Organometallic Synthesis

Compounds with metal–carbon bonds usually require specialized approaches to their syn thesis that differ from those discussed above for traditional Werner-type coordination complexes. This relates to the usually low oxidation state of the target compounds, that makes many of them air-sensitive (requiring the use of an inert atmosphere), the distinctive types of ligands involved, and the tendency for these compounds to be insoluble in or to react with water (leading to nonaqueous solvents being required). Solvents such as diethyl ether, tetrahydrofuran or toluene are more likely to be employed in this field. It is also com mon to employ sealed glass reaction vessels flushed with nitrogen or argon gas or else an inert atmosphere ‘glove box’, which is a large glass-fronted container fitted with portholes and rubber gloves that allow work to be carried out separated from the atmosphere in an inert gas (such as dinitrogen or helium) environment. Historically, organometallic compounds have been known for at least as long as Werner type complexes, with coordinated ethene first reported by Zeise in Denmark in 1827, and metal carbonyls like Ni(CO)₄ prepared by Mond in France in the 1890s, although it is true that the vast number of examples date from the 1950s and beyond. The two gross classes of ligands met in organometallic chemistry are: simple-bonded type, such as M CH₃,thatbehaveinmanywayslikeaconventionalmetal–donorbond;andmulti-centred-bonded systems such as occur with an alkene that binds symmetrically side-on and involves its-electrons in the linkage to the metal centre. This has been discussed earlier in Chapter 2.5. As an aid to understanding the outcomes of organometallic reactions and in synthesis, there is a convenient way in which the stability of a compound can be predicted, called the 18-electron rule. In the light elements of the p block, we traditionally invoke an octet (or 8-electron) rule to probe stability. This assumes that an s and three p valence orbitals are used in bonding, and allows us to understand why CH₄ is stable whereas CH₅ is not. In d-block chemistry, it is possible to use what is in that case an 18-electron rule (using an s, three p and five d orbitals) to help predict stability of a complex. This is used mostly for organo metallic complexes,and has values in ceitlimits the number of combinations of metal and ligand that lead to the desired electron count. but is invariably met in more specialized and advanced courses. It works best for low-valent metals with small neutral high-field ligands like carbonyls, but is less effective for high-valent metal ions involving weak-field ligands, and thus is not usually invoked for traditional Werner-type coordination complexes. Metal carbonyls represent a key class of organometallic compounds, and are often a starting point for other chemistry. They tend to be monomers dimers or small oligomers, such as Ni (CO)₄ and Mn²(CO)10 the latter involving a formal metal–metal bond. Most metal carbonyls are made by reduction of simple salts or oxides in the presence of CO, or direct reaction of CO with finely-divided metals at elevated pressure. Examples appear in (6.43) and (6.44).

The carbonyl ligands are able to be substituted by some other ligands or else the coordination sphere can be expanded by addition reactions with other compounds. It is also straightforward to prepare compounds that incorporate carbonyl and other ligands such as amine ligands. This can sometimes be achieved directly such as in (6.45).

The product (with en = 1.2-ethanediamine chelated) can be isolated readily. The transition metal carbonyls are usually far more robust than those formed by main group elements; for example H₃B(CO) decomposes below room temperature, whereas Cr(CO)₆ can be sublimed without decomposition. An early example of organometallic synthesis was the reaction of [PtCl₄]²⁻ with thylene in dilute aqueous hydrochloric acid solution, which yields the classical-bonded orange complex [Pt(C₂H4)Cl₃]- where the Pt and two carbon atoms form an equilateral triangle (or in other words, the ethene is bound symmetrically side-on to the platinum(II) centre). A vast range of alkene complexes has been reported subsequently including the tris(n2 ethylene) nickel (0) which features three side-on ethylene molecules arranged in a trigonal pattern around the metal atom. Bonding in these complexes requires molecular orbital theory for a satisfactory explanation. This assigns-bonded character through a filled molecular orbital of the alkene overlapping symmetrically with an empty metal orbital, and-bonded character through overlap of an empty ∗ molecular orbital on the alkene with a filled metal d orbital. Another important class of compounds are the so-called ‘sandwich’ compounds featuring a metal bound (or ‘sandwiched’) between two flat aromatic anions, the best known of which is the cyclopentadienyl ion (C₅H₅). These compounds can be prepared by reactions such as (6.46) performed in a nonaqueous solvent such as diethyl ether.

This compound is robust– it is air stable able to be sublimed without decomposition and resistant to strong acids and bases. It can undergo reversible one-electron reduction chemically or electro chemically with a significant change in colour from yellow to blue that strongly suggests that it is a metal-centred reduction. An array of other related compounds are known including those featuring larger aromatic ring systems such as Cr(C₆H₆)2. Examples with metals from most parts of the Periodic Table, even f-block elements are now established.

Metal–alkyl σ-bonded compounds can be formed conveniently by reactions employing alkyl halides or alkyl magnesium bromides, such as example (6.47), performed in diethyl ether.

These reactions are facilitated by the presence of spectator ligands such as phosphines and carbonyls. Phosphines, common co-ligands in organometallic compounds are usually conveniently introduced through direct reaction with metal halides.

Organometallic compounds also undergo reactions of coordinated ligands readily. A simple example involves the susceptibility of coordinated carbon monoxide towards nu cleophilic attack. The [Mo (CO)₆] complex reacts with methyllithium (6.48) with the new ligand produced at one site also able to undergo additional reactions not described here.

 Areactionwithasimilaroutcomeinvolvesmigrationofoneligandtoattackanotheradjacent ligand (a 1.1-migratory insertion) promoted by the availability of another ligand to occupy the site vacated by the first movement. In example (6.49) below, an initial R-M-C=O component of the molecule is converted to an M-C(R)=O component leaving a vacant coordination site, filled by an added phosphine ligand in this case.

The hydride ion (H−) is an efficient small ligand in organometallic chemistry. The first transition metal hydrides were prepared using the Hieber base reaction, exemplified in (6.50). The hydroxide adds to the carbon of one CO ligand to produce an intermediate that rapidly loses carbon dioxide leaving the hydride ion to occupy the coordination site.

These are but a few examples of an array of reactions available to organometallic systems. Clearly, the level of difficulty is greater in performing many organometallic reactions compared with Werner-type coordination chemistry reactions through the special equipment that must be employed because air and/or protic solvents can lead to unwanted reactions although the reactions themselves do not necessarily impose any greater inherent complexity. The storage and handling of products may pose problems however due to their reactivity, particularly in redox reactions. A detailed examination of their chemistry may be pursued through advanced and/or specialist texts.

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مواضيع ذات صلة

Pretty in Red? - Colour and the Spectrochemical Series

Peak Performance– Illustrating Selected Physical Methods

Probing the Life of Complexes– Using Physical Methods

Reactions of Coordinated Ligands

Metal-directed Reactions

Reactions Involving the Metal Oxidation State

Catalysed Reactions

Chiral Complexes

Substitution Reactions without using a Solvent

Substitution Reactions in Nonaqueous Solvents

Reactions Involving the Coordination Shell

Block f : Inner Transition Metals (Lanthanoids and Actinoids)