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A lot of the organometallics are rather... interesting compounds to work with. The most famous (among those who care, anyway) is tert-butyllithium or t-BuLi. It is the textbook example of a pyrophoric substance, demonstrated to pretty much every chemistry major as an air-sensitive chemical requiring special handling (Syringe and cannula transfers, gas-tight septa, argon/nitrogen blankets, that kind of thing). Despite the inherent hazards of the stuff, it's widely used in the industry (hence widespread notoriety) to add butyl groups to an organic molecule (most organometallics are useful for this kind of carbon-carbon bond formation), and you can buy it by the - airtight - gallon canister.

There are, surprisingly (except to most organic chemists), even more dangerous compounds in the average organic chemistry lab. Along the pyrophoric line, many of the multi-methyl-metallics are violently pyrophoric (even compared to BuLi), including trimethylaluminum, dimethylzinc, and dimethylmagnesium. All of these are also extremely poisonous (anything these compounds can do to your wonder-drug-in-progress, they can also do to various key structures in your own body), with dubious honors reserved for dimethylmercury. Because many of these, especially the organoalkaline compounds, react violently with water, they also automatically rate at least a 3 on the "reactivity" scale.

When leads me to wonder, because I wonder about these things from the safety of my office chair in a completely unrelated field; just how bad can it get? Specifically, is there any compound with an accepted use in laboratory or industrial application that is nasty enough to max out the entire NFPA-704 diamond? The closest I can find is trimethylaluminum, at a 3-4-3 (heath-fire-reactivity). T-BuLi is a 3-3-4. I can't find NFPA data on straight diazomethane (which may be because nobody in their right mind ever works with the stuff in its pure gaseous form; it's always used in a dilute diethyl ether solution, and even then is never sold or shipped that way) but it would probably be a finalist, as the gas is acutely toxic, autoignites at room temperature, and detonates on standing (something for everyone!). I'm thinking that these two inorganic families - light alkyl-metallics and organo-polyazides - would be the most likely candidates to produce a compound so toxic, so flammable, and so readily reactive, yet so interesting to chemistry, that the NFPA would see fit to rate it, and would give it highest honors.

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    $\begingroup$ There's t-BuOOH but one source (cited by the Wikipedia stub article) says it's 4-4-4 while another source says it's 1-4-4... $\endgroup$
    – BoltClock
    Feb 12, 2013 at 8:26
  • $\begingroup$ @BoltClock and other sources have it at different values… it depends how it is conditioned (in solvent, or adsorbed on porous substrates, …). As the question say, noöne in their right mind would sell it in a form where it is 4-4-4 (and noöne would probably buy it, because it would hardly be usable!) $\endgroup$
    – F'x
    Feb 12, 2013 at 8:37
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    $\begingroup$ Diborane is a pleasant 4-4-3. Seriously, that stuff is nuts. Nice example of 3c2e bonding though. Hydrazine is also 4-4-3. $\endgroup$
    – Richard Terrett
    Feb 12, 2013 at 10:36
  • $\begingroup$ I found a MSDS for diazomethane but it is not a very good one. However, all signs point to 4-4-4. $\endgroup$
    – Ben Norris
    Feb 12, 2013 at 11:39
  • $\begingroup$ Tert-butylhydroperoxide looks promising. One nitpick (which doesn't disqualify it as I didn't require it) is that it doesn't react strongly to water (it gets its 4 for reactivity from its strong oxidation qualities and its propensity as a peroxide to explode spontaneously). You still can't use water to fight this fire, but you can use foam. $\endgroup$
    – KeithS
    Feb 12, 2013 at 18:24

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Answering my own question based on the comments, tert-butyl-hydroperoxide is at least one such chemical. As stated on this MSDS from a government website, it's a 4-4-4, with additional special warning of being a strong oxidizer. The only thing that it does not do that could make the 704 diamond any worse is react strongly with water. It is in fact water soluble, though marginally, preferring to float on top (and therefore traditional water-based fire suppression is ineffective, but foam/CO2 will work).

If anyone else can find a chemical that, in a form that is used in the lab or industrially, is a 4-4-4 that is a strong oxidizer and reacts strongly with water, that's pretty much "as bad as it gets" and they'll get the check.

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  • $\begingroup$ There seems to be some question about the rating of tert-Butyl hydroperoxide. Perhaps in part because in practice it is never supplied or used undiluted? The NOAA page itself links to the ICSC which lists a 70% aqueous solution as 1-4-4-OX. $\endgroup$
    – feetwet
    Dec 13, 2016 at 2:51
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Diborane. NIOSH gives NFPA 4-4-4-W:

NFPA 704 4-4-4-W

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  • $\begingroup$ Pentaborane(9) has the same diamond, at least on Wikipedia. $\endgroup$
    – Mithoron
    May 3 at 17:02
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I think chlorine trifluoride deserves a mention

One of the issues with relying on published NPFA triangles to judge the answer to this question is that some of them don't seem to be very reliable given the known properties of compounds.

