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Organometallic Chemistry 537

The key cyclic intermediate in this process is shown in A number of useful electron counting rules, analogous
Scheme 13. to the 18e rule for mononuclear compounds, enables us
to predict the structures to be expected among metal clus-
R ters. Nobelist Roald Hoffmann has developed a series of
R
analogies between metal fragments and organic fragments
M CH 2
M that is another useful structural and conceptual tool. For
R R example, Os(CO) 4 is said to be isolobal with CH 2 , and so
the triosmium cluster shown in Scheme 14 is isolobal with
cyclopropane.
R R
R R
F. Organometallic Species in Biology
M
M
M C bonds are proving to be important in biology. The
Scheme 13.
best-known case is vitamin B-12, one derivative of which
is shown in Scheme 16. The form shown is one of several
The Shell Higher Olefins Process (SHOP) is an ex-
derivatives that contains a metal–carbon bond. In one other
ample of a large-scale commercial process that relies on
naturally occurring organometallic derivatives of B-12, an
a metathesis step. In SHOP, ethylene is oligomerized to
adenosyl group can replace the methyl group. Lack of
alkenes having a chain length of around C 16 by a homo-
B-12 leads to the fatal disease pernicious anemia. The
geneous nickel catalyst. The chain lengths of the resulting
reason is that coenzyme B-12 is a cofactor in a number of
mixture can usefully be manipulated via metathesis.
vital steps in human biochemistry, for example synthesis
of deoxy-ribose for DNA, and the synthesis of the amino
E. Compounds with Metal–Metal Bonds acid methionine. B-12 contains cobalt, and its biochem-
ical role involves making and breaking of Co C bonds.
Some compounds containing a metal–metal bond occur in
It has also been found that Ni C and Ni H bonds are
organometallic chemistry. M M bonds are important in
important in bacterial biochemistry, where they allow cer-
“clusters,” such as Os 3 (CO) 12 , which contains a triangle
tain bacteria to live on H 2 and CO as energy and carbon
of metal atoms. The CO ligands are shown only as lines
source and to produce methane. Methane, or “marsh gas,”
radiating from the triosmium core in the diagram. Clusters is a common natural product; almost all of it appears to be
contain three or more metals with at least two M M bonds formed by a bio-organometallic route.
(see Scheme 14).
Os CONH 2
Os
CH 3
CONH 2
Os
H 2 NOC
N
Scheme 14. H 2 NOC CO N CONH 2
N
The second situation in which M M bonds are impor-
N
tant is the case of dinuclear species. Multiple bonding is
more often found in dinuclear cases, for example Re 2 Cl 2− HN
8
(Scheme 15), but there are not as yet very many metal–
metal multiply bonded organometallic compounds. O N
CONH 2
N
2 HO
Cl Cl O
Cl Cl O
P
Re Re O
Cl Cl O O
Cl Cl CH 2 OH
Scheme 15. Scheme 16.