The tetramanganese-calcium cluster of the oxygenevolving complex of photosystem II adopts electronically and magnetically distinct but interconvertible valence isomeric forms in its first light-driven oxidized catalytic state,S 2 .T his bistability is implicated in gating the final catalytic states preceding O À Ob ond formation, but it is unknown how the biological system enables its emergence and controls its effect.Here we show that the Mn 4 CaO 5 cluster in the resting (darkstable) S 1 state adopts orientational Jahn-Teller isomeric forms arising from adirectional change in electronic configuration of the "dangler" Mn III ion. The isomers are consistent with available structural data and explain previously unresolved electron paramagnetic resonance spectroscopic observations on the S 1 state.T his unique isomerism in the resting state is shown to be the electronic origin of valence isomerism in the S 2 state,establishing afunctional role of orientational Jahn-Teller isomerism unprecedented in biological or artificial catalysis.

This review comments on the homogeneous HER mechanisms for catalysts carrying S-non-innocent ligands in the light of experimental and computational data.

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Spin-state energetics of transition metal complexes remain one of the most challenging targets for electronic structure methods. Among single-reference wave function approaches, local correlation approximations to coupled cluster theory, most notably the domain-based local pair natural orbital (DLPNO) approach, hold the promise of bringing the accuracy of coupled cluster theory with single, double, and perturbative triple excitations, CCSD(T), to molecular systems of realistic size with acceptable computational cost. However, recent studies on spin-state energetics of iron-containing systems raised doubts about the ability of the DLPNO approach to adequately and systematically approximate energetics obtained by the reference-quality complete active space second-order perturbation theory with coupled-cluster semicore correlation, CASPT2/CC. Here, we revisit this problem using a diverse set of iron complexes and examine several aspects of the application of the DLPNO approach. We show that DLPNO-CCSD(T) can accurately reproduce both CASPT2/CC and canonical CCSD(T) results if two basic principles are followed. These include the consistent use of the improved iterative (T 1 ) versus the semicanonical perturbative triple corrections and, most importantly, a simple two-point extrapolation to the PNO space limit. The latter practically eliminates errors arising from the default truncation of electron-pair correlation spaces and should be viewed as standard practice in applications of the method to transition metal spin-state energetics. Our results show that reference-quality results can be readily achieved with DLPNO-CCSD(T) if these principles are followed. This is important also in view of the applicability of the method to larger single-reference systems and multinuclear clusters, whose treatment of dynamic correlation would be challenging for multireference-based approaches.

The electronic and geometric structures of the water-oxidizing complex of photosystem II in the steps of the catalytic cycle that precede dioxygen evolution remain hotly debated. Recent structural and spectroscopic investigations support contradictory redox formulations for the active-site Mn 4 CaO x cofactor in the final metastable S 3 state. These range from the widely accepted Mn IV 4 oxo-hydroxo model, which presumes that OÀ O bond formation occurs in the ultimate transient intermediate (S 4 ) of the catalytic cycle, to a Mn III 2 Mn IV 2 peroxo model representative of the contrasting "early-onset" OÀ O bond formation hypothesis. Density functional theory energetics of suggested S 3 redox isomers are inconclusive because of extreme functional dependence.Here, we use the power of the domain-based local pair natural orbital approach to coupled cluster theory, DLPNO-CCSD(T), to present the first correlated wave function theory calculations of relative stabilities for distinct redox-isomeric forms of the S 3 state. Our results enabled us to evaluate conflicting models for the S 3 state of the oxygen-evolving complex (OEC) and to quantify the accuracy of lower-level theoretical approaches. Our assessment of the relevance of distinct redox-isomeric forms for the mechanism of biological water oxidation strongly disfavors the scenario of early-onset OÀ O formation advanced by literal interpretations of certain crystallographic models. This work serves as a case study in the application of modern coupled cluster implementations to redox isomerism problems in oligonuclear transition metal systems.