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Wandering Through the Hydrides

If you want to get a feel for chemistry, one way might be to wander through the periodic table, picking one particular type of compound and seeing how things change as you go from element to element. (That’s a good part of how Mendeleev figured the whole thing out, actually). But you’ll want to pick carefully. Chlorides, for example, are rabble, as Primo Levi put it memorably. He was right: one chloride is often very much like another, even when the elements involved are very different – it’s as if they’ve been pulled down to a lowest common denominator. I make some exception for the beautiful green of nickel (II) chloride, or the startling metallic purple of the anhydrous chromium (III) salt, but really, if you can’t get something neat looking from the nickel or chromium compounds, you’re really in a bad way. Most other chlorides are nondescript white powders. Boring!
Pick hydride instead. Hydrides are, if anything, a bit too exciting sometimes, since they tend to be rather reactive. With water, the usual reaction of a metal hydride is to strip off a proton immediately, giving you the hydroxide of the metal and plenty of lively hydrogen gas bubbling off. Sometimes the whole process is joyful enough for the whole mess to burst into flame.
That’ll happen to you with the alkali metal hydrides, over on the left-hand side of the table. Organic chemists the world over know sodium hydride the best. It’s a fine strong base – kinetically rather slow, but it deprotonates and spares not. By contrast, you hardly ever see much lithium hydride around. Potassium hydride makes an appearance every so often, though, to muttered curses, since it’s usually stored as a slurried suspension in mineral oil and is correspondingly painful to weigh and dispense. It’s lively stuff, too, and will set things on fire for you with great thoroughness. The higher alkali hydrides are things I’ve never seen, and I have no desire to, if their ferocity steps up like potassium does versus sodium.
The next row over (the alkaline earths) feature a compound that gets used every so often in the lab, calcium hydride. It’s a good drying agent, because of that water reactivity, and solvents are distilled from a spoonful or two of it to make them anhydrous. But has anyone ever seen or handled magnesium hydride? I sure haven’t. It’s one of the things that the hydrogen-storage people mess around with, apparently, but you just don’t come across it in an organic chemistry lab. Ditto for the other alkaline earths: barium hydride? Never seen it or even thought about it. I don’t have access to Scifinder for the time being, since I have no desire for a second mortgage on my house just now, but I see from the web that apparently some people are using the stuff. Maybe it has a great future in organic synthesis, but it sure has no past.
This same pattern holds as you go across the elements. You come across some things that are used all the time (boron hydride, better known as borane), and some that are encountered once in a while (aluminum hydride, copper hydride). There are some well-studied chemicals that are just too reactive and nasty for most people, like phosphine or stannane, and some that are too reactive and nasty for any sane person at all, like mercuric hydride. Some of the nasty ones that are used much more outside of organic chemistry. People who do semiconductor work, for example, know all about the arsenic and germanium hydrides, for example, while few organic chemists have ever touched them.
And then there are some that I’m not sure anyone ever messes with at all. They don’t seem to be particularly worse than their neighbors; they just sort of seem to be overlooked. Chemists in the audience – ever thought about titanium hydride? Me neither. Chromium hydride? Never once, until this evening (I wonder what color it is?) These are simple compounds, but even among the simple ones you keep finding all these streets that no one ever walks down. . .

23 comments on “Wandering Through the Hydrides”

  1. Jose says:

    Several years ago, I had similar thoughts. CsH and RbH have never been prepared, or reported other than computational/theoretical studies…. I feel pretty certain it was tried with rather frightening results!

  2. Jal-Frezi says:

    The only lab fire I’ve ever had was down to that hideous potassium hydride sludge (mixed with my own naivety and a sink with traces of waste acetone), but there was a JOC note last year about an air-stable, solid 1:1 mix of KH and paraffin that might make the sludge a thing of the past.
    The ref is JOC(2006),71(23), 8973
    Has anyone tried this stuff and more importantly I wonder if anyone sells it…

  3. schinderhannes says:

    Hi Derek,
    you forgot to mention carbon hydride, a very dangerous highly flammable gas (some also call it methane or natural gas 🙂
    Plus all ist higher hómologues, the hydro-carbons.

  4. Wavefunction says:

    Nitrogen hydride is also not entirely uninteresting

  5. eugene says:

    Yeah, I know what you mean Jal-Frezi, my potassium hydride came as a sludge too, so I washed off all the stupid mineral oil with hexanes several times to get that pure white and dry powder that I can now easily weigh out and add to my mixtures. In the glove box of course. I prefer my organolithiums as white solids too. A little pyrophoric though… But (more) air stable KH would be great to have of course.

  6. aa says:

    lets not forget the hideous chalcogen hydrides… H2Se must have a disgusting odor if it is anything like MeSeH… Has anyone ever used this or H2Te?

  7. Hap says:

    Has anyone made KH-impregnated silica gel? Someone had reported (and patented, I think), BuLi on silica gel as a relatively stable and manipulable compound, and I would think that you could do KH on silica as well.
    Taber reported paraffin-suspended compounds as well – I repmember because silver carbonate suspended in paraffin was reported in the LaClair synthesis(?) of hexacyclinol/whatevertheycalleditol.

