compounds are not water-sensitive; tetramethylsilicon, for example, does not react with water or air at room temperature.

The synthesis of s- and p-block organometallic compounds 

Synthesis of these compounds can often be accomplished by one of several general reaction types. The most important of these are outlined below.

Formation of alkyllithium and Grignard reagents 

The highly active metals combine with a halogen-substituted hydrocarbon to produce simple organometallic compounds. For example, methyllithium, an important reagent in organic synthesis, is produced commercially by following the reaction:

2Li + CH3Cl LiCH3 + LiCl

With other active metals, such as magnesium, aluminum, and zinc, the reaction generally yields the organometallic halide. A common reaction of this type is the synthesis of a Grignard reagent, an alkylmagnesium halide that finds wide use in organic synthesis (the s indicates that the metal is in solid form).

Double displacement 

The synthesis of organometallic compounds by double displacement involves organometallic (MR) and binary halide (EX, where E is a metal or metalloid and X is a halogen) starting materials. It provides a convenient synthetic procedure that is widely used in the laboratory and to a lesser extent on a commercial scale. As the following examples illustrate, the organic group on the more active metal is transferred to the less active metal or metalloid. In this context the most common highly active metals are lithium, aluminum, and magnesium.

4Li(CH3) + SiCl4 4LiCl + Si(CH3)4

Al2(CH3)6 + 2BF3 2AlF3 + 2B(CH3)3

Redistribution 

Double displacements involving the same central element are often referred to as redistribution reactions. A commercially important example is the redistribution of silicon tetrachloride and tetramethylsilicon (also known as tetramethylsilane) at elevated temperatures.

The products from this reaction can be separated by distillation. This reaction is performed industrially where (CH3)2SiCl2 is removed from the equilibrating mixture and then hydrolyzed to produce the intermediates for silicone polymers, which have the form -(Si(CH3)2-O)-n (see above Inorganic polymers).

Hydrometallation 

The addition of a metal hydride to a multiple bond is called hydrometallation, and it leads to the formation of a metal-carbon bond.

M-H + H2C=CH2 MH2C-CH3

This reaction is driven mainly by the high C-H bond strength relative to most E-H bond strengths. Two important hydrometallation reactions are hydroboration and hydrosilation, illustrated, respectively, by the following examples.

In the hydroboration and hydrosilation of an unsymmetrical alkene, the boron or silicon adds to the carbon atom having less bulky substituents, and the smaller hydrogen atom adds to the carbon atom having bulky substituents--(CH3)2C in the above equations. Hydroboration was discovered and developed in the United States by Herbert C. Brown, who received the Nobel Prize for Chemistry in 1979 for this research. Both hydroboration and hydrosilation are widely used in the synthesis of complex organic molecules. In these applications, the B-C or Si-C bond is generally cleaved in a subsequent step to produce a product that is free of boron or silicon.

Reduction 

All organometallic compounds are potential reducing agents, and those of the electropositive elements are very strong reducing agents because the metal gives up electrons to the carbon, resulting in a polar M-C bond with a partial positive charge on the metal and a negative charge on the carbon. Organometallic compounds of highly electropositive elements such as lithium, sodium, and aluminum ignite spontaneously and sometimes explode on contact with air or other oxidizing agents. The useful organometallic reagents Li(CH3), Zn(CH3)2, B(CH3)3, and Al2(CH3)6 are spontaneously flammable in air (pyrophoric). Accordingly, techniques have been developed to handle these and other pyrophoric compounds. Glass reaction vessels sealed from the atmosphere and purged with nitrogen gas are commonly used for handling air-sensitive organometallic compounds in the laboratory. Large quantities of pyrophoric compounds such as Al2(C2H5)6 are routinely handled with ease in the chemical industry by using closed metal reactors for the production of these and other much less reactive compounds. Organometallic compounds have reduced reactivity when the metallic component is not highly electropositive and when the metal is completely surrounded by attached groups. For example, elevated temperatures are required to initiate combustion with Si(CH3)4 and Sn(CH3)4, and at room temperature they can be handled in air.

