I was at dinner the other day, and sitting across from me was a Mechanical Engineering grad student; we got to talking about his research, which studies combustion. In particular, he mentioned that one of the things they study is carbon monoxide (necessitating CO detectors all over the lab!). He had an interesting story of how when they introduce CO into the combustion system, their viewing windows filmed up with an orange coating very quickly. This could be traced back to the source of the CO gas, in a high-pressure steel canister.
CO is a very good ligand to metals and metal ions — this is in fact why it’s so dangerous, as it binds very tightly to the iron in the haemoglobin in your blood, and doesn’t fall off. With no room for oxygen to bind and get distributed through the body, our cells quickly run out of metabolic pathways. Anyway, CO binds tightly to a lot of metals; in particular, to the iron in the steel canister. When five CO molecules have attached to a single iron atom, the electronic shell is filled, and the complex is free to separate from the bulk metal. The “iron carbonyl” so obtained can actually be generated as a bulk compound, a dense liquid. However, there’s nothing to bind the complexes to each other save for the weak Van der Waals interaction, so the liquid is quite volatile. In my dinner companion’s experiment, enough iron carbonyl was forming and staying in the vapor phase that it entered the combustion chamber with the stream of CO gas. Once there, the carbon monoxide was fully oxidized to carbon dioxide, and the iron converted to rust, which was the orange coating on the viewports.
This sort of carbonyl chemistry is not unique to iron; many metals will form such complexes: nickel carbonyl requires only four CO molecules to saturate, whereas chromium and others need form a more standard six-coordinate geometry. Many metal carbonyls form colored crystals (transition metals are a great way to get colored compounds), and some are liquids; nickel carbonyl boils at 43 degrees C!
Looking at these structures, one might be concerned that thermal decomposition would release dangerous carbon monoxide. However, it turns out that these (generally highly toxic) compounds are more dangerous than that: they are great ways to get unoxidized metal atoms into living tissue, as the aggregate complex is electrostatically bland and can diffuse readily through many materials. Once in the body, the metals themselves are quite active, both catalytically and in terms of disrupting biological structures such as DNA. The wikipedia page for nickel carbonyl claims that it is immediately fatal to humans at concentrations near 30 ppm.
All in all, a fun and fascinating branch of chemistry, bringing unexpected behavior from relatively commonplace materials. Just be careful and use proper conditions and technique.