Carbon Monoxide. Toxic Gas and Fuel for Anaerobes and Aerobes: Carbon Monoxide Dehydrogenases - The Metal-Driven Biogeochemistry of Gaseous Compounds in the Environment - page 38

reaction with soda lye at elevated temperatures and pressures. In technical and
industrial processes CO can be produced by reaction of elemental carbon with air
and water vapor producing a mixture of CO and H 2 called “synthesis gas” or
“water-gas” [ 1 ].
CO is an attractive partner for

π

-electron-rich metals due to its chemical
properties. The HOMO of CO is centred at C and it therefore binds preferentially
to metals with the C and not the electron lone pair at O, which is in an orbital of
lower energy. Because CO is an unsaturated soft ligand, it is able to donate
˃ -electrons to the metal and to accept metal d π -electrons by a process termed
back bonding. While electron donation along the ˃ -bond removes electron density
from C, the accepted metal d -electrons increase the electron density both at C and

O. Thus, when CO binds to a metal its C becomes more positive and its O
more negative, thereby polarizing the molecule. The stability of the formed
metal-carbonyl depends on (i) the electron configuration of the metal; (ii) the
other ligands coordinating the metal, and (iii) on the arrangement of the ligands.
Metals of low

π

-basicity form carbonyls, which are sensitive to nucleophilic attack
as the C is particularly positively charged and different nucleophiles, including
hydroxyl ions react with metal carbonyls. In addition to forming metal carbonyls,
CO inserts into metal alkyl bonds producing metal acyls. Metal carbonyl and metal
acyl formation with CO are found in industrial processes and microbial physiology
and enzymology [ 2 ].
Two industrial processes with analogies to the enzyme chemistry described in
the following make use of the reactivity of metal carbonyls [ 2 ]. First, the water-
gas shift reaction alters the CO:H 2 ratio of synthesis gas by reacting CO with
water to CO 2 and H 2 . When CO binds to a metal it becomes activated for the
nucleophilic attack of an OH group. The formed metallacarboxylic acid liberates
CO 2 and leaves a metal hydride species, which may be protonated to H 2 .This
reaction is analogous to the chemistry catalyzed by carbon monoxide dehydroge-
nases, except that the enzymes do not produce H 2 and keep the two electrons and
two protons separated. Second, in the Monsanto- and Cativa-processes acetic acid
is produced by reaction of CO with methanol. The active catalysts are precious
transition metals of the 4d and 5d row of the periodic system, which catalyze a
condensation reaction analogous to the reaction catalyzed at the Ni,Fe-cluster of
acetyl-CoA synthases. The overall reaction is a carbonylation of methanol, which
is relying on the tendency of CO to react with the metal-bound methyl group
by migratory insertion. The formed metal-acetyl group is reductively eliminated
to give acetic acid as product. Both processes are used to convert CO on a
large scale.
CO oxidation produces CO 2 , which is a linear molecule with formal C

π

Odouble
bonds. CO 2 is overall non-polar, but due to the different electronegativities of C
and O, it has a positively polarized C atom and negatively polarized O atoms.
Reduction of CO 2 needsactivationbydecreaseoftheC-Obondorder,whichis
also apparent by a decrease of the O-C-O bond angle from 180 to 133 in the
one-electron reduced CO 2 . The carbon centred lowest unoccupied molecular orbital
allows reduction and nucleophilic attack at the C with transfer of electron density

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