A coordination nanosheet composed of [Fe(tpy)₂]²⁺ (tpy = 2,2':6',2''-terpyridine) units, showing reversible electrochromism at the ligand-based cathodic potential, has been prepared through a liquid/liquid interfacial synthesis. The noble metal-free nanosheet exhibited a CO evolution rate of 114.3 mmol·g⁻¹·h⁻¹ with the selectivity up to 99.3% under visible light irradiation in the presence of water, which is in the front rank of heterogeneous catalysis for CO₂ photoreduction. Such robust photocatalytic performance is due to efficient ligand-based electron transfer through long-lived π radical anion tpy^·− with a lifetime more than 25 min, as evidenced by in situ electron paramagnetic resonance (EPR) and ultraviolet–visible–near infrared (UV–vis–NIR) spectroscopy studies. Fe(II) cation in [Fe(tpy)₂]²⁺ mainly contributes to enhancing reduction potentials of ligand and stabilizing π radical anion tpy^·−. This ligand-based electron transfer with the aid of metal cation represents a promising strategy for selective CO₂ photoreduction, especially towards gaining CO from CO₂.