The geometry of aluminium carbide is attached. Why isn't it completely linear (in accordance to VSEPR), but bent on the Al atoms?
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$\begingroup$ @Marko Where did you get such a structure? Aluminium carbide isn't molecular and in its structure there are no multiple bonds. $\endgroup$– MithoronFeb 1, 2015 at 11:56
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2$\begingroup$ That structure (originally from ChemSpider) is incorrect. $\endgroup$– Fred SeneseFeb 1, 2015 at 12:44
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$\begingroup$ It was from ChemSpider, I am not so imaginative. $\endgroup$– EJCFeb 1, 2015 at 16:13
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1 Answer
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That structure is incorrect. Several online chemical distributors have similar structures posted in their entries for aluminum carbide; so does PubChem! Some sources show the structure as linear; some show it as bent.
That structure basically assumes that aluminum forms primarily covalent bonds with an "incomplete octet" in VSEPR structures, rather like boron. But aluminum atoms are a lot larger and more metallic than boron atoms, and its compounds will have comparatively more ionic character. $\rm Al_4C_3$ isn't a molecular compound.
Cotton & Wilkinson's Advanced Inorganic Chemistry says that aluminum carbide "reacts instantly with water to produce methane, and X-ray studies have shown it to contain discrete carbon atoms (C-C = 3.16 Å); for these reasons it is sometimes considered to be a "methanide", that is, a salt containing $\rm C^{4-}$, but this is probably an oversimplification."
This is the structure that's posted on Wikipedia:
It says that "Aluminium carbide has an unusual crystal structure that consists of two types of layers. It is based on AlC$_4$ tetrahedra of two types and thus two types of carbon atoms. One is surrounded by a deformed octahedron of 6 Al atoms at a distance of 217 pm. The other is surrounded by 4 Al atoms at 190–194 pm and a fifth Al atom at 221 pm." This is attributed to Greenwood's Chemistry of the Elements, but I don't see it in the edition I have.
