What is the electronegativity for an ionic bond?
The electronegativity difference ΔEN between the atoms in an ionic bond must be greater than 1.6.
Bonds have no electronegativity. The atoms have the electronegativities. The differences between the electronegativities of the atoms determine the ionic character of the bond.
Bonds range from 100 % covalent to 100 % ionic, with every value between. There is no sharp dividing line between ionic and covalent. We usually say that a bond has, say, 90 % ionic character and 10 % covalent character.
For a B-H bond, ΔEN = 2.20 -2.04 = 0.16. We would call this a covalent bond. It has 1 % ionic character.
For an Fr-F bond, ΔEN = 3.98 – 0.7 = 3.7. We would call this an ionic bond. It has 100 % ionic character.
The dividing line comes when ΔEN = 1.7. It corresponds to 51 % ionic character.
It is customary to say that anything with more than 50 % ionic character is "ionic". We say that magnesium chloride is an ionic compound (ΔEN = 1.85), even though the Mg-Cl bond is only 57% ionic.