Although $\ce{CO_2}$ has four lone pairs on the two oxygen atoms, but it still does not form addition compounds by donating lone pairs. Why?

$\ce{CO}$ on the other hand, does form addition compounds by donating the lone pair from C atom.


A fairly simplistic argument is that in CO, the lone pair on oxygen is of a much lower energy than the lone pair on carbon (because oxygen has a greater effective nuclear charge and therefore pulls electrons closer to itself). Therefore it is quite rare (very rare indeed) to find any compound that has the oxygen acting as a donor atom; CO only really reacts via its lone pair on C.

Carbon dioxide only has low-energy lone pairs on O, and is consequently a lousy Lewis base.

One could easily refine this by bringing in MO schemes, but at 3 a.m. in the morning, this is good enough to convince me.

  • $\begingroup$ Wik sez CO2 complexes do form in certain circumstances. Your argument about the oxygens not being the electron donors seems to hold for CO2 too, though -- looks like it's pretty much only ever one of the C-O bonds that actually does the electron donating. $\endgroup$ – hBy2Py Dec 18 '16 at 19:15
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    $\begingroup$ Actually, protonated CO2 is known, too. I think my argument is a little bit shaky with respect to CO. The better explanation is probably simply that the HOMO has larger coefficients on C (which can be linked to the relative energies of the AOs - but the link isn't quite as direct as I wrote above). But it'll do. I don't pretend that this is a rigorous answer. $\endgroup$ – orthocresol Dec 19 '16 at 3:55
  • $\begingroup$ @orthocresol A rigorous answer would be better. $\endgroup$ – Shoubhik Raj Maiti Dec 21 '16 at 14:01

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