化学专业英语之有机金属化合物——金属配合物
ORGANOMETALLICS—METAL π COMPLEXES
Metal π complexes are characterized by a type of direct carbon-to-metal bonding that is not a classical ionic, σ, or π bond . Numerous molecules and ions, e.g., mono- and diolefins, polyenes, arenes, cyclopentadienyl ions, tropylium ions, andπ-allylic ions, can form metal π complexes with transition-metal atoms or ions. These are classified as organ metallic complexes, because of their direct carbon-metal bond, and as coordination complexes, because the nature and characteristics of the tt ligands are similar to those in coordination complexes. In 1827, Zeise reported that ethylene reacts with platinum (II ) chloride to form a salt K (C2H4)PtCl3(l), but it was not until after the elucidation of the structure of ferrocene (2) in 1953 that attention was redirected to Ziese's salt, which was the first reported metal π unreactivecomplex.
Generally, metal tt complexes can be classified into three main groups; olefin-, cyclopentadienyl-, and arene-metal π complexes; mixed complexes are categorized according to structural or chemical analogies within these groups. Allyl π complexes are designated as olefin π complexes in this review. Study of metal π complexes has contributed to the elucidation of the mechanisms of Ziegler-Natta polymerization, the oxo reaction, and catalytic hydrogenation, and to the development of the Wacker process which is used for the oxidation of olefins1.
The following nomenclature for metal it complexes is used : (1) Organic π ligands precede the metal atom. (2)Organic π ligands precede inorganic 7t ligands. (3)Inorganic π ligands, e.g., carbonyl or nitrosyls, generally follow the metal atom; halides also follow the metal but
precede carbonyls or nitrosyls. (4)A prefix, e.g., di, is preferred rather than bis in describing sandwich-type π complexes, e.g., dibenzenechromium. (5) The symbol π can be used preceding a ligand in order to distinguish π-complex bonding from a, ionic, or other bonding. The symbol η(eta or hapto)precedes a ligand and indicates the number of C—M bonds in the ligand.
Monoolefins , dienes, polyolefins, and acetylenes serve as ligands to transition metals and form olefin π complexes. Typical examples of olefin π complexes are monoolefin ligands, e.g., potassium η2-ethyleneplatinum trichloride (1); and cyclopentadienylium. –η3-cycloheptatrienylium molybdenum dicarbonyl (3); diene ligands, eg, η4-butadieneiron tricarbonyl(4 ).
Certain of the delocalized π-electron ring systems of aromatic molecules overlap with dxy and dy3 metal orbitals as do the π electrons of alkenes with metal d orbitals2. The following aromatic rings can form π complexes;
The C5H5- ,C6H6,and C8HS arenes are the most common in arene k complexes that are characterized by π-bonded rings alone or π-bonded rings that are associated with one ring and other ligands, eg, halogens, CO, RNC, and R3P. Typical examples are the di-η5-cyclopentadienyl complexes , ie, metallocenes , eg , di-η5-cyclopentadienyliron (2 ). In di-η4-5-cyclopentadienyliron ,ie, ferrocene, the 6-π-electron system of the C5H5- ion is bonded to the metal. Other aromatic ring systems are mono-η5-cyclopentadienylmetal nitrosyl and carbonyl complexes.
Properties
The π-Complex Bond. Metal π complexes are among those that are least satisfactorily described by crystal-field theory (CFT) or valence-bond theory (VBT). The nature of the bonding can be treated more completely and quantitatively by molecular-orbital theory (MOT) or ligand-field theory (LFT). The ligand-field theory originally was advanced as a corrected CFT. The LFT relies on the use of molecular orbitals and often is used interchangeably with the MOT. The usual approach is to use the linear combination of atom
ic orbitals (LCAO) method. It is assumed that when an electron in a molecule is near a particular nucleus, the molecular wave function is approximately an atomic orbital that is centered at the nucleus. The molecular orbitals are formed by adding or subtracting the appropriate atomic orbitals. For transition metals .the "3d, 4s, and 4p orbitals are the atomic orbitals of interest. The ligands may have σ-and π-valence orbitals. Once the appropriate atomic orbitals have been selected for the metal and ligands, the proper linear combination of valence atomic orbitals is determined for the molecular orbitals. The determination of orbital overlaps that are possible, ie, meet inherent symmetry requirements, is done by application of the principles of group theory. At this point, the procedure becomes arbitrary in that approximate wave functions must be selected for use in the calculations of the overlap integrals and coulomb integrals3. Finally, an arbitrary charge distribution is chosen and the orbital energies and interaction energies are calculated, and a solution of the secular equation for the energies and coefficients of the atomic wave functions can be determined. A new initial charge distribution is repeated until consistent values are obtained.
Reactions
Metal π complexes react with a wide range of chemical reagents. However, the reactions of the π-olefin-, π-cyclopentadienyl-, and it-arene-metal complexes are distinctly characteristic of each group, π Cyclopentadienyl complexes, ie, metallocenes ,exhibit a high degree of aromaticity and undergo many typical aromatic substitution reactions. However, the π arene complexes do not exhibit a discernible degree of aromaticity.
Although most physical properties, particularly the structure of metal tt complexes, are interpreted by use of the basic principles of coordination chemistry, these established principles do not explain suitably some reaction anomalies of the different groups of metal π complexes.
Olefin π Complexes. Reactions involving olefin x. complexes similarly are characteristic of uncomplexed and complexed olefinic functions. Generally, reactions involving the former are not very different from those observed for free olefins. However, reactions of the latter are altered significantly by π-complex formation. Among the reactions of interest are additio
n, elimination, and substitution.

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