  8. Wavefunction says:

    I doubt if anyone who has smelt H2Te has lived to tell the tale.
    Consider the following charming conversation between Linus Pauling and Matthew Meselson:
    LP: Well, Matt, you know about tellurium, the group VI element below selenium in the periodic chart of the elements?
    MM: Uh, yes. Sulfur, selenium, tellurium …
    LP: I know that you know how bad hydrogen sulfide smells. Have you ever smelled hydrogen selenide?
    MM: No, I never have.
    LP: Well, it smells much worse than hydrogen sulfide.
    MM: I see.
    LP: Now, Matt, Hydrogen telluride smells as much worse than hydrogen selenide as hydrogen selenide does compared to hydrogen sulfide.
    MM: Ahh …
    LP: In fact, Matt, some chemists were not careful when working with tellurium compounds, and they acquired a condition known as “tellurium breath.” As a result, they have become isolated from society. Some have even committed suicide.
    MM: Oh.
    LP: But Matt, I’m sure that you would be careful. Why don’t you think it over and let me know if you would like to work on the structure of some tellurium compounds?
    I would love to smell some H2Po if it exists.

  9. DRogers says:

    And let’s not forget the infamous dihydride of oxygen…
    “Dihydrogen Monoxide (DHMO) is a colorless and odorless chemical compound, also referred to by some as Dihydrogen Oxide, Hydrogen Hydroxide, Hydronium Hydroxide, or simply Hydric acid. Its basis is the unstable radical Hydroxide, the components of which are found in a number of caustic, explosive and poisonous compounds such as Sulfuric Acid, Nitroglycerine and Ethyl Alcohol.”

  10. Stephen says:

    Alkali metal rxns w/water are a fun trend to watch.

  11. kiwi says:

    KH is a bit of an unpredictable beast, sometimes the dry powder is fine, sometimes its rather angry. its pretty easy to just strip it off in the flask with 2x pentane washes, vac it down and weigh. probably could make the solid paraffin dispersion by that method quite quickly/cheaply

  12. Spiro says:

    To Jal-Frezi about KH in paraffin
    >Has anyone tried this stuff and more importantly I wonder if anyone sells it…
    I tried that stuff : it does work very well and is user-friendly. You can scale up safely, and even better, scale down to as little as a 0.1 mmol scale reaction without a dry box. Weigh your 5 mg or 5 kg of KH (in paraffin) in air and put it in your reactor, argon-flush, and add your dry solvent.
    Now, I wish I had all my air-sensitive reagents in paraffin….
    By the way, it is sold by Wilmington Pharma Tech.

  13. Tot. Syn. says:

    Have any of you seen the prep for NaH? I thought about this a few weeks ago, and had a look on Wikipedia, where they describe the process as bubbling hydrogen gas through liquid sodium. Erk!!! I’d presume the procedure for KH is the same, so I can definitely understand why CsH and RbH aren’t available from Aldrich…

  14. > they describe the process as bubbling hydrogen gas through liquid sodium
    One wonders why they don’t have undergrad students do that in lab…

  15. Harry says:

    Well, I can talk about my experiences making Sodium Amide in 20 kg batches. Much fun to be had in dropping 2 lb bricks of Sodium metal into a stirred reactor full of liquid ammonia with just a touch of Ferric Nitrate for a catalyst. Wait til the Hydrogen evolution dies down and chuck in another until you’ve added the required amount. Once thats done you simply heat a bit to drive off the excess ammonia and dump the dry Sodamide into antistatic bags and seal. I guess you might consider Sodamide to be a sort of mixed hydride.
    I also made Lithium Amide in the same way, and like the hydrides, Lithamide is tame compared to Sodamide. I’d never ever want to make Potassium Amide.
    Ahh… good times… good times.
    My $0.02, YMMV.

  16. milkshake says:

    I have seen old lab preparation for NaH, it used liquid Na dispersion in hot xylenes and hydrogen.
    NaH is made in huge quantities for steel metalurgy use (without the solvent I think). The trick is how to prepare a fluffy NaH powder rather than hard chunky stuff that is difficult to dig out from the reactor. Impure hydrogen obtained from petrochemical precesses supposedly gives nicer NaH product than electrolytically-pure hydrogen.

  17. Yogi says:

    This post on hydride is filled with great insights. You’re absolutely right about some of the hydride the semi-conductor industry uses for metal-vapor deposition. I found out the other day on Wikipedia that a new agent being used in semiconductor is isobutyl germane (iso-BuGeH3) which appear to be an easy to handle liquid. I was wondering what kind of reducing power this would have and potential selectivity vs something like DIBAL. Has anyone seen any use of this organo-germanium in chemistry? I’m really curious

  18. Jose says:

    Yogi- via google, strange little review of organogermanium chemistry.

  19. Yogi says:

    Thanks a lot Jose I’m going to have a look at that review. Not to many schemes…

  20. Morten says:

    LiD packs quite a punch in the Teller-Ulam configuration. I know lithium deuterium technically isn’t a hydride but you organic types rarely seem to care about isotopes.
    My only question now is why do they call it an H-bomb when it’s mainly deuterium and tritium?

  21. ****** says:

    From: High School Chemistry and Math Teacher
    Question: Can metal hydrides be used to reduce (no pun intended) or mitigate (or treat) galvanic corrosion in brass — particularly along solder joints? What hydrides could be delivered to a corroded joint and yield a reduction of oxidized metal species of Ag, Pb, and Zn?
    CuH2 + ZnO (etc.)
    I am presently treating corrosion with polysiloxanes, I have also considered graphite impregnation.

  22. dave weston says:

    I just found this by accident. I made Rb Abnd Cs hydrides for my PhD in about ’93. Direct combination. Pretty easy really.
    They tend to detonate slightly in air though.
    Nopw play with MgH2. Cool.

  23. Doug Taber says:

    KH 50% by weight in paraffin is now available from SigmaAldrich, product #708860. As mentioned in a post above, this material is easy to handle both on a small scale and on a large scale.

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