Abstract
The relationship between chemical structure and spin state in a transition metal complex has an important bearing on mechanistic bioinorganic chemistry, catalysis by base metals, and the design of spin crossover materials. The latter provide an ideal testbed for this question, since small changes in spin state energetics can be easily detected from shifts in the spin crossover equilibrium temperature. Published structure-function relationships relating ligand design and spin state from the spin crossover literature give varied results. A sterically crowded ligand sphere favors the expanded metal–ligand bonds associated with the high-spin state. However, steric clashes at the molecular periphery can stabilize either the high-spin or the low-spin state in a predictable way, depending on their effect on ligand conformation. In the absence of steric influences, the picture is less clear since electron-withdrawing ligand substituents are reported to favor the low-spin or the high-spin state in different series of compounds. A recent study has shed light on this conundrum, showing that the electronic influence of a substituent on a coordinated metal ion depends on its position on the ligand framework. Finally, hydrogen bonding to complexes containing peripheral N‒H groups consistently stabilizes the low-spin state, where this has been quantified.
Highlights
In principle, controlling the spin state of a coordinated transition ion through ligand design should be a straightforward problem of coordination chemistry
spin crossover (SCO) in d5 or d6 metal ions like iron(III) or iron(II), which involves a ∆S = ̆2 spin state change, has a greater effect on their molecular structure than the ∆S = ̆1 SCO undergone by d4 and d7 centers
Bonds to harder N, O-donor ligands tend to be more sensitive to metal ion spin state than softer P, S or halide ligand donors [24]
Summary
In principle, controlling the spin state of a coordinated transition ion through ligand design should be a straightforward problem of coordination chemistry. Which is not.generalization is that high-spin complexes, with their increased radical character, are Another more prone towards single-electron reactivity than low-spin metal centers. Another generalization is that high-spin complexes, with their increased radical character, are relevant in biological and synthetic oxidation complexes, catalysis, which high-valentare Another generalization is that high-spin withmost theircommonly increasedinvolves radical character, more prone towards single-electron reactivity than low-spin metal centers. This is ironprone or manganese intermediates reactivity whose spin are malleable, depending on the co-ligands more towards single-electron thanstates low-spin metal centers This is relevant relevant inThe biological and synthetic oxidation catalysis, which most commonly involves high-valent two-step radical rebound mechanism typically adopted by involves these reactions involves an or in present. Spin state flip [14]
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