The influence of metal and anion electronic states on the stability of high valent metals

Creators: Morrell, Colin T.¹; Davis, Victoria K.¹; Dulock, Nicholas V.¹; Patheria, Eshaan S.¹; See, Kimberly A.¹

1. California Institute of Technology

Abstract

The interplay between transition metal d-electrons and anion p-electrons drives reactivity and structural trends not only in molecular compounds, but also in extended solids. Here, in honor of his 90th birthday, we related the foundational work by Prof. Harry B. Gray on the oxo wall in molecular compounds to the layered-to-pyrite transition observed in metal chalcogenide extended solids. In both cases, we can consider a ligand-to-metal charge transfer process as the mechanism for producing reduced metal centers with relatively oxidized ligands/anions. We focus the review on this process and the inability to stabilize high valent metals in specific anion frameworks — namely Fe in a sulfide. Such concepts are critical for understanding our ability to electrochemically oxidize metal sulfides to yield high valent Fe, which in sulfides is Fe³⁺, and/or oxidized sulfide anions as persulfides, S²⁻₂. Happy, happy birthday, Harry!

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Acknowledgement

We thank Prof. Harry B. Gray and Dr. Jay R. Winkler as students and mentees. Discussions of the oxo wall in the advanced inorganic chemistry course here at Caltech inspired the writing of this review along with informal lunch discussions outside Broad Cafe. We thank Harry for helpful impromptu discussions in his office. We thank Harry for his lifetime of incredible inorganic chemistry, but also first-class mentorship to both students and professors alike. Your unwavering support is both incredibly empowering and inspiring. The field would not be the same without you, and Caltech certainly would not be the same without you! Happy, happy birthday Harry! We love you! Here’s to many more. We include a photograph taken with Harry for the review in Fig. 8.

Funding

This work is supported by the National Science Foundation (NSF) under Award No. DMR-2340864. C.T.M. acknowledges support from the National Science Foundation Graduate Research Fellowship under Grant No. DGE-1745301.

Supplemental Material

MMC S1. Tabulated information on the iron sulfide binary materials and coordination information on select alkali metal iron sulfide ternary materials (PDF)

Data Availability

Crystal structure .cif files were obtained from the Inorganic Crystal Structure Database (ICSD release 2025.1): TiS₂ (ICSD-91579), troilite FeS (ICSD-31963), mackinawite FeS (ICSD-238590), pyrite FeS₂ (ICSD-235825), marcasite FeS₂ (ICSD-42415), Fe₇S₈ (ICSD-42491), Fe₃S₄ (ICSD-117136), and Li₂FeS₂ (ICSD-68380), NaFeS₂ (ICSD-37026), KFeS₂ (ICSD-202380), RbFeS₂ (ICSD-633208), CsFeS₂ (ICSD-25697), Na₅FeS₄ (ICSD-62579), and Na₃FeS₃ (ICSD-8161) [66]. Three dimensional visualization of structures was enabled through the use of VESTA [93].

Conflict of Interest

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: The authors declare that they have no known competing financial interests that could have appeared to influence the work reported in this paper. The authors’ relationships with Prof. Harry Gray inspired the content of this article.

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