Lewis Metal Atom Charges

A frequent question is, why does the Lewis structure start off with some metal atoms having negative charges?  For example, if in an organometallic complex, a magnesium or zinc atom has four bonds to neighboring ligands, then the Lewis structure will assign the charge on the metal atom of -2.  Everyone knows that the metal should have a positive charge, so what's going on?

The answer is simple.  Imagine a complex [M(NH3)4]2+, where M is a metal atom that normally exists as the 2+ ion, e.g., Mg or Zn, and, for the purpose of building the Lewis structure, the metal atom forms four bonds, one to each of the four NH3 groups.

Each two-electron N-Zn bond is constructed from the lone pair on a nitrogen atom, donating to an empty orbital on the zinc atom.  This puts one electron on the zinc atom, and removes one electron from the nitrogen atom.  So each -NH3 becomes -NH3+, and the zinc becomes Zn2-.  Normally, this Lewis structure is best for starting a MOZYME calculation.  As soon as MOZYME starts, the more correct polarized bond structure forms and the metal atom becomes electropositive.  If you don't like the idea of a Zn2-, then use METAL to convert the zinc into a purely ionic entity, i.e., Zn2+.  This should not affect the results in any significant way.


See also: MOZYME introduction