Dative Bonds

[Hinuku]

Hey,

I need some help regarding dative bonds. I"m having trouble writing the lewis structures for the more complicated ions like CO3^2-. I would really appreciate some help ;p

[Daedalus]

Think about it in terms of electrons and elements. Carbon is neutral with four electrons; Oxygen is neutral with 6 electrons. The carbonate ion overall has a -2 charge, meaning it must have 20 electrons in total. Carbon is the central atom and it tends to form four bonds, that is, bonds totalling eight electrons - this will become easier to see with organic chemistry, but it's something that just needs to be learned I guess. Then if you have to have 8 electrons around the carbon distributed in three oxygens, and still account for a -2 charge, you can sort of use this process (draw it out on paper to make more sense of it):

Imagine each oxygen forming a single bond with the carbon. You get 3 bonds and 7 electrons only for the carbon, which wants 8; each oxygen then also has 7 electrons. Except you have two extra electrons (it's a -2 ion) which you can add to any two of the oxygens, one each. Then you have two oxygens with 8 electrons and a -1 charge (because of the additional electrons not matched by protons), and one oxygen with 7 electrons only. Clearly this oxygens needs to form a double bond, with four electrons - two from the oxygen, two from the carbon (the second bond involves a single additional electron from the carbon). Now you have two single bonds, one double bond; each atom has 8 electrons around it in the Lewis structure so they're "happy" (did your teachers use that to explain how this works?) - and two of the oxygens have a -1 charge.

In practice the double bond can be with either of the oxygens - resonance structures - so the electrons get delocalized, creating an effective -2/3 charge on each oxygen and a more stable molecule than might be expected. The bond lengths are somewhere between single and double, and the molecule is symmetric. For the lewis structure part of the syllabus though you only need to be able to draw these structures, and the main thing this involves is working it out. Set out the facts that have to be accounted for and work out the explanation - if all else fails, try again. It gets easier with practice!