Carbanion character 

Organometallic compounds containing highly active (electropositive) metals, such as lithium, magnesium, aluminum, and zinc, react rapidly and completely with water with the liberation of a hydrocarbon. For example, dimethylzinc liberates methane gas along with solid zinc hydroxide.

Zn(CH3)2 + 2H2O Zn(OH)2 + 2CH4

The partial negative charge of an organic group bonded to a highly active metal results in a distinctive pattern of reactivity that is frequently referred to as nucleophilic or carbanion character. Thus, the above hydrolysis of dimethylzinc can be viewed as a transfer of a slightly acidic H+ from water to the strongly basic carbanion CH3- in dimethylzinc.

Alkyllithium, alkylaluminum, and alkylmagnesium compounds are the most common carbanion reagents in laboratory-scale synthetic chemistry; carbanion character is greatly diminished for the less metallic elements boron and silicon. The nucleophilic character of organometallic compounds of active metals has many synthetic applications, which are outlined in Figure 26. For example, the organic group in organometallic compounds of active metals attacks the carbonyl carbon of a ketone, and upon hydrolysis a tertiary alcohol results (see Organic compounds: Alcohols, phenols, and ethers). Similarly, aldehydes can be converted to secondary alcohols by reaction with an organometallic reagent followed by hydrolysis. Also shown in Figure 26 are double displacement reactions for preparing sulfones (R2SO2) and sulfoxides (R2SO) by treating thionyl chloride (SOCl2) or sulfuryl dichloride (SO2Cl2) with an alkyllithium or Grignard reagent.

One consequence of the carbanion character of organometallic compounds containing active metals is the protolysis (proton-transfer) reaction that takes place with very weak protonic acids, including water. Alcohols react in a manner similar to that of water, and this provides a convenient way of introducing an alkoxide (OR) substituent into an organometallic compound.

(C2H5)3Ga + HOCH3 [(C2H5)3GaOHCH3] 

(C2H5)2Ga(OCH3) + C2H6

The rate of reaction decreases with bulky organic groups on the alcohol. For example, tert-butyl alcohol,

reacts slowly with most active organometallics, and it is therefore employed in the laboratory to safely destroy reactive organometallic wastes.
-hydrogen elimination-->

-hydrogen elimination 

This is a common organometallic reaction in which a hydrogen atom on a carbon atom that is one position removed from the metal (the position) transfers to the metal with the liberation of an alkene. The following example shows the formation of ethene, C2H4.

This reaction is the reverse of the addition of an M-H bond to an alkene (hydrometallation, discussed above), and under some conditions significant equilibrium concentrations of both reactants and products are observed. The -hydrogen elimination reaction is thought to proceed through the transition state depicted in Figure 27. As might be expected from this reaction mechanism, compounds with an accessible central metal atom tend to undergo -hydrogen elimination readily. For example, it occurs with trialkylaluminum compounds but not with tetraalkylsilicon compounds, in which reaction of the silicon atom is hindered by the bulky substituents.

d- and f-block organometallic compounds 

The nomenclature for organometallic compounds of the d-block elements, also known as transition metals (see Figure 22), follows rules similar to those described for the main-group organometallics. Groups attached to the metal (ligands) are named first, followed by the metal. When a ligand is attached to the metal by more than one carbon atom, the number of sites of attachment is indicated by n in n.

Metal carbonyls 

Many of the early discoveries in d-block organometallic chemistry involved the metal carbonyls--i.e., compounds consisting of a metal atom bonded to one or more carbon monoxide (CO) ligands. Mond's discovery of the first simple metal carbonyl, tetracarbonylnickel, Ni(CO)4, at the end of the 19th century was quickly followed by a series of discoveries in his laboratory and elsewhere showing that most of the d-block metals form neutral homoleptic carbonyls. (The term homoleptic refers to identical groups attached to a central atom.) The remarkable volatility of tetracarbonylnickel, whose boiling point is 43 C, prompted one of Mond's contemporaries to state that "Mond put wings on metals." The ease of formation of tetracarbonylnickel (from metallic nickel and carbon monoxide) and its high volatility led Mond to develop an efficient industrial process for separating nickel and cobalt, which often occur together in their ores. This process requires diligent safety measures because tetracarbonylnickel is extremely toxic.

