Pentaleno [1,6-ab]indene is the pentalene analog of indeno[1,2,3-ef ]heptalene described above in section 3.3. The three fused five-membered rings form a highly strained bowl shape which, to our knowledge, has not been synthesized. Through substitution at these rings with either amino or hydroxyl groups, a small S 1 -T 2 inversion is achieved.