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I came across a question in which the following reaction was taking place:

$$ \ce{Mg4C3 + H2O -> CH3CH \bond{#} CH} $$

Each $\ce{Mg}$ atom must be present in a +2 oxidation state, thus the total charge of the cation is +8. We thus have a $\ce{C3^{8-}}$ anion. The only structure of this $\ce{C3^{8-}}$ anion I could come up with was three carbons attached to each other in a linear fashion, with the terminal carbons each bearing a -3 charge and the central carbon bearing a -2 charge.

From this anion I would have assumed that the product formed upon hydrolysis is propane, but the product formed in the above reaction is propyne, just like when $\ce{Mg2C3}$ undergoes hydrolysis.

I'm guessing that the explanation for propyne being formed is the actual lattice structure of $\ce{Mg4C3}$, but I couldn't find any literature referencing this reaction or the structure of $\ce{Mg4C3}$.

The question is taken from:

  • V.K. Jaiswal, s-block elements, Problems in Inorganic Chemistry, Thirteenth Edition, Page 331, question 36
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  • $\begingroup$ @Poutnik agreed, but if the 3 carbon atoms don't form a single anion, how will we get a product which consists of 3 interconnected carbon atoms? $\endgroup$ – Ashish Ahuja Nov 12 '20 at 18:08
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    $\begingroup$ I have just quoted Wikipedia, I have not claimed anything. Note Mg2C3, not Mg4C3. $\endgroup$ – Poutnik Nov 12 '20 at 18:23
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    $\begingroup$ Typo? Should it be Mg2C3? $\endgroup$ – Waylander Nov 12 '20 at 18:41
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    $\begingroup$ Yeah, seems a blatant mistake. $\endgroup$ – Mithoron Nov 12 '20 at 18:46
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    $\begingroup$ I don’t know if this is enough to prove that this is not a typo but this paper mentions Mg4C3 $\endgroup$ – Robin Singh Nov 13 '20 at 3:34

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