The structure of metal carbonyls 

The carbonyl ligands in the tetracarbonylnickel molecule project toward the vertices of a tetrahedron (see Figure 24), and thus the structure is referred to as tetrahedral. Similarly, the six carbonyl ligands in hexacarbonylchromium project toward the vertices of an octahedron.

Many other metal carbonyls contain two or more metal atoms, such as decacarbonyldimanganese and octacarbonyldicobalt, shown here.

In all these structures carbon monoxide is connected to the metal through its carbon atom. When more than one metal atom is present, as in octacarbonyldicobalt, the carbon of the carbonyl ligand may bridge between metal atoms.

The formulas of most homoleptic metal carbonyl compounds conform to the rule that each metal atom in the metal carbonyl molecule have 18 valence electrons. The valence electrons represent the outer electrons on the metal plus those from the ligand. For example, the electron count for Fe(CO)5 includes 8 from the iron atom (it is in group 8 and hence has 8 valence electrons) and 2 from each carbonyl ligand, giving a total of 18 electrons. This 18-electron rule applies to many organometallic compounds of the d-block metals other than carbonyls, but there are exceptions in the organometallic chemistry of the d-block metals. The most notable exceptions are metals on the far left of the d block (e.g., V(CO)6 contains 17 electrons) and on the right of the d block where the platinum metals--rhodium (Rh), iridium (Ir), palladium (Pd), and platinum (Pt)--often exhibit a 16-electron count in their organometallic compounds.

Zero-oxidation-state metal carbonyls 

The central metal in a neutral metal carbonyl, such as those described above, is assigned an oxidation state of zero, quite unlike the case in simple inorganic compounds in which positive oxidation states are the norm, as, for example, Fe3+ in FeCl3 or Ni2+ in NiBr2. Unlike the free metals, which also have a zero oxidation state, many carbonyls are soluble in a variety of simple organic solvents and are highly reactive. Because of these chemical and physical properties, the metal carbonyls are convenient starting materials for the synthesis of compounds with the metal atom in a zero or low oxidation state. One simple reaction is the substitution of other ligands such as triethylphosphine, P(Et)3, for CO (Et is a common abbreviation for the ethyl group, -C2H5).

Cr(CO)6 + P(Et)3 Cr(CO)5P(Et)3 + CO

Because most other ligands are far bulkier than carbon monoxide and also because of differences in bonding properties, it is often not possible to replace all the carbonyl ligands by groups such as triethylphosphine, and a mixture of products with varying degrees of substitution can result.

The remarkable ability of the carbonyl ligand to stabilize compounds with metals in the zero oxidation state has led to detailed studies of M-CO bonds. A minor part of the M-CO bonding can be attributed to the tendency of the carbonyl ligand to donate a pair of electrons to the metal, which is the principal mode of interaction for most ligands with metal cations in classical coordination compounds such as [Co(NH3)6]3+. The second mode of interaction with the metal is the simultaneous back-donation of electron density from the metal to the carbonyl ligand, which is called back bonding and is illustrated in Figure 28.

Metal carbonyl anions 

More remarkable than the formation of zero-oxidation-state metal carbonyls is the reduction of many of these carbonyl compounds to metal carbonyl anions, in which the metal has a negative oxidation state. The following example demonstrates that the two-electron reduction by sodium metal is accompanied by the loss of the two-electron donor carbonyl ligand, and so the 18-electron count on iron is preserved; the solvent is tetrahydrofuran (THF).

The stabilization of very electron-rich complexes, such as [Fe(CO)4]2-, is attributed to the back bonding that shifts electron density from the metal to the carbonyl ligands, and this view is substantiated by C-O bond distances and other physical data. The metal atom in these carbonyl anions is assigned a negative oxidation state (-2 in the above example). This formalism does not acknowledge the delocalization of electron density from the metal to the ligand, but the chemical properties of the carbonyl anions do suggest that some of the negative charge resides on the metal. For example, a metal carbonyl anion can be protonated with the H+ ion, which generally attaches to the central metal and not a carbonyl ligand, as in the following example.

