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Wikipedia's The nature of metallic bonding; In 2D says:

Graphene is an example of two-dimensional metallic bonding. Its metallic bonds are similar to aromatic bonding in benzene, naphthalene, anthracene, ovalene, etc.

Note: I've left the block quote here for continuity, but it's convincingly argued in comments and answer(s) that "metallic" is not the best way to describe bonding in graphene.

Most 2D materials currently studied tend to fall into two categories;

(as an aside, these 2D materials usually also form van der Walls stacks analogous to graphite)

Question: Of these or others, are there any 2D materials whose bonding can be best described as metallic rather than covalent?

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  • $\begingroup$ nature.com/articles/…. ; pubs.acs.org/doi/10.1021/acsnanoscienceau.2c00017 $\endgroup$
    – Ronith
    Commented May 29 at 2:38
  • $\begingroup$ @Ronith "...which can range from metallic to semiconducting..." In my experience, when 2D material scientists talk about the experimental electronic collective behavior in this way (either probed optically or electrically), it is not the same thing as talking about the definition of the interatomic bonding. You can go from insulating to semiconducting to metallic behaviors for example simly by doping, and that does not substantially change the nature of the majority of the bonds. So a simple search for "2D+metallic" isn't enough here. $\endgroup$
    – uhoh
    Commented May 29 at 3:37
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    $\begingroup$ This Wikipedia article is lame. Graphene isn't metallic even is one would only considers its band structure. Even calling it "material" is a misnomer. People have some tendency to call everything that conducts electricity well to be metallic, or every molecule a material, apparently. $\endgroup$
    – Mithoron
    Commented May 29 at 15:53
  • $\begingroup$ @Mithoron yes I'm coming around to that conclusion. $\endgroup$
    – uhoh
    Commented May 30 at 1:59

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Yes, there are many of them.

Some examples from Wikipedia:

The presence of the Fermi level and electrons in the conduction band of h-MBenes (MBenes in hexagonal conformation ), cause the intrinsic metallic character of MBenes 7 . Thanks to this metallic character and therefore a significant delocalization of electrons, MBenes have excellent electrical conductivity , an interesting character for the different applications cited in the next major part 5 , 7 , 8 .

With a high electron density at the Fermi level, MXene monolayers are predicted to be metallic.[1][53][54][55][56] In MAX phases, N(EF) is mostly M 3d orbitals, and the valence states below EF are composed of two sub-bands. One, sub-band A, made of hybridized Ti 3d-Al 3p orbitals, is near EF, and another, sub-band B, −10 to −3 eV below EF which is due to hybridized Ti 3d-C 2p and Ti 3d-Al 3s orbitals. Said differently, sub-band A is the source of Ti-Al bonds, while sub-band B is the source of Ti-C bond. Removing A layers causes the Ti 3d states to be redistributed from missing Ti-Al bonds to delocalized Ti-Ti metallic bond states near the Fermi energy in Ti2, therefore N(EF) is 2.5–4.5 times higher for MXenes than MAX phases.[1]

https://en.wikipedia.org/wiki/MXenes#

https://fr.wikipedia.org/wiki/MBenes

Note that all of them, including graphene, also have covalent bonds.

The bonding in a monolayer of metallic element is possibly described as only metallic, but those need additional support from a substrate.

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  • $\begingroup$ This is really interesting - I have not heard of these materials before, thank you! I will now dig in and do some reading, but can you help me to begin to understand what "...all of them, including graphene, also have covalent bonds." means? Does graphene have both metallic and covalent bonds (i.e. two different kinds, this doesn't seem likely) or do you mean that all of the bonds can be considered as metallic and also covalent? Thanks! $\endgroup$
    – uhoh
    Commented May 29 at 12:13
  • $\begingroup$ @uhoh I mean both kinds of bonds. Each carbon atom is coordinated to 3 others in graphene. If there weren't any covalent bonds, the most compact arrangement with 6 neighbors would be formed (if it somehow was still 2D). $\endgroup$
    – Paul Kolk
    Commented May 30 at 0:04
  • $\begingroup$ Well that's interesting, but citation needed! My understanding is that all bonds in a graphene sheet are the same, cf. Each atom in a graphene sheet is connected to its three nearest neighbors by σ-bonds and a delocalised π-bond They certainly all have exactly the same length - the free-standing sheets are flat and to my knowledge the atoms lie on a nice, hexagonal honeycomb net. $\endgroup$
    – uhoh
    Commented May 30 at 1:48
  • $\begingroup$ So actually maybe the Wikipedia block quote in my question is wrong, and graphene's bonds are better described as covalent. Otherwise, ya it might collapse into a hexagonal close packed arrangement, flat or 3D (just for example). Perhaps I should revise my title and remove the "besides graphene" to avoid confusing future readers (update: I've just done so and added a note in the question explaining why) $\endgroup$
    – uhoh
    Commented May 30 at 2:02
  • $\begingroup$ @uhoh On the one hand, You are saying: "all bonds in a graphene sheet are the same", but then (correctly) mention $\sigma$ and $\pi$-bonds, so two types. $\endgroup$
    – Paul Kolk
    Commented May 30 at 9:27

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