Where Is the Fluoro Wall?: A Quantum Chemical Investigation.

Julian D Rolfes,Maurice Van Gastel,Frank Neese

doi:10.1021/acs.inorgchem.9b03474

Julian D Rolfes, Maurice Van Gastel + Show 1 more

Open Access

https://doi.org/10.1021/acs.inorgchem.9b03474

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Abstract

Despite their isoelectronic properties, fluoro and oxo ligands exhibit completely different chemical behavior. Formally speaking, the first is known to exclusively form single bonds, while the latter is generally observed to form double (or even triple) bonds. The biggest difference, however, lies in what is known among inorganic chemists as the Oxo Wall: the fact that six-coordinate tetragonal transition metal oxo complexes are not observed beyond group 7 elements. While the Oxo Wall was explained a few decades ago, some questions regarding the nature of the Oxo Wall remain unanswered. For example, why do group 8 oxo complexes with high oxidation states not violate the Oxo Wall? Moreover, why are transition metal fluoro complexes observed through the whole transition metal series? In order to understand how the small difference between these two isoelectronic ligands can give rise to such different chemical behaviors, we conducted an extensive computational analysis of the geometric and electronic properties of model fluoro and oxo complexes with metals around the Oxo Wall. Among many insights into the details of the Oxo Wall, we mostly learned that the oxygen 2p orbitals are prone to meaningfully interact with transition metal d orbitals, because they match not only spatially but also energetically, while for fluorine the p orbital energies are lower to an extent that interaction with transition metal d orbitals is much reduced. This in turn implies that in those instances where the metal d orbitals principally accessible for interaction are occupied, the oxygen 2p orbitals are too exposed to be stable.

Highlights

The Oxo Wall is a widely known and accepted concept among the inorganic chemical community.[1]
While transition metal oxo complexes are a prominent motif in biological oxidation processes,[2,3] the concept of the Oxo Wall dates back to 1962 when Ballhausen and Gray developed a molecular orbital energy level scheme that correctly described the electronic structure of the vanadyl ion.[4]
Through an extensive computational analysis of the geometric and electronic structure of a series of model complexes around the Oxo Wall, we solidified the theoretical foundation of the Oxo Wall concept

Summary

Introduction

The Oxo Wall is a widely known and accepted concept among the inorganic chemical community.[1]. While transition metal oxo complexes are a prominent motif in biological oxidation processes,[2,3] the concept of the Oxo Wall dates back to 1962 when Ballhausen and Gray developed a molecular orbital energy level scheme that correctly described the electronic structure of the vanadyl ion.[4] The description of chromyl and molybdenyl ions followed shortly after, where the metal−oxo interaction was represented as a triple bond for the first time.[5] This notion is based on elementary molecular orbital considerations in which the bond order is deduced by subtracting the number of electrons in antibonding orbitals of a given bond from the number of electrons in the bonding counterparts of these orbitals and dividing the result by two.[6] For example, in the six-coordinate tetragonal oxo complex Mo(V)OCl52− the single d electron is found in the nonbonding dxy orbital, leaving the two other d orbitals of t2g symmetry (dxz and dyz) amenable to π interactions with the px/y orbitals from the oxo ligand on the z axis (Figure 2). A total of six electrons are present in the two bonding π-orbitals and the bonding σ-orbital (interaction between oxo pz and metal dz[2] orbital), while all antibonding counterparts are empty, leaving a metal−oxo triple bond

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