[Fe(CO)4]2- + HCl [HFe(CO)4]- + Cl-

Owing to their high reactivity, carbonyl anions are useful starting materials for the synthesis of other organometallic compounds, and this accounts for their applications in organic synthesis. For example, [Fe(CO)4]2- is used to extend a carbon chain by transfer of the carbonyl substituent, producing aldehydes, ketones, or carboxylic acids (see Figure 29).

Compounds with metal-carbon bonds 

The variety of hydrocarbon ligands found in d-block organometallic chemistry (see Figure 22) range from simple -bonded alkyl ligands, double-bonded carbenes, and triply bonded carbynes to a host of polyene ligands, some of which are described in the remainder of this section.

Simple alkyl ligands 

A simple alkyl ligand forms an M-C single bond, and in doing so the alkyl group acts as a one-electron monohapto ligand. Many such compounds are known, but they are less common in the d block than in the s and p blocks. This may in part be a result of the modest M-C bond strengths. Another reason for the limited stability of alkyl ligands in many d-block complexes is a set of reactions that can be quite rapid, such as -hydrogen elimination, CO insertion, and reductive elimination. As described below, these reactions transform simple hydrocarbon ligands into other groups.

The -hydrogen elimination reaction is an important feature of the hydrocarbon chemistry of both d- and p-block organometallics. The reaction consists of the abstraction of a hydrogen atom from the organic ligand with the formation of two products, one of which contains a metal-hydrogen bond and the other of which is an alkene.

LnM-CH2CH3 LnM-H + H2C=CH2

(Ln represents the n ligands not involved in the hydrogen elimination.) The reverse of this reaction, alkene insertion into the M-H bond, is illustrated by the hydroboration and hydrosilation reactions discussed above. Both the -hydrogen elimination and addition of M-H across a C=C double bond are thought to proceed through a cyclic intermediate involving a three-centre, two-electron bond where a hydrogen atom bridges between the carbon and the metal atoms (see Figure 27).

Since a compound with no hydrogen atoms on the -carbon atom cannot undergo -hydrogen elimination, benzyl, CH2(C6H5), and methyltrimethylsilyl, CH2Si(CH3)3 (shown below) ligands are generally more robust than ethyl ligands when attached to d-block metal atoms. Similarly, the lack of -hydrogen atoms on the methyl group accounts for the greater stability of complexes containing the methyl ligand rather than the ethyl ligand.

A reaction frequently referred to as CO insertion leads to carbon-carbon bond formation between the carbon atom of a carbonyl ligand and the carbon atom of an alkyl ligand, which is the methyl group in the following example.

The CO insertion reaction is involved in all of the transformations of [Fe(CO)4R]- to organic molecules shown above in Figure 29.

Another type of reaction that can transform an attached organic ligand (as well as other groups) is reductive elimination.

The converse of reductive elimination is oxidative addition.

The reactions discussed above, insertion of C=C into an M-H bond, -hydrogen elimination, and CO insertion, are often employed for the synthesis of organic molecules in the laboratory and in industry. They also account for some of the individual steps in some important catalytic cycles described below in the section Organometallic compounds in catalysis.

Alkylidene ligands 

Alkylidene ligands, such as CH2, CHR, or CR2, form the M=C d-p double bonds (i.e., bonds between the d orbitals of the metal and the p orbitals of the carbon), and their metal compounds are often called carbenes. The first stable metal carbene complex was discovered in the laboratory of the German chemist Ernst O. Fischer in Munich (then in West Germany) by the reaction of a metal carbonyl with an alkyllithium compound. In this reaction, the alkyl group is transferred (as an R- group) to the carbon atom of the coordinated carbonyl, and subsequent addition of carbocation reagent results in attachment of an R+ group to the oxygen. 

This type of carbene complex is common for the atoms of metals in groups 6-8, and they are called Fischer carbenes. The Fischer carbenes can be modified by electron-rich groups. For example, the attack of an amine on the electron-poor carbon atom of a Fischer carbene results in the displacement of the OR group to yield a new carbene (Me represents the methyl group, -CH3).

