Iridocene (involving iridium) would presumably be still more potent, but is not very well-studied due to its instability.
ˈɪɹɪdˌoʊsiːn ɪnvˈɑːlvɪŋ ɪɹˈɪdiəm wʊd pɹɪzˈuːməbli biː stˈɪl mˈoːɹ pˈoʊtənt, bˌʌt ɪz nˌɑːt vˈɛɹi wˈɛlstˈʌdid dˈuː tʊ ɪts ɪnstəbˈɪlᵻɾi.