Carbon suboxide ${\mathrm{C}}_3{\mathrm{O}}_2$ has 

Carbon suboxide has linear structure with $\mathrm{C} - \mathrm{C}$ bond length equal to $1.30\text{Å}$ and C - O bond length equal to $1.20\text{Å}$

$\mathrm{O}=\mathrm{C}=\mathrm{C}=\mathrm{C}=\mathrm{O}\leftrightarrow \mathrm{O}-\mathrm{C}\equiv \mathrm{C}-\mathrm{C}\equiv \mathrm{O}$