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For my contribution to Sciencegeist’s toxic blog carnival, I decided to write about a decidedly enigmatic compound. Nickel tetracarbonyl is a transition metal complex but also a foul-smelling (and, given the context of this blog post, naturally highly toxic) gas. These are not generally two molecular properties that coincide. It also forms quite easily when nickel metal comes into contact with carbon monoxide.
Having not personally worked with it, I nevertheless respect and admire nickel tetracarbonyl from afar. However, this is a compound which can provoke intensely personal reactions from people who have had the opportunity to get a little more hands-on. I came across possibly one of the most vivid of these in our Chemistry in its element podcast series when Bernie Bulkin described his initiation into a lab working on metal carbonyl complexes:
‘One of the first things I was given to read when I started was the summary of the toxicological effects of nickel carbonyl. I learned, with some concern, that at 30ppm it was certainly fatal, and even a significantly lower dose of 3ppm caused death in 50% of a group of test animals.
‘When you breathe it in, it decomposes, giving you a dose of carbon monoxide and depositing some nickel on your lungs. If you survive the first few hours, the nickel causes a form of pneumonia, coughing, breathlessness, extreme fatigue. This lasts for several days, often resulting in cardiovascular or renal failure and death. I was relieved to find that the safety precautions in the lab were extremely rigorous.’
The concept of receiving a bolus dose of carbon monoxide – deadly enough in its own right as described in Patrick’s blog yesterday – plus the added spice of nickel-plated lungs, was enough to imprint an instant respect for the compound in my mind.
So why on Earth might we want to make or use this compound, given its extreme potential to cause harm?
Nickel as a metal is industrially important – it is hard, shiny and reasonably resistant to corrosion (except by carbon monoxide of course…). Alloyed into steel or plated over the surface it endows the metal with these useful properties too, so much so that it was used to make coins (before being largely replaced by iron, which is cheaper and doesn’t cause the same kind of skin irritation that some people experience when handling nickel). Some coins, particularly the US five-cent pieces known as ‘nickels’, still contain nickel alloyed with copper or plated on the surface of a steel blank.
Nickel’s hardness and corrosion resistance also makes it ideal as the basis for superalloys used to make jet engine turbine blades. These are generally grown as single crystals of the metal for optimum performance at the high temperatures and force loadings of a working jet engine.
So where does nickel carbonyl fit in?
Nickel is rarely found in ores on its own – it is usually combined with its transition metal neighbours iron and cobalt. In fact, the name nickel comes from colloquial German for ‘devil’ (think of ‘Old Nick’ in English folklore) and cobalt derives from the word for a gremlin or hobgoblin, reflecting their role as annoying impurities in iron ores. So a method for separating the metals would not only deliver the desirable nickel, but improve processes for purifying iron and cobalt as well.
Having discovered nickel carbonyl by accident, Ludwig Mond – a German chemist – found that nickel reacts with carbon monoxide much more quickly than does either iron or cobalt. As a savvy businessman, he realised the potential of this observation and, in the late 19th century, developed it into the Mond process for extracting nickel from mixed ores. Reacting impure nickel with CO releases it as gaseous nickel carbonyl and leaves behind the impurities. The nickel can then be reclaimed by heating the complex until it decomposes. This process is still used when the purest nickel (greater than 99.99% pure) is required.
From a research chemistry point of view, nickel complexes form a variety of useful catalysts. Many of these are prepared from nickel carbonyl in some form, owing to the ease of displacing the carbonyl ligands. However, the chemist aspiring to prepare such catalysts would be well advised to seek out alternative sources of nickel in which someone else has already done the carbonyl substitutions – nickel plated lungs and death by suffocation or pneumonia is certainly not to be recommended…