Why do Magnesium and Lithium form *covalent* organometallic compounds?

Question

Lithium and magnesium are Group 1 and Group 2 elements respectively. Elements of these groups are highly ionic, and I've never heard of them forming significantly covalent _inorganic_ compounds. Yet these elements form a variety of organometallic compounds ($\\ce{PhLi}$, the whole family of Grignard reagents, etc). Organometallic compounds have significant covalent character (i.e., the bond can be called covalent) in the carbon–metal bond. What's so special about carbon that makes these elements form covalent bonds?

Answer 1

Remember that a true ionic bond is defined by the difference in two elements' electronegativity (EN) values >1.7. Carbon has EN = 2.55 and Mg has EN = 1.31. Therefore since the difference is 1.57 (<1.7), the bond is not entirely ionic. Therefore there is some degree of covalent character to the bond between these 2 elements.

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