[paul cotter (OP)]

In Vogel's organic preparations(can't find my copy so not sure of edition) mention is made of the use of this substance as a synthon with the warning that only experienced chemists would use it, due to it's hazards. I have been unable to discover what those hazards are. I know the free acids are known for having a bad smell but many oxidisations such as the Swern, Corey Kim and Moffatt use or produce dimethyl sulphide which would stink out any lab without a good fume cupboard.

[Origin]

Here is the SDS for the chemical:  https://www.sigmaaldrich.com/US/en/sds/aldrich/225495?userType=anonymous 
I noticed one of it's attributes is it spontaneously ignites when exposed to air, so that could be a issue....

[paul cotter (OP)]

Interesting, I have never seen a reference to it being pyrophoric before. I suspect this may apply to the 1-4 dioxane complex mentioned and not the salt, but I could be wrong. Even so many reagents regularly used in an organic lab are pyrophoric such as the ubiquitous butyl lithium and even Raney nickel can inflame if allowed to dry out. 

[chiralSPO]

It can also decompose, releasing significant quantities of CO gas.

I wouldn't be surprised if the complex itself were pyrophoric (it is formally Fe(-II), or Fe^2-, which is ready to dump electrons into the first oxidant it can find)

Another option is that, through slow decomposition and loss of CO, it may become enriched with Fe(0) nanoparticles which are pyrophoric (similar to those formed by the decomposition of ferrous oxalate.)

I have never worked with the compound in question, but I have plenty of experience working with cobalt carbonyls, including compounds containing a tetracarbonylcobaltate(-I) anion. Although I did not observe any samples catch on fire, these compounds readily reacted with oxygen in the atmosphere, and I do recall one sample of finely divided dicobalt octacarbonyl which began smoking immediately upon (accidental) exposure to air.

[paul cotter (OP)]

Thank you, chiralSPO, I have not seen you on the forum for quite some time and it is a pleasure(for me) to interact again. Going on memory, the only known metal-carbonyl-hydrides are those of iron and cobalt and they are unstable gases with allegedly bad aromas. Did you ever acidify the tetracarbonyl cobaltate? I have an interest in what is the worst possible chemical odour!!  Iron tetracarbonyl hydride is said to be toxic but I doubt if it is in the same league as nickel tetracarbonyl.

[Bored chemist]

About a million years ago as as student I did some practical with a ferrocene derivative. I'm fairly sure it was a carbonyl  ferrocene complex.
It involved reduction using sodium amalgam.

As far as I recall the product smelled like camphor- as does ferrocene itself.

[chiralSPO]

Thank you, chiralSPO, I have not seen you on the forum for quite some time and it is a pleasure(for me) to interact again.

I had some "real life" duties that took priority over this forum for a while. I may return more regularly. I'm glad to hear that my return is appreciated (at least by some). :-)

Did you ever acidify the tetracarbonyl cobaltate? I have an interest in what is the worst possible chemical odour!!  Iron tetracarbonyl hydride is said to be toxic but I doubt if it is in the same league as nickel tetracarbonyl.

I did often quench these reactions and clean them up with a mix of aqueous hydrochloric acid and isoproanol. I don't recall ever noticing a particularly strong odor when doing so, but I also went to pretty extreme lengths to avoid inhaling any vapors (as the reaction also released carbon monoxide).

About a million years ago as as student I did some practical with a ferrocene derivative. I'm fairly sure it was a carbonyl  ferrocene complex.
It involved reduction using sodium amalgam.

As far as I recall the product smelled like camphor- as does ferrocene itself.

I suspect it could have been CpFe(CO)₂⁻, aka Fp⁻. This can be prepared by the reduction of Fp₂ by extremely strong reducing agents.

I did this once as well, though I used NaK instead of the amalgam. I don't recall the complexes having a smell, though cracking the dicyclopentadiene to make the Fp₂ was quite a smelly endeavor!