Periodic table trends are helpful, but actual Electronegativity values are even more so. What about the Mg to C bond? The trend says that C is more electronegative but by how much? Knowing Mg is 1.2 and C is 2.5 (on the Pauling scale) says this is a very polar covalent bond in which the C is electron-rich and thus reactive. When we get into the weeds, e.g. B and Al in reducing reagents, knowing these numbers will explain the very different reactivities of compounds like sodium borohydride and lithium aluminum hydride.