Atoms in Highly Symmetric Environments: H in Rhodium and Cobalt Cages, H in an Octahedral Hole in MgO, and Metal Atoms Ca-Zn in C20 Fullerenes

Zikri Altun,Carl Trindle,Erdi Ata Bleda

doi:10.3390/sym13071281

Abstract

An atom trapped in a crystal vacancy, a metal cage, or a fullerene might have many immediate neighbors. Then, the familiar concept of valency or even coordination number seems inadequate to describe the environment of that atom. This difficulty in terminology is illustrated here by four systems: H atoms in tetragonal-pyramidal rhodium cages, H atom in an octahedral cobalt cage, H atom in a MgO octahedral hole, and metal atoms in C20 fullerenes. Density functional theory defines structure and energetics for the systems. Interactions of the atom with its container are characterized by the quantum theory of atoms in molecules (QTAIM) and the theory of non-covalent interactions (NCI). We establish that H atoms in H2Rh13(CO)243− trianion cannot be considered pentavalent, H atom in HCo6(CO)151− anion cannot be considered hexavalent, and H atom in MgO cannot be considered hexavalent. Instead, one should consider the H atom to be set in an environmental field defined by its 5, 6, and 6 neighbors; with interactions described by QTAIM. This point is further illustrated by the electronic structures and QTAIM parameters of M@C20, M=Ca to Zn. The analysis describes the systematic deformation and restoration of the symmetric fullerene in that series.

Highlights

IntroductionLocal geometry and the strength of bonds are rationalized by, for example, the tetravalency of carbon, the divalency of oxygen, and similar assignments to other common atoms
Some structures are borrowed from reported X-ray crystallography studies, while other structures are found by geometry optimization in ωB97XD/cc-pVDZ
We have addressed H atoms in H2 Rh13 (CO)24 3− trianion, for which each H is enclosed in a square-pyramidal rhodium cage; H atom in HCo6 (CO)15 1− anion, for which the H atom in trapped an octahedral cobalt cage, H@MgO in which the H atom occupies a hole from which O atom is absent

Summary

Introduction

Local geometry and the strength of bonds are rationalized by, for example, the tetravalency of carbon, the divalency of oxygen, and similar assignments to other common atoms. Departures from such simple counts are exceptional (though not rare) and require special description. Molecules with larger than normal numbers of bound partners, i.e., “hypervalent compounds”, were identified by Musher [1]. Many of these species may be written as Xn GYm where G is a member of groups 14, 15, and 16. The question of the nature