Modern inorganic chemistry

GROUP IV 179

Full sp carbon

hybrid orbital \ \ ^ ^ \ / ^ ^ Singly-filled p

orbita Is on oxygen

Singly- fil led p

orbital on carbon

Empty p orbital on carbon Full p orbital on oxygen

monoxide is in fact found to have donor properties and forms donor

compounds, for example with diborane (p. 145) it splits the molecule

by donating to the borane, BH 3.

2CO + B 2 H 6 ^ 2OC-»BH 3

It also forms compounds known as carbonyls with many metals.

The best known is nickel tetracarbonyl, Ni(CO) 4 , a volatile liquid,

clearly covalent. Here, donation of two electrons by each carbon

atom brings the nickel valency shell up to that of krypton (28 + 4x2);

the structure may be written Ni( <- C=O) 4. (The actual structure is

more accurately represented as a resonance hybrid of Ni( <- C=O) 4

and Ni(=C=O) 4 with the valency shell of nickel further expanded.)

Nickel tetracarbonyl has a tetrahedral configuration,

O

C

C

O

Other examples are iron pentacarbonyl, Fe(CO) 5 , and chromium

hexacarbonyl, Cr(CO) 6 , which have trigonal bipyramidal and octa-

hedral configurations respectively.

Carbon monoxide burns with a characteristic blue flame in air

or oxygen. The reaction

2CO + O 2 -> 2CO 2 :AH=- 283 kJ mol" i

is very exothermic and as expected, therefore, carbon monoxide
reacts with heated oxides of a number of metals, for example copper,
lead, iron, reducing them to the metal. For example :

PbO + CO -> Pb + CO 2 |

Carbon monoxide forms addition compounds. With chlorine in
sunlight or in the presence of charcoal in the dark,carbonyl chloride