Can CO2 be a base (in coordinate covalent bonds)

[yestomeds]

The Lewis dot structure for carbon dioxide seems to be a double bond b/w the C and the 2 O's.
As well as 4 extra e- (2 e- pairs) hanging around each of the oxygens.

So that means carbon dioxide would have nonbonding electrons present - the prerequisite for being a Lewis Base (gives away an entire pair of e-)... so why is TPR saying that CO2 doesn't make a good ligand (base)?

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[Irish Luck]

This is the current scenario: :::O-C=O: <-> ::O=C=O:: <-> :O=C-O:::

(+/- charges on O not drawn because it was confusing the diagrams)

The molecule is resonance stabilized and thus doesn't want to get rid of it's electron base pairs.

If the Oxygen was to give away the electrons, it would become more unstable.

 

[amy_k]

Is it just me who sees a shocked-face emoticon instead of Oxygen? Anyway, IrishLuck is right. The electron pairs on the oxygens in CO2 are not readily available for donation as is needed for a lewis base, since the actual structure is a resonance hybrid with pi electrons distributed throughtout the molecule.

It does however more often act as a Lewis acid, for example, the reaction with water shown here: http://wps.prenhall.com/wps/media/objects/3312/3392119/blb1611.html
The empty p orbital on the carbon caused by the resonance structures can accept an electron pair from a lewis base such as water or metal oxides.

I believe there are some circumstances where CO2 can act as a lewis base and donate one of the lone pairs to a lewis acid, although not so readily as it acts as an acid, so its not a particularly good one (so it doesn't make a good ligand which is what the question asks).

 

[yestomeds]

Thanks you guys.

So basically drawing a Lewis structure is insufficient - it seems to show 'paired' electrons that are freely dangling off of an atom (I.e. Can act as a Lewis base). Just drawing a LDS means you could be real wrong... Meaning you have to draw out resonance structires for any molecule in order to know if they could act as bases or not (bases in the sense of a "ligand")?!