Carbon, Mercury and Bonding

name         Amy
grade        other
location     AZ

Question -   Why does a carbon form a covalent bond with 
mercury instead of an ionic bond?
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The "standard" answer is that carbon is not sufficiently 
electronegative to fully accept the two 6s2
valence electrons: [Xe] + 4f14 5d10 6s2 and so shares them more 
equally to form a covalent bond.
Note, however, that this is an "after the fact" rationalization, 
rather than an "explanation" before there is any data. We know, for 
example, that dimethyl mercury behaves more like a covalently bonded 
organic molecule --
that is -- relatively low melting and boiling points compared to 
ionic salts. Having said that, we try to put molecules into 
classifications, ionic, covalent, etc. but there is a continuum 
bonding types especially with elements in the middle of the periodic 
table, so do not be too dogmatic about the bonding "boxes". They are 
convenient abstractions, but not immutable laws of chemistry.

Vince Calder
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