The attack of an electron-rich amine (indicated by - in the above equation) on the carbon atom in the Fischer carbene is attributed to the significant electronegativity of the middle to late d-block metals, which makes the carbon atom of the carbene electron-poor (indicated by + in the above equation).

The reactions of Fischer carbene complexes with alkynes has considerable utility in organic synthesis. For example, naphthyl compounds (i.e., those derived from the fused ring system C10H8) can be synthesized by the reaction of methoxy phenyl Fischer carbenes with an alkyne.

The chromium can be removed from the organic product by mild oxidation. Another route to a carbene is the deprotonation of an alkyl ligand, producing a carbene that contains only hydrocarbon ligands.

These ligands that contain only carbon and hydrogen are commonly attached to metal atoms from the early part of the d block such as titanium (Ti) and tantalum (Ta). The complexes are known as Schrock carbenes for their discoverer Richard Schrock of the United States. The chemistry and spectroscopy of the Schrock carbenes indicate that these compounds have the opposite polarity of the Fischer carbenes. The carbon behaves as if it were electron-rich, because the M+=C- bond is polarized so as to put negative charge on the metal-bound carbon, for the early d-block metals readily give up electrons. As a result, the carbon attached to the metal atom in a Schrock carbene reacts with electron-seeking reagents, such as Me3Si+Br+, at the carbene carbon (Cp is a common abbreviation for the cyclopentadienyl ligand, -C5H5).

An interesting reaction of the Schrock carbenes is the alkene metathesis reaction:

This reaction appears to proceed through a four-membered ring intermediate containing carbon atoms and the metal atom.

Alkylidyne ligands 

Alkylidyne ligands have the general formula CH or CR. They are bound to the metal by an MC triple bond involving one bond and two d-p bonds. The simplest member of this series is methylidyne, CH, and the next simplest, CCH3, is ethylidyne. One route to methylidynes, discovered in Fischer's laboratory, involves the abstraction of an alkoxide group (OR) from a Fischer carbene by BBr3.

Alkene and alkyne ligands 

An alkene ligand contains a bond between carbon atoms, C=C, which can serve as an electron pair donor in a metal complex, as in the case of Zeise's salt (see above General considerations: Historical developments). This complex may be prepared by bubbling ethene, C2H4, through an aqueous solution of [PtCl4]2- in the presence of divalent tin, Sn(II), which aids in the removal of the chloride ion (Cl-) from the coordination sphere of the divalent platinum, Pt(II).

The alkene ligand bonds to the metal centre by both electron donation and acceptance, similar to the situation with carbon monoxide. Electron donor and acceptor character between the metal and the alkene ligand appear to be fairly evenly balanced in most ethene complexes of the d metals.

The allyl ligand, -CH2-CH=CH2, can bind to a metal atom in either of two configurations: as an 1-ligand or an 3-ligand. Because of this versatility in bonding, 3-allyl complexes are often highly reactive. Examples of 1- and 3-allyl complexes are, respectively, shown here.

Acetylene, H-CC-H, has two bonds and hence is a potential four-electron donor. Substituted acetylenes form very stable polymetallic complexes in which the acetylene can be regarded as a four-electron donor. An example is 2-diphenylethynehexacarbonyldicobalt, in which four of the six electrons in the triple bond of the ethyene ligand, R-CC-R, are shared with the two cobalt atoms (Ph represents the phenyl ligand, -C6H5). As in this example, the alkyl or aryl groups (R) on the acetylene impart stability to the metal complex--in contrast to simple acetylene (HCCH) complexes, where the hydrogen atoms are reactive.

Polyene ligands 

Diene (-C=C-C=C-) and larger polyene ligands present the possibility of several points of attachment to a metal atom. The resulting polyene complexes are usually more stable than the equivalent monohapto complex with individual ligands. For example, bis(4-cycloocta-1,5-diene)nickel is more stable than the corresponding complex containing four ethene ligands.