According to one of its suppliers ClF3 has a rating of 4-0-3. So it would not count very highly here. This seems about as inaccurate as describing dimethylmercury as only mildly toxic. I mean how in hell can it have a zero on the fire score and only a 3 on the toxicity score?

Here is how John D Clarke describes it in Ignition (his history of rocket fuels):

It is hypergolic with every known fuel, and so rapidly hypergolic that no ignition delay has ever been measured. It is also hypergolic with such things as cloth, wood, and test engineers, not to mention asbestos, sand, and water—with which it reacts explosively. It can be kept in some of the ordinary structural metals—steel, copper, aluminum, etc.—because of the formation of a thin film of insoluble metal fluoride which protects the bulk of the metal, just as the invisible coat of oxide on aluminum keeps it from burning up in the atmosphere. If, however, this coat is melted or scrubbed off, and has no chance to reform, the operator is confronted with the problem of coping with a metal–fluorine fire. For dealing with this situation, I have always recommended a good pair of running shoes.

So for example, while the compound itself is arguably not flammable, absolutely everything else is flammable in its presence. And given that those reactions tend to produce far more benign compounds like HF gas, it can hardly be considered as only a 3 on the toxicity scale either (though, perhaps, if you are dead already from the corrosive or explosive effects of its reactions, the toxicity is not a major concern).

To put it mildly, ClF3 is one of the most dangerous compounds that is commercially available on any reasonable metrics and damn those published NPFA triangles.

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  • $\begingroup$ True, there's also dioxygen difluoride that has 4 health,0 fire and 4 reactivity. It seems pretty accurate for this one,but chlorine trifluoride seems far more dangerous than a 3 on reactivity. $\endgroup$
    – Bruh Moments
    Feb 20, 2022 at 1:19
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    $\begingroup$ A chemical that reacts explosively with water, drenches everything in unquenchable fire, and not to mention is highly toxic, should definitely be treated as if it were 4-4-4. $\endgroup$
    – user16217248
    Jan 27 at 2:44
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Ethyl nitrite is rated 4-4-4 but it does not have anything on the white square. Pentaborane has a good 4-4-4-W. However it's not an oxidizer.

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  • $\begingroup$ Wikipedia's article on pentaborane rates it a 3-3-2; not sure what form it's in for that rating. Anyway, source? If you can back up your pentaborane rating you'll get the check; it may not be an oxidizer, but a 4 for reactivity plus a "no water" hazard means it'll do practically everything else but. $\endgroup$
    – KeithS
    May 4, 2015 at 16:26
  • $\begingroup$ As for ethyl nitrite, that's been used as a cold remedy for centuries (though OTC sale was banned by the FDA in 1980 as being "not safe or effective"), and is rated a 2-4-4 by NIST. Notwithstanding a few doozies from the medical community and a mentality in previous generations that "if it tastes bad/hurts/makes you puke, it must somehow be good for you", anything nasty enough to get a Health-4 rating from the chemists would be a pretty obvious poison over on the medical side. $\endgroup$
    – KeithS
    May 4, 2015 at 16:39
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    $\begingroup$ NIOSH rates pentaborane 4-4-2, no W noted. $\endgroup$
    – feetwet
    Dec 13, 2016 at 2:48
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It seems that diborane (see answer by feetwet) is the only compound rated with H4 F4 R4 code by NIOSH, though there are two substances lacking just a single point to join a 4-4-4-club:

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    $\begingroup$ Interesting about picric acid. Back in grad school, one of my fellow grad students worked in a lab that had a big bottle of picric acid on the shelf. Next to it was a big bottle of TNT. They wanted to make explosive thin film particle detectors. I saw those bottles and never visited that lab again. Thankfully, it was in another building. Later, they had an explosion of the “nitrogen tri-iodide” they had made (for potential use with their detectors) and were drying under suction and with stirring. Luckily, no detonation of the picric acid or TNT. $\endgroup$
    – Ed V
    Jul 25, 2022 at 22:12
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I cannot seem to find any more 4-4-4 chemicals (except monomethylhydrazine),except the ones that are shown above. I have found ones that are very close:

Nickel tetracarbonyl :(4 health,3 flammability,3 reactivity)

Dichlorosilane (4 health,4 flammability,2 reactivity,water hazard)

Diethyl ether peroxide (2 health,4 flammability,4 reactivity)

Dimethylcadmium (4 health,4 flammability,2 reactivity, water hazard)

Monomethylhydrazine

(4 health,4 flammability,4 reactivity)

Tert-butyl hydroperoxide

(4 health,4 flammability,4 reactivity, oxidizer)

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