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I was asked to comment on the planarity of the carbon suboxide molecule. I thought that the Lewis structure of the molecule looks somewhat like an Allene and must have similar bonding. The two terminal pi bonds must be out of plane and hence the molecule would be non planar. But the answer given in my book states that it is planar.
I couldn’t find a satisfactory description of its structure anywhere. The molecule would be linear, but that doesn’t imply that the molecule should be planar.

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When we talk about a molecule being linear or planar, we only consider the positions of the nuclei and ignore the electron orbitals. Sure, π-bonds might be out of plane, but that does not count. If it would be otherwise, we would have to consider that any atom except hydrogen has p-orbitals, and some of those are out of plane, and so no molecule ever would have the right to be called planar.

So yes, $\ce{C3O2}$ is kinda like an allene, with all its carbons having sp-hybrid orbitals, which inevitably makes the molecule linear and hence planar.

One might point out that the molecule is relatively easy to bend, but that's another story.

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  • $\begingroup$ What if it polymerizes? $\endgroup$ – Oscar Lanzi Apr 24 '18 at 23:02
  • $\begingroup$ What of it? A polymer is a different thing with different structure and different everything. $\endgroup$ – Ivan Neretin Apr 25 '18 at 4:44
  • $\begingroup$ One would have to imagine quite some bending for the molecule to be non-planar. $\endgroup$ – Martin - マーチン Apr 25 '18 at 14:12
  • $\begingroup$ Lay it obliquely onto a cylinder, maybe? $\endgroup$ – Oscar Lanzi Apr 26 '18 at 0:55
  • $\begingroup$ I wonder how that translates to the vibrational quantum states. $\endgroup$ – Ivan Neretin Apr 26 '18 at 5:20

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