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

Classification of organometallic compounds
is given by
L1W=nD(M-Me)=L1H 1 (M.gJ+nMI 1 (Me.g)-L1H 1 (Me"M.gJ
In general. .1H~(Me"M.fll is small compared with the enthalpy of atomization of
the element. ~Hi(M,g), so that the latter term is dominant. It is found that within
a series of compounds with given n, ~H- and hence D(M-Mel qualitatively
follow the trend in atomization enthalpy for the elements . .1H~'c(M,g). The
strongest bonds to carbon are thus formed by those elements which are most
strongly bound in their standard states (Fig. 1.4 ).
While the strengths of single metal-carbon bonds vary from strong (e.g.
B-C) to rather weak (e.g. Pb-C), they are of a similar order of magnitude
to the strengths of single bonds C-X (X = C. N. O. CI. S etc.) which are present in
familiar organic compounds.
(b) MULTIPLE BONDINC: TO CARBOf\;. Carbon forms strong multiple bonds with
itself and with nitrogen and oxygen. Multiple bonding between carbon and other
main group elements, however. (except S and Se) is uncommon. In phosphorus
yields (p. 136). R 1 P=CH 2 • the n-component probably includes some P(3d)
character, as in the isoelectronic phosphine oxides, R 1 P=0. Otherwise there are
only curiosities such as the phospha- and arsabenzenes (p. 144) and the silaethenes
(p. 124). Except at high temperatures in the gas phase, compounds such as
R 2 C=SiR~ do not exist as monomers unless their aggregation to polymers or
cyclic oligomers is prevented, perhaps by introducing very bulky substituents.
Transition elements, however, form complexes in which formal metal-carbon
double or triple bonds are present. Examples are (0C) 5 W=C(OMe)Me (p. 2 33) and
(But0) 3 W -CEt (p. 377). The n-components of these bonds are thought to arise
through interaction between metal ( n-1 )d anei carbon 2p orbitals.
c Si Ti
Outer Orbi tais, 3d--- 5d
empty arbitals which make
tittle or no contribution to 3p--- 4p 5p
bonding, being too high in
energy 3s--- 4s
5s
3d
Valence Orbitals,
which may or may not be
fully occupied and which
are important in bonding
4p HHH
?~>-#-1t-#- 3p HHH 4s H
2s----*- 3s H 3d M~HHH
3p hHH
Inner(Core) Orbitals,
2p HHH
which are filled and ore too 1 s H 3s H
2s H
tow in energy to contribute 2p H H 11
significantly to the bonding 2s H
1 s H
1s H
Fig. 1.3 Division of orbitals into outer. valence and core orbitals.
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