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Organometallic Chemistry 531

Hundreds of H-bridged species of this type are known, but to form a complex or coordination compound [ML n ] m+ ,
the simplest is B 2 H 6 (Scheme 3). such as [Co(NH 3 ) 6 ] . The square brackets denote that
3+
the complex molecule (if m = 0) or ion (if m = 0) is an en-
H H
H tity that retains its identity and can be regarded as a single
B B molecule or ion. The M L bonds are relatively strong. A
H H H key feature of coordination chemistry is that the presence
of the ligands modify the properties of the central atom
Scheme 3.
or ion, and the presence of the atom or ion modifies the
Diborane is one of the most useful main group organ- properties of the ligand. The extent of these mutual modi-
ometallic species, because it adds to alkenes in the unusual fications of metal and ligand can vary from minor to very
anti-Markownikov direction to give organoboron deriva- profound. When we consider organometallic complexes,
tives that can be converted into any of a number of useful we find that this same mutual effect of metal on ligands
organic compounds. and vice versa is also a very marked feature of the chem-
istry. One additional general property of complexes best
(5)
RCH CH 2 + 1/2B 2 H 6 = RCH 2 CH 2 BH 2 illustrated by organometallic compounds is the possibil-
ity that ligands, usually of chemically different types, can
RCH 2 CH 2 BH 2 + H 2 O 2 = RCH 2 CH 2 OH (6)
react with one another within the coordination sphere of
RCH 2 CH 2 BH 2 + Br 2 = RCH 2 CH 2 Br (7) the metal (i.e., while still attached to the metal) in ways
that are not observed for the free ligands L in the absence
Almost all other types of reagents that add to alkenes do
of a metal.
so in the opposite direction to give the commercially less
The commonest structural arrangements of the ligands
useful branched derivatives.
around the metal in organometallic complexes in octahe-
A development of interest in the area of main group
dral (1), square planar (2) or tetrahedral (3). Less regular
organometallics was the isolation of compounds with mul-
arrangements can often be described as distortions of 1–3.
tiple bonds between elements such as P and Si, e.g.,
RP PR or R 2 Si SiR 2 . Such heavy elements had been L L
thought to be incapable of double bonding, and attempts L L L L
to make compounds like these had always led to the forma- M M M
tion of polymers such as (PR) n . The solution turned out to L L L L L
be the use of very bulky R groups, which prevented poly- L L L
merization. The resulting double bonds prove to have elec- 1 2 3
tronic structures interestingly different from those present
in such long-known compounds as R 2 C CR 2 . A ligand such as Cl or H 2 NCH 2 CH 2 NH 2 has more
−
than one lone pair of electrons (four on Cl , one on each
−
of the two N atoms). In such a situation, two M L bonds
II. TRANSITION METAL can be formed. Chloride ion commonly bridges two met-
ORGANOMETALLIC COMPOUNDS als (4), while H 2 NCH 2 CH 2 NH 2 more commonly chelates
to a metal to form a ring (5)(L n M refers to a metal and un-
A. With Metal Carbon Bonds specified ligands). Chelate complexes are often more sta-
ble (thermodynamically and kinetically) than analogous
The transition metals, the elements from groups 3–10 of
species with monodentate ligands (i.e., ligands that have
the periodic table, have been more intensively studied in
only one point of attachment to a metal).
recent years than the main group elements. Transition-
metal organometallic chemistry grew out of coordination Cl NH 2
chemistry, of which it is a subfield. Coordination chem- CH 2
L n M ML n L n M
istry was founded in its modern form by Alfred Werner
from 1896 and deals with compounds of metals and metal Cl NH 2 CH 2
ions with ligands (symbolized in a general fashion as L). 4 5
From the point of view of coordination chemistry, it is
immaterial whether or not these ligands are bound by an Complexes that differ in the arrangement of the ligands
M C (or M H) bond, and indeed the majority of co- around the central metal are different compounds and
ordination compounds have M NorM O bonds to the differ in properties. For example, cis-[PtCl 2 (NH 3 ) 2 ](6)is
ligands. Metal ions are Lewis acids; that is, they can accept an active anti-tumor drug, but trans-[PtCl 2 (NH 3 ) 2 ](7)is
one or more pairs of electrons from one or more ligands L not. Both 6 and 7 are square planar complexes, but they