化学专业英语之有机金属化合物——金属配合物

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 πbond . Numerous molecules and ions, e.g., mono- and diolefins, polyenes, arenes, cyclopentadienyl ions, tropylium ions, andπ-allylic ions, can form metal πcomplexes 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 (C

2H

4

)PtCl

3

(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 complex.

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 πcomplexes in this review. Study of metal πcomplexes 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 πligands precede the metal atom. (2)Organic πligands 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-typeπ 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 πcomplexes. Typical examples of olefin πcomplexes 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 d

xy and d

y3

metal orbitals as do the π electrons

of alkenes with metal d orbitals2. The following aromatic rings can form π complexes;

The C

5H

5

- ,C

6

H

6

,and C

8

H

S

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 R

3

P. 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 C

5H

5

- ion is bonded to the metal. Other aromatic ring systems are

mono-η5-cyclopentadienylmetal nitrosyl and carbonyl complexes.