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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?

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The character of the bond is determined by the electronegativity of the atoms.

Speaking of bonds as purely ionic or covalent is not always correct - usually it is more correct to say that a bond has ionic or covalent characteristics.

So comparing the difference in electronegativities gives us the following:

$$\begin{array}{cc}\hline \text{Difference in electronegativity} & \text{Type of bond} \\ \hline < 0.4 & \text{Non-polar covalent} \\ 0.4 \mathrm{-} 1.7 & \text{Polar covalent} \\ >1.7 & \text{Ionic} \\ \hline \end{array}$$

At the upper end of the polar covalent spectrum, the bonds frequently have both covalent and ionic characteristics.

For organometallic compounds, the difference in electronegativity between Li and C is $1.57$. So while this is still in the polar covalent range, it is also close to ionic. Similarly, the difference between Mg and C is $1.24$ - again, a very polar covalent bond.

Compare this to the difference between H and C ($0.35$) - a non-polar covalent bond.

So to answer your question, the thing that is "special" about carbon is that it has a fairly low electronegativity compared to the chalcogens and halogens. Granted, bonds with carbon are also going to be weaker than in say LiCl, but that's what makes organometallic compounds actually work to form carbon-carbon bonds.

(The electronegativity values came from wikipedia's great chart)

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  • $\begingroup$ "usually it is more correct to say that a bond has ionic or covalent characteristics."-->yeah, that's why I was careful to talk in terms of "covalent character", though I missed a spot. Great answer, now I feel stupid. I'll wait before accepting this, though--let others have a chance. $\endgroup$ – ManishEarth May 17 '12 at 13:36
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Why do Magnesium and Lithium form covalent organometallic compounds? I think this question touches a more general question in chemistry: Why do some metallic elements form 'covalent' bonds at all? And the other version of this question would be why sodium or potassium does not usually form covalent bonds. The answer for these questions would become long and winding very easily since it's going to touch on how electrons move and shift in forming a chemical bond, covalent or otherwise.

So I think a similar question that can have an easier answer would be: When does an element form 'covalent' bond with another element? Answer: If the electronegativities of the two elements differ by less than 1.9, the formed bond would be covalent. But then again, the answer is not absolute and serves only as a guideline. The case in point is Na-C bond, which has a delta(Eneg)=1.6 but is considered an ionic bond.

Back to the original question. Why do Magnesium and Lithium form covalent organometallic compounds? A hand-waving answer would be that in forming a Li-C or Mg-C bond the electrons are not moved all the way from metal to carbon, but only partially to somewhere between the metal and carbon.

I hope these had touched the topic a little bit.

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The atomic and ionic radii of Li are very small. Therefore, the nuclear attraction is strong compared to the rest of the alkali metals, which leads to a greater polarising power in Li ion, and the formation of covalent bonds.

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