_P.-Powell-auth.-Principles-of-Organometallic-Chemistry-Springer-Netherlands-1988

General survey
syntheses. They are also involved as intermediates in many catalytic processes,
some of which are used for the large scale conversion of carbon monoxide,
hydrogen, alkenes and other small molecules into useful organic chemicals.
1.3 Properties
Most organometallic compounds resemble organic rather than inorganic compounds
in their physical properties. Many possess discrete molecular structures,
and hence exist at ordinary temperatures as low melting crystals, liquids or gases
( see Table 1.1). Commonly they are soluble in weakly polar organic solvents such
as toluene, ethers or dichloromethane. Their chemical properties vary widely and
their thermal stability, for example, depends markedly on their chemical
composition. Thus tetramethylsilane (Me 4 Si) is unchanged after many days at
500°C, whereas tetramethyltitanium decomposes rapidly at room temperature.
Similarly there are wide differences in their kinetic stability to oxidation; some
(e.g. Me 4 Si, Me 2 Hg, *Cp 2 Fe) are not attacked at room temperature by the oxygen
in air, whereas others (e.g. Me 3 B, Me 2 Zn, Cp 2 Co) are spontaneously inflammable.
1.4 Classification of organometallic compounds J,y bond type
Organometallic compounds may conveniently be classified by the type of
metal-carbon bonding which they contain. Carbon is a fairly electronegative
element (2.5 on the Pauling scale) and hence might be expected to form ionic
bonds only with the most electropositive elements, but to form electron-pair
covalent bonds with other elements. The Periodic Table may be divided into very
approximate regions into which the various types of organometallic compound
predominantly fali (Fig. 1.1). As is usually found in inorganic chemistry, such a
classification is far from rigid and the regions overlap considerably. It will be
noticed that these regions are very similar to those observed in the classification of
hydrides into (a) ionic, (b) volatile covalent, (c) hydrogen-bridged and (d) metallike.
As we shall see later, the organic compounds of the d-block transition
elements often involve not only a- but also n- or b-bonding which is not
commonly found amongst compounds of the main group elements. In detailed
discussion of their chemistry, therefore, d-block transition metal organometallic
compounds are better taken separately from those of the main group elements.
Moreover, the chemistry of organic derivatives of the transition elements may be
dominated more by the ligand (especially when it occupies in effect severa! coordination
positions) rather than by the Periodic Group.
1.4.1 Ionic organometallic compounds
Organometallic compounds containing metal ions generally are formed only by
the most electropositive elements. Thus methylpotassium crystallizes in a close
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