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Most metals are composed of pure elements or are alloys (mixtures of pure elements.)

Are there metals that are composed of one type of molecule that is metallically bonded together?

The difference between a metal that is composed of one type of compound and an alloy is that in an alloy different elements are attached by metallic bonding but in a metal composed of one compound covalent or ionic bonding attaches multiple atoms together into compounds and then these compounds form metallic bonds with each other.

I have read that Gallium in its pure elemental state forms pairs which are then metallically bonded together which sort of is what I'm wondering about except that I'm wondering if metals can form out of more complicated compounds.

I would also expect there to be a lot of compounds which on their own do not form metals but do not degrade the metallicity of alloys they are put in very much.

I think a more specific and low level question might be: are there covalent compounds that can make metallic bonds with other things?

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    $\begingroup$ Metallic bond kinda contradicts the very idea of molecules (it is hard to form covalent bonds when you are immersed in a sea of electrons). Gallium with its "almost molecules" is likely the best we can get. Also, you might want to think of large metal clusters. And then there are steel (which contains carbon, which is not a metal on its own) and other alloys containing some non-metallic elements. $\endgroup$
    – Ivan Neretin
    Apr 6, 2016 at 20:40
  • $\begingroup$ How do you feel about the various allotropes of selenium? $\endgroup$
    – Jon Custer
    Apr 6, 2016 at 21:05
  • $\begingroup$ related chemistry.stackexchange.com/questions/35278/… $\endgroup$
    – Mithoron
    Apr 6, 2016 at 23:32
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    $\begingroup$ Organic metals may be a common reality in the not-too-distant future. $\endgroup$
    – Nicolau Saker Neto
    May 8, 2016 at 1:43
  • $\begingroup$ @NicolauSakerNeto I think you should expand this into an answer since the implication of Heeger's discoveries are that the answer is "yes". $\endgroup$
    – matt_black
    May 8, 2016 at 9:37

5 Answers 5

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According to A Conducting Crystal Based on A Single-Component Paramagnetic Molecule, [Cu(dmdt)2] (dmdt ) Dimethyltetrathiafulvalenedithiolate) J. Am. Chem. Soc., 2002, 124 (34), pp 10002–10003 :

Single-component molecular metals should greatly extend the development of new types of molecular conductors. For example, while the first metallic molecule-based magnet has been reported recently, [reference 8] it may be possible to create such metallic molecular magnets from single-component molecules containing paramagnetic ions such as Cu2+ and Co2+.

Reference 8 is Coexistence of ferromagnetism and metallic conductivity in a molecule-based layered compound Nature 408, 447-449

The temperature dependence of the resistance exhibits a metallic behaviour over the entire temperature range that we investigated (down to 2 K).

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Sort of, if you define metals as substances that exhibit some metallic behaviour

Metallic elements are, well, the metals. But other substances can exhibit many of the properties of those metals.

One well-known example if what you get when you dissolve a lot of sodium in liquid ammonia. Beyond a certain concentration, a new metallic-like phase if formed where electrons are solvated in the liquid (which contains ammonia molecules). The bronze, metallic colour of this phase is seen in this Periodic Video.

Many metallic properties are dependent on the delocalisation of electrons. Some conjugated polymers can have delocalised electrons and metallic levels of conductivity when doped. Whether these should count as metals is debatable but they do exhibit some metallic properties. And they won a Nobel prize for Heeger, et al. See Wikipedia for some discussion.

So there are molecular systems with some metallic properties, though their bonding is not necessarily because of the electron delocalisation.

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Yes

Adding to the other examples, there is an elemental diatomic metal.

No that's not a typo.

I'm sure you learned your diatomics in school: H2, N2, O2, F2, Cl2, Br2, and I2. They probably gave you the rather awkward acronym "HOFBrINCl" as well. In college you may have learned that at high enough temperatures S2 and possibly even P2 can exist in equilibrium with other species.

But at STP there's still one missing from that list, right between Cl2 and Br2. That missing diatomic is gallium metal. In fact, the covalent Ga2 pairs are why gallium melts at such a low temperature.

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Yes mercury can form an amalgam with ammonium that is $\ce{H3N-Hg-H}$ although it decomposes at room temperature.

See https://en.wikipedia.org/wiki/Amalgam_%28chemistry%29#Ammonium_amalgam

It might be possible to do this with more chemicals and mercury.

Theoretically cations like Ammonium might have a metallic nature but I wouldn't know the details of that.

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Under sufficiently high pressure ammonium, $\ce{NH4}$, could form a metal. Such a metallic species may be present in the interiors of Uranus and Neptune [1](https://doi.org/10.1093/mnras/114.2.172).

Reference

1. M.F.M. Bernal, H.S.W Massey, "Metallic Ammonium", Monthly Notices of the Royal Astronomical Society V114(2) (April 1954), Pages 172–179; doi: 10.1093/mnras/114.2.172 (open access).

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