π‑Back-Bonding Interaction Depending on the Bridging Chain Lengths of Chelated N‑Heterocyclic Carbene Platinum Units in Heterometallic Trinuclear Complexes Affecting Their Electrochemical Property

Newly synthesized heterometallic trinuclear M₂Pt complexes (M = Rh, Ir) containing a platinum moiety having a chelated bis-N-heterocyclic carbene (bisNHC) ligand with a variety of alkylene chain lengths of the bridging part showed two reversible reduction waves in cyclic voltammetry. Only the second reduction potentials were affected by the alkyl chain lengths, which afforded different dihedral angles between the imidazolylidene rings and the platinum coordination plane resulting in the variation of π-back-donation from the platinum center to the carbene carbon atoms.