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We report the single crystal XRD and MicroED structure, magnetic susceptibility, and EPR data of a series of CaMn₃^IVO₄and YMn₃^IVO₄complexes as structural and spectroscopic models of the cuboidal subunit of the oxygen‐evolving complex (OEC). The effect of changes in heterometal identity, cluster geometry, and bridging oxo protonation on the spin‐state structure was investigated. In contrast to previous computational models, we show that the spin ground state of CaMn₃^IVO₄complexes and variants with protonated oxo moieties need not beS=9/2. Desymmetrization of thepseudo‐C₃‐symmetric Ca(Y)Mn₃^IVO₄core leads to a lowerS=5/2 spin ground state. The magnitude of the magnetic exchange coupling is attenuated upon oxo protonation, and anS=3/2 spin ground state is observed in CaMn₃^IVO₃(OH). Our studies complement the observation that the interconversion between the low‐spin and high‐spin forms of the S₂state is pH‐dependent, suggesting that the (de)protonation of bridging or terminal oxygen atoms in the OEC may be connected to spin‐state changes.

We report the single crystal XRD and MicroED structure, magnetic susceptibility, and EPR data of a series of CaMn₃^IVO₄and YMn₃^IVO₄complexes as structural and spectroscopic models of the cuboidal subunit of the oxygen‐evolving complex (OEC). The effect of changes in heterometal identity, cluster geometry, and bridging oxo protonation on the spin‐state structure was investigated. In contrast to previous computational models, we show that the spin ground state of CaMn₃^IVO₄complexes and variants with protonated oxo moieties need not beS=9/2. Desymmetrization of thepseudo‐C₃‐symmetric Ca(Y)Mn₃^IVO₄core leads to a lowerS=5/2 spin ground state. The magnitude of the magnetic exchange coupling is attenuated upon oxo protonation, and anS=3/2 spin ground state is observed in CaMn₃^IVO₃(OH). Our studies complement the observation that the interconversion between the low‐spin and high‐spin forms of the S₂state is pH‐dependent, suggesting that the (de)protonation of bridging or terminal oxygen atoms in the OEC may be connected to spin‐state changes.