Palladium(ii) N-heterocyclic allenylidene complexes with extended intercationic PdPd interactions and MMLCT phosphorescence.

Extended intercationic PdPd contacts of 3.30 angstrom in the crystal structure and distinct MMLCT transitions absorbing at 528 nm and emitting beyond 600 nm in solutions have been revealed with cyclometalated Pd(ii) N-heterocyclic allenylidene complexes. The Pd(ii)-based MMLCT excited states are responsive to concentrations, temperatures, mechanical force and organic vapors.