When Copper Falls: Overriding the Irving-Williams Stability Trend Through Outer-Sphere Hydrogen Bonding

The Irving-Williams series (Mn < Fe < Co < Ni < Cu > Zn) is a well-established thermodynamic stability trend among high-spin divalent first-row transition-metal complexes. However, complete isostructural Mn-Zn series are rare, and those that enable direct evaluation of a defined secondary coordination sphere are even rarer. As a result, whether outer-sphere noncovalent interactions can alter this periodic trend without changing the primary coordination environment has remained unresolved. Here, we present a complete isostructural octahedral series, (allLH)2MCl2 (M = Mn-Zn), supported by a flavin-derived bidentate ligand, allLH. An outer-sphere N-H···Cl hydrogen-bonding network provided by allLH locks all complexes into a single meridional isomer, enabling direct thermodynamic comparison across the row. In stark contrast to the classical Irving-Williams series, Cu(II) becomes the least stable species in the (allLH)2MCl2 series. Experimental and computational analyses show that this reversal arises from a finely balanced noncovalent constraint imposed by the rigid hydrogen-bonded scaffold. This scaffold retains the octahedral geometry of (allLH)2CuCl2 but only partially accommodates Jahn-Teller distortion, leaving the resulting stabilization insufficient to offset the accompanying attenuation of outer-sphere hydrogen-bond stabilization. This study demonstrates that a well-established metal-ion stability trend can be overcome by designing outer-sphere noncovalent interactions without altering the primary coordination environment.