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.


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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 Jul 27 '16 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 Jul 28 '16 at 11:34
  • $\begingroup$ @NicolauSakerNeto - are elements diagonal from each other on the periodic table always related in this way (cation size)? $\endgroup$ – Dissenter Jul 28 '16 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 Jul 29 '16 at 12:00
  • $\begingroup$ see this:-chemistry.stackexchange.com/questions/34099/… $\endgroup$ – Nilay Ghosh Jul 30 '16 at 4:56

The crystal structure of calcium carbide (Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 0-08-037941-9.) 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 $-Ca-C\equiv C-$ polymeric chains.

  • $\begingroup$ Can somebody make my reference a linkable one? The old link died, I tried to use a reference from the Wikipedia article on calcium carbide (ref. 11), but the reference comes out not linkable. $\endgroup$ – Oscar Lanzi Nov 12 '17 at 23:05

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