Stories About Chemistry
69. Strange Whims of Carbon Monoxide
There is nothing tricky about this compound: one atom of carbon and one of oxygen. It is called carbon monoxide. It is very poisonous and takes part in chemical reactions reluctantly; such is a brief character of the substance with the simple formula CO.
...Believe it or not, an event of no great interest occurred in 1916 at a German chemical plant. Somebody decided to make use of a very old steel cylinder (in which a mixture of two gases, hydrogen and carbon monoxide, had been kept under pressure for about five years running). It was opened, the gases were let out and at its bottom was found some light brown liquid with an unpleasant, kind of “dusty” odour.
The liquid proved to be a known, but very rare chemical compound consisting of one iron atom and five molecules of carbon monoxide. Its name, as given in chemical handbooks, was iron pentacarbonyl Fe(CO)5.
(Incidentally, speaking of the fates of scientific discoveries. Iron pentacarbonyl was obtained on exactly the same day, June 15, 1891, by two scientists: Berthelot in France and Mond in England. Such coincidences are not so frequent, are they?)
An investigation of how the substance had formed in the cylinder showed that nothing supernatural had happened. The hydrogen had made the inner iron surface of the container very active by reducing the iron oxides to the metal. Under pressure, the carbon monoxide had reacted with the iron. After studying the mechanism of the reaction the chemists of the same plant designed an apparatus in which kilograms of the substance could be produced.
As a matter of fact, the pentacarbonyl had found practical usage. It had proved to be not a bad antiknock agent (we seem to be getting them by the dozen, don’t we?). A special fuel containing iron pentacarbonyl was produced and it was called Motaline. But motor cars did not use Motaline very long. The pentacarbonyl decomposed too easily into its components, and the resulting iron powder clogged the piston rings of the engines. And just at this time TEL was discovered...
Now make a mental note of the easy decomposability of iron pentacarbonyl, and let us turn our attention to other matters for a little while.
A large number of carbonyls are now known: of chromium and molybdenum, of tungsten and uranium, of cobalt and nickel, of manganese and rhenium. These compounds have various properties: some are liquids, others solids; some decompose easily, others are fairly stable.
But they all have a common and very curious property: the usual conception of valence does not apply to carbonyls.
It will be recalled that in complex compounds different numbers of neutral molecules combine with the metal ions. That is why coordination numbers are used instead of valence in the chemistry of complex compounds. The coordination number shows how many molecules, atoms, or complex ions are linked to the central atom.
Carbonyls are still stranger fruits of nature’s invention. In them neutral atoms are combined with neutral molecules. The valence of the metals in these compounds must be considered equal to zero because carbon monoxide is a neutral molecule.
This is another paradox of chemistry, and, to tell the truth, it has found no theoretical explanation so far.
And so we now leave off our little excursion into theory.
Practice found the metal carbonyls a dainty morsel.
As catalysts, for one thing.
But carbonyls have much more important uses.
Reverting to the plant in the storehouse of which the old cylinder was found, at the bottom of which a strange liquid was observed that turned out to be iron pentacarbonyl, that...
To make a long story short, that went into production on almost an industrial scale. But once, while the operator in charge of the synthesis apparatus was day dreaming, the pentacarbonyl began leaking out. The vapours of the substance deposited on a steel sheet which happened to be lying nearby. The operator soon discovered the leak and quickly eliminated it, but in doing so he accidentally pushed the sheet off the platform and it fell to the ground floor.
Hunters have a saying: “Once a year even a stick will fire." The steel sheet, which had lain peacefully in the sun for so long, exploded when it hit the ground.
A special “Inquest Commission” held more than one sitting before the experts were able to conclude that the sheet had “exploded” because it had been coated with a very fine iron powder. All finely divided powders in general are likely to explode; for example, explosions have been known to occur with flour dust and powdered sugar.
The iron powder had formed on the sheet as a result of the decomposition of the pentacarbonyl.
The preparation of very fine metallic powders by the decomposition of metal carbonyls strongly attracted the interest of scientists.
They established that such powders have very specific properties. Their particles are very small, just over a micron in diameter. For instance, iron powder can be obtained in the form of fluffy iron “wool” consisting of strong metallic chains. Deposited on a hot surface, carbonyls form a very strong and thin film on it. Such powders and films possess very valuable magnetic and electrical properties and this has gained them extensive usage in radio engineering and electronics.
Carbonyl powders are also attractive to powder metallurgy.