Single-site catalyst systems useful for polymerizing olefins are disclosed. The catalyst systems comprise an organometallic complex and an activator. The complex includes a Group 3-10 transition metal, M, and at least one indenoindolyl ligand that is pi-bonded to M. The activator is a reaction product of an alkylaluminum compound and an organoboronic acid. Catalyst systems of the invention significantly outperform known catalyst systems that employ a metallocene complex and similar aluminoboronate activators.