Cycloocta-1,5-diene (cod), a fairly common ligand in organometallic chemistry, is introduced into the metal coordination sphere by ligand displacement reactions; e.g.,

PdCl2(NCPh)2 + cod codPdCl2 + 2NCPh.

Metal complexes of cod are often used as starting materials because the cod ligand can bind in various ways to the metal and the complexes are intermediate in stability. Many of them are sufficiently stable to be isolated and handled, but cod and similar ligands can be displaced by stronger ligands. For example, Ni(cod)2 reacts with CO to form Ni(CO)4 and the free cod molecule. This reaction is a convenient source of the extremely toxic Ni(CO)4, for it can be generated directly in a flask where it is then available to undergo a subsequent reaction.

Cyclic polyene ligands 

These rings, which have alternating double and single bonds, are among the most important ligands in organometallic chemistry; the most common members of this group range from cyclobutadiene (C4H4) to cyclooctatetraene (C8H8). Their organometallic compounds include the metallocenes ferrocene (Figure 23) and bisbenzenechromium and bis(cyclooctatrienyl)uranium (commonly called uranocene), shown here.

A metallocene consists of a metal atom between two planar polyhapto rings (as in ferrocene), and because of this structure they are informally called sandwich compounds. Cyclic polyenes are also known to form complexes in which they bind to a metal atom through some but not all of their carbon atoms.

The cyclobutadiene ligand is a four-electron donor. It is unstable as the free (i.e., uncombined) hydrocarbon, but it is known to exist in stable complexes, including Ru(C4H4)(CO)3.

This is one of many cases in which coordination to a metal atom stabilizes an otherwise unstable molecule. Because of its instability, cyclobutadiene must be generated in the presence of the metal to which it is to be coordinated. This can be accomplished in several ways, one of which is the dimerization of a substituted acetylene. Interestingly, the C4R4 is bound to the cobalt in preference to the trimerization product, C6R6.

The cyclopentadienyl ligand (C5H5, abbreviated Cp) has played a major role in the development of organometallic chemistry. Examples of cyclopentadienyl complexes are given above, such as ferrocene. Also, Cp was an ancillary ligand in the above equation illustrating the dimerization of a substituted acetylene. In some metal cyclopentadienyl compounds, the metal is bonded to only one of the five carbon atoms, and in these complexes the Cp is designated as a monohapto, 1-, ligand, which contributes one electron to form a bond with the metal, as in

Others contain a trihapto (3-) cyclopentadienyl ligand, which donates three electrons. The most common case, however, is when Cp is a pentahapto ligand contributing five electrons. Two of the bonding modes for Cp are illustrated in the following structure, which contains both 3- and 5-C5H5 ligands.

The bis(5-cyclopentadienyl) sandwich complexes of iron, cobalt, and nickel are readily prepared by the reaction of sodium cyclopentadienide with the corresponding d-metal halide.

Because of their great stabilities, the 18-electron group-8 compounds ferrocene, ruthenocene, and osmocene maintain their ligand-metal bonds under rather harsh conditions, and it is possible to carry out a variety of reactions on the cyclopentadienyl ligands while they are attached to the central metal. In some cases, they undergo reactions similar to those of simple aromatic hydrocarbons, such as Friedel-Crafts substitution, which is a characteristic reaction of benzene, C6H6.

It is also possible to replace the hydrogen atom on a C5H5 ring with a lithium atom using the highly reactive reagent butyllithium.

LiC4H9 + Fe(5-C5H5)2 Fe(5-C5H5)(5-C5H4Li) + C4H10

This lithiated product is an excellent starting material for the synthesis of other ring-substituted products.

A closely related set of so-called bent sandwich compounds, in which the Cp rings are not parallel, are important in the organometallic chemistry of the early and middle d-block elements and the f-block elements (lanthanides and actinides). The Schrock carbene Ta(5-C5H5)2(CH3)(CH2), shown above, is one such example. Bent sandwich compounds are important in the organometallic chemistry of the f-block elements, but to achieve stability the pentamethylcyclopentadienyl ligand, C5(CH3)5, is generally employed with these elements, as, for example, in the following uranium compound.

Metal clusters 

Metal cluster compounds contain metal-metal bonds. The focus here is on compounds having three or more metals in a closed array. Carbon monoxide is the most common ligand in organometallic cluster compounds, but many other organometallic ligands are bound to clusters, and the presence of several metals leads to bonding arrangements for the ligand that are not possible for monometallic compounds. A variety of metal arrays are seen in cluster compounds. Triangular , tetrahedral , and octahedral clusters are common, and much larger metal arrays are known. The structures of many clusters, which can be precisely determined by single-crystal X-ray diffraction, provide some clues to the way in which ligands are bound to the surfaces of bulk metal particles. The latter are more difficult to structurally characterize than are molecular clusters. For many d-block clusters there is a strong correlation between their structure and the number of valence electrons (from the metal atoms and the ligands). This set of correlations for clusters is similar to the 18-electron rule for mononuclear organometallics, and these guidelines are often called Wade's rules after the British chemist Kenneth Wade, who first recognized that a triangular cluster such as Ru3(CO)12 usually has 48 valence electrons, a tetrahedron such as Co4(CO)12 60 electrons, and an octahedron such as Rh6(CO)12(3-CO)4 86 electrons. In some cases it is possible to synthesize clusters in a stepwise manner. An interesting example of this type is the buildup of a ruthenium nitride cluster; in the process of cluster building, the nitrogen ligand is progressively encapsulated by metal atoms (see Figure 30).

Organometallic compounds in catalysis 

Catalysts are substances that increase the rate of a reaction but are not consumed in the reaction. Catalysts are widely encountered in nature, industry, and the laboratory. Many of the catalysts utilized in the chemical industry and the laboratory are organometallic compounds.

Hydrogenation 

A catalytic cycle involving organometallic species is shown in Figure 31. The overall result of the process is to add molecular hydrogen, H2, across the double bond of an alkene, and so the process is referred to as the hydrogenation of an alkene. The reactants, H2 and ethene (C2H4), enter the cycle by reaction with the complex to produce in succession a hydrido complex (B) and an alkene complex (C). In the final step, from (D) to (A), the hydrogenated product leaves the loop with the regeneration of the coordinatively unsaturated Rh(I) complex. The cycle continues as long as hydrogen gas and ethene are supplied. The rhodium complexes in solution are the catalysts and are not used up in the reaction. Rhodium, which is more expensive than platinum, can be used even in catalytic processes where the products are inexpensive, because the rhodium is not consumed. Modifications of this type of catalyst are also employed in the production of pharmaceuticals such as levodopa, which is used to treat Parkinson's disease.

Hydroformylation 

Hydroformylation involves the addition of carbon monoxide and hydrogen to an alkene to form an aldehyde containing one more carbon atom than the original alkene.

This catalytic reaction is employed in the petrochemical industry, where Co2(CO)8 or various rhodium catalysts are utilized. The catalytic cycle proceeds through a series of organometallic intermediates. The aldehydes produced by hydroformylation are normally reduced to alcohols that are used as solvents, as plasticizers, and in the synthesis of detergents. The scale is enormous, amounting to millions of tons of product per year.

Alkene polymerization 

Polyalkenes, the most common and useful class of synthetic polymers, are often prepared by use of organometallic catalysts, either in solution or supported on a solid surface. In the 1950s, the German chemist Karl Ziegler developed a catalyst for ethene polymerization based on a catalyst formed by the reaction of TiCl4 with Al(C2H5)3. Soon thereafter Giulio Natta of Italy made use of this type of catalyst for the polymerization of propene to produce polymers with highly regular structures. The intimate details of the reactions of these commercial catalytic processes are not entirely understood, but there are strong indications from more easily studied soluble organometallic catalysts that alkenes coordinate to a metal centre and then insert into a hydrocarbon chain, producing a longer-chain hydrocarbon attached to the metal centre. Repetition of this process leads to extremely-long-chain hydrocarbon polymers, which include many of the most familiar plastics, such as polyethylene and polypropylene. These plastics are used in consumer items ranging from milk containers and plastic bags to artificial limbs and car bumpers. (D.F.S.)