Hydrogen Bonds Dictate the Coordination Geometry of Copper: Characterization of a Square‐Planar Copper(I) Complex

6,6′′‐Bis(2,4,6‐trimethylanilido)terpyridine (H 2 Tpy NMes ) was prepared as a rigid, tridentate pincer ligand containing pendent anilines as hydrogen bond donor groups in the secondary coordination sphere. The coordination geometry of (H 2 Tpy NMes )copper(I)‐halide (Cl, Br and I) complexes is dictated by the strength of the NH–halide hydrogen bond. The Cu I Cl and Cu II Cl complexes are nearly isostructural, the former presenting a highly unusual square‐planar geometry about Cu I . The geometric constraints provided by secondary interactions are reminiscent of blue copper proteins where a constrained geometry, or entatic state, allows for extremely rapid Cu I /Cu II electron‐transfer self‐exchange rates. Cu(H 2 Tpy NMes )Cl shows similar fast electron transfer (≈10 5   m −1  s −1 ) which is the same order of magnitude as biological systems.