Magnesium silicide, or #Mg_2Si#, is an example of a compound formed by magnesium and silicon. The bond in #Mg_2Si# is considered to be polar covalent with some ionic character.
The difference in electronegativity between magnesium and silicon is about 0.6, which is very close to the 0.5 value set as threshold for covalent bonding.
#EN_(Si) - EN_(Mg) = 1.9 - 1.3 = 0.6#
This is why the bond is considered to be polar covalent. You can think of the difference in electronegativity as an indicator of what predominant character a bond between two atoms will have
As a powder, magnesium silicide crystalizes in a face-centered cubic structure. #Si^(4-)# ions will occupy the corners and the face-centered positions of the unit cell, while #Mg^(2+)# ions will occupy eight spots in the interior of the cell.