In general,

• Metallic bonds have low difference in electronegativity and low average electronegativity in a bond.
• Ionic bonds have moderate to high difference in electronegativity and moderate values of average electronegativity in a bond.
• Covalent bonds have moderate to high average electronegativity and can exist with moderately low difference in electronegativity.

A strong metallic bond will be the result of more delocalized electrons, which causes the effective nuclear charge on electrons on the cation to increase, in effect making the size of the cation smaller.