Metallo-Organic Compounds

organic compounds containing a metal atom that is directly bonded to a carbon atom.

The metallo-organic compounds may be divided into two groups:

(1) Compounds of nontransition metals and some compounds of transition metals. These compounds contain single (σ) metal-carbon bonds.

(2) Compounds of transition metals (including metal carbonyls) formed by filling the s-, p-, and -orbitals of the metal atom by the π-electrons of various unsaturated systems, such as aromatic, olefin, acetylene, allyl, and cyclopentadienyl systems.

The most thoroughly studied metallo-organic compounds of the first group include the derivatives of lithium, sodium, potassium, beryllium, magnesium, zinc, cadmium, mercury, boron, aluminum, tellurium, germanium, tin, lead, arsenic, and antimony. The properties of these compounds are determined by the nature of the M—C bond (M is a metal atom), which depends mainly on the nature of the metal but also on the type and number of the organic radicals bonded to the metal atom. In the metallo-organic compounds of the alkali metals, the M—C bond is strongly polarized, in which case the metal atom carries a partial positive charge, whereas the carbon atom carries a partial negative charge:

Therefore, metallo-organic compounds of this type are very reactive: they are vigorously decomposed by water and are very sensitive to oxygen. In practice, these compounds are used only in solutions (hydrocarbons, ether, tetrahydrofuran, and other solvents) and are protected from moisture, CO₂, and atmospheric oxygen. Compounds of the alkaline earths (magnesium and calcium), as well as zinc, cadmium, boron, and aluminum, have analogous properties. For example, such compounds as (CH₃)₂Zn, (CH₃)₃B, and (C₂H₅)₃A1 are flammable in air. More stable compounds of these elements are mixed metallo-organic compounds, which contain a metal bonded to an organic radical and to one or two acid groups—for example, (C₂H₅)₂AlCl and C₂H₅AlCl₂. Increasing electronegativity of the metal leads to decreasing polarity of the M — C bond, and the compounds of such metals as mercury, tin, and antimony are covalent. These materials are volatile liquids or crystalline substances that are resistant to the action of oxygen and water. Upon heating, these compounds decompose, with the formation of the metal and free organic radicals—for example, (C₂H₅)₄pb → Pb + 4C₂H₅.

Metallo-organic compounds of the first group may be prepared by reaction of metals with alkyl halides or aryl halides:

n-C₄HBr → n-C₄H₉Li + LiBr

by the addition of metal hydrides or salts to double bonds:

3CH₂ = Ch₂ + AlH₃→(C₂H₅)₃Al

by the reaction of diazo compounds with metal salts:

2CH₂N₂ + HgCl₂→ClCH₂HgCH₂Cl + 2N₂

by the reaction of metallo-organic compounds with metal halides, metals, and one another:

3C₆H₅Li + SbCl₃ → (C₆H₅)₃Sb + 3LiCl

(C₂H₅)₂Hg + Mg → (C₂H₅)₂Mg + Hg

(CH₂ = CH₄Sn + 4C₆H₅Li → (C₆H₅)₄Sb + 4CH₂ + CHLi

The metallo-organic compounds of transition metals of the first group have a tendency toward homolytic cleavage (alkyl derivatives of silver, copper, and gold); the aryl and alkenyl derivatives of these metals are more stable. The acetylides are very stable, as are the methyl compounds of platinum, such as (CH₃)PtI and (CH₃)₄Pt.

In the metallo-organic compounds of the second group, the metal atom interacts with all the carbon atoms of the π-electron system. Typical representatives of this class are ferrocene, dibenzene chromium, and butadiene iron tricarbonyl. The classical theory of valence proved inadequate for recently produced compounds of this type.

The metallo-organic compounds played a large role in the development of concepts concerning the nature of the chemical bond. Such compounds are used in organic synthesis (the organolithium and organomagnesium compounds are the most important in this area). Many metallo-organic compounds are being used as antiseptic, medicinal, and physiologically active materials, antiknock compounds (for example, tetraethyl lead), antioxidants, and stabilizers for polymers. The preparation of pure metals from the carbonyls and metallo-organic compounds is of great importance in the production of semiconductors and application of metallic coatings. The metallo-organic compounds are intermediate products in a number of the most important industrial processes that are catalyzed by metals, their salts, and complex metallo-organic catalysts (for example, hydration and cyclopolymerization of acetylene; anionic, including stereospecific, polymerization of olefins and dienes; and carbonylation of unsaturated compounds).