On the importance of cyanide in diiron bridging carbyne complexes, unconventional [FeFe]-hydrogenase mimics without dithiolate: An electrochemical and DFT investigation

•Sulfur-free diiron mimicry of the hydrogenase enzyme needs cyanide ligand.•CN− role in catalytic H2 production involves metal-ligand cooperation.•H2 production catalysis without dithiolate can exploit carbynes as diiron strap.•Ordinary phosphines in place of CN− lead to a thermodynamic sink for H2 evolution.•CN− does a non-trivial job: strictly related ligands cannot perform its function.