Every Grignard reagent (R-Mg-X) binds two ether molecules to the magnesium as shown in this interactive example.

You can (left) click and drag to rotate the molecule, use the scroll wheel to enlarge or shrink. Right clicking opens a menu with a lot of controls, but the most convenient are linked in the scripts below the structure.

The carbonyl compound can coordinate to Mg (replacing an ether); the Lewis acidity of Mg makes the carbon more electrophilic.

Finally, formation of the new C-C bond results in a magnesium alkoxide: