Is carbon suboxide planar? 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.
When we talk about a molecule being linear or planar, we only consider the positions of the nuclei and ignore the electron orbitals. Sure, &pi;-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.