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According to my professor, magnesium and lithium form covalent bonds with carbon but calcium does not. He did not elaborate.

Why is this so? The electronegativities are:

Mg — 1.31
Li — 0.98
Ca — 1
C — 2.55

So a $\ce{C-Mg}$ bond would have an electronegativity difference of 1.24 and a $\ce{C-Li}$ bond would have an electronegativity difference of 1.57. Both are considered covalent. So how come a $\ce{C-Ca}$ bond wouldn't be covalent? There exists an electronegativity difference of 1.55 — comparable to the difference in the $\ce{C-Li}$ bond.

I know that Mg likes to be tetra-coordinated (Grignards) but I don't think this has much to do with this discussion. So, what makes Mg and Li special compared to Ca?

Summary of possible explanations others have come up with

  1. $\ce{Mg^2+}$ has an ionic radius 72 pm, $\ce{Ca^2+}$ has an ionic radius of 100 pm, and for $\ce{Li+}$, it's 76 pm. So perhaps the greater charge density of $\ce{Mg^2+}$ and $\ce{Li+}$ allow them to pull electrons toward themselves a bit more, thus making the bond more covalent in character? That being said, $\ce{Li+}$ has a single positive charge while $\ce{Ca^2+}$ has a double positive charge, and I haven't calculated the actual charge densities for any of these ions so I don't think we can come to a definitive conclusion here.

http://www.wiredchemist.com/chemistry/data/metallic-radii

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    $\begingroup$ related en.wikipedia.org/wiki/Group_2_organometallic_chemistry Covalent that, ionic this, is sort of silly. On the other hand article I linked does not mention calcium Griniard analogues at all - organocalcium chemistry looks like dwarf in comparison with organomagnesium. $\endgroup$
    – Mithoron
    Commented Jul 27, 2016 at 23:21
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    $\begingroup$ Magnesium forms a rather smaller cation, which would likely be better at polarising the fluffy electron cloud of the would-be carbanion and forming a more covalent bond. Magnesium has a diagonal relationship with lithium, too, which also forms a very small cation and has a significant amount of covalent bonding to carbon in organolithiums. $\endgroup$
    – Nicolau Saker Neto
    Commented Jul 28, 2016 at 11:34
  • $\begingroup$ @NicolauSakerNeto - are elements diagonal from each other on the periodic table always related in this way (cation size)? $\endgroup$
    – Dissenter
    Commented Jul 28, 2016 at 16:05
  • $\begingroup$ As far as I'm aware diagonal relationships are only really invoked for elements in the second and third periods, and I think it has to do in part with the anomalously small size of the 2p orbital. $\endgroup$
    – Nicolau Saker Neto
    Commented Jul 29, 2016 at 12:00
  • $\begingroup$ This question is similar to: Why do Magnesium and Lithium form *covalent* organometallic compounds?. If you believe it’s different, please edit the question, make it clear how it’s different and/or how the answers on that question are not helpful for your problem. $\endgroup$
    – Paul Kolk
    Commented Jul 31 at 16:30

1 Answer 1

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The crystal structure of calcium carbide gives a subtle hint of covalent bonding. It looks like a rock salt structure, but note that the carbon atoms in the anions have their lone pairs directed towards the calcium atoms and so are favorably oriented for covalent overlap. The structure might be considered either a rock salt structure or a series of parallel $\ce{–Ca–C#C –}$ polymeric chains.

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    $\begingroup$ Sorry ! The question was not related with $\ce{CaC2}$. It was about the nature of the bond between Magnesium and Carbon $\endgroup$
    – Maurice
    Commented Nov 25, 2023 at 17:05
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    $\begingroup$ I mentioned calcium carbide to illustrate that the bonding does not go from covalent to ionic in one blow just because we go oe period down in the group. MgC2 has those chains too, albeit stacked differeny. $\endgroup$
    – Oscar Lanzi
    Commented Nov 25, 2023 at 19:34

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