For example, calcium carbide (CaC$_2$) has covalent C‒C bonds and ionic Ca$^{2+}$‒ C$_2^{2-}$ bonds.

  • $\begingroup$ I looked up ionic compound in Wiki and no compound is 100% ionic or covalent... $\endgroup$ – f p Jun 5 '13 at 20:38
  • $\begingroup$ Technically, that is true. However in CaC$_2$ the C‒C bond is mostly covalent and the C‒Ca bond is mostly ionic. $\endgroup$ – Max Radin Jun 9 '13 at 2:44
  • $\begingroup$ How about ambi-valent or ambivalent. $\endgroup$ – Dale Apr 10 '15 at 18:24
  • $\begingroup$ atomic salt (NaCl) vs molecular salt (NH3-SO4-) vs an atom and a molecule forming a salt (CaC2). You have the third case. Sorry it isn't jargon, but the description exists. $\endgroup$ – Dale Nov 11 '15 at 18:17

There may be a strong case to call this a "salt", like other ionic compounds. Very often, the anion (negative ion) of a salt is an ion comprised of different atoms with covalent bonds. For example, $\ce{ SO_4^{2-} }$ is the sulfate anion, and the bonds between $\ce O$ and $\ce S$ are covalent to some degree. So $\ce{ Na_2SO_4 }$ is an ionic compound under this definition. Thus, any compound with some component that is strongly ionic can be called an ionic compound; i.e. a salt, regardless of the bonding structure of the components.

  • $\begingroup$ Thanks for your answer. I am wondering if there is a word that will distinguish ionic compounds that do have covalent bonds from ionic compounds that don't. $\endgroup$ – Max Radin Jun 9 '13 at 2:40
  • $\begingroup$ @Max Radin: I do not know for certain, but it would seem unlikely. There are just too many anions (negatively charged ions) that have lots of covalent bonds ($\ce{SO_4^{2-}},\ce{NO_3^-}$, and so on). There are even some cations (positively charged ions) with this property ($\ce{NH_4^+}$). Molecules with these ions are generally considered ionic. $\endgroup$ – user467 Jun 12 '13 at 0:44

The problem with your case is that it covers a very wide range of possibilities. The simplest cases are probably calcium carbide or any cation’s peroxide/superoxide ($\ce{KO2}$, $\ce{BaO2}$) which are salts for all intents and purposes.

On the other end of the scale consider a protein consisting of hundreds of amino acids with the occasional lysine-glutamate, lysine-aspartate, arginine-glutamate or arginine-aspartate salt-bridge type bond. For all intents and purposes, that is a covalent compound although it includes intramolecular ionic interactions. There are probably simpler cases, too, where a Brønsted base and a Brønsted acid are present in the same molecular backbone and form an ionic interaction between themselves. I am just too lazy to open SciFinder to look for an example.

We can and probably should treat the two extreme ends of the scale separately. We can distinguish between them a priori by asking ourselves whether there is a contiguous chain of mostly covalent bonds between each side of the ionic interaction. If there is no link between the two ionic fragments, we should call the compound an ionic one (usually ‘salt’). If there is one, we are dealing with a molecule that we may choose to further specify as zwitterionic.

There is no real reason to classify these compounds in a separate group altogether. Ammonium chloride differs so little from potassium chloride much like sodium iodide is almost like sodium perchlorate. At least, the difference in behaviour inside these pairs is a lot less than e.g. between sodium sulphide and zinc sulphide (although the latter is already well on the way towards covalency).

On the molecular side, classifying small zwitterions as such is likely enough to distinguish them from uncharged molecules. And glycine, for example, is only zwitterionic ($\ce{^{+}H3N-CH2-COO-}$) in very polar solvents such as water. In less polar and non-protic solvents, the neutral from ($\ce{H2N-CH2-COOH}$) is encountered.


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