Spin Frustration Determines the Stability and Reactivity of Metal–Organic Frameworks with Triangular Iron(III)–Oxo Clusters

Abstract Density functional theory (DFT) is the standard approach for modeling MIL‐101(Fe) and related Fe‐based metal–organic frameworks, typically assuming a ferromagnetic high‐spin configuration. However, this widely adopted approach overlooks a key electronic feature: Spin frustration in the triangular ‐O) nodes. Using flip‐spin, broken‐symmetry DFT, we identify the true ground state as an antiferromagnetic state that standard DFT fails to capture. We demonstrate that neglecting spin frustration in MIL‐101(Fe) leads to structural distortions, incorrect energetics, and misleading predictions of stability and reactivity. By explicitly accounting for spin frustration, we recover the correct structure and rationalize the temperature‐dependent and CO binding. Spin frustration enhances fixation at room temperature, while its loss upon partial reduction suppresses this activity but promotes CO adsorption via ‐backbonding. These findings challenge current computational conventions and highlight spin frustration as a critical electronic feature in these frameworks.