Answer:

Actually bonds that are

ionic, or close to it, are possible. But they require a little ingenuity. Instead of combining atoms of very different electronegativity, we can build multiatomic ions in which the charge is (formally) buried or highly delocalized making covalent bonding unfavorable. We use molecular orbital structures rather than electronegativity to drive the charge separation.

Explanation:

provided by compounds in which alkali metals are disproportionated into cations and anion (see Alkalide). Because the ionization energy of alkali metals (at least from sodium on down) are so small, the cation may be stabilized by forming a complex where the (formal) positive charge is deeply buried and again, no obvious means exists for covalent bonding. The alkalide anion must be bonded by essentially ionic attraction to the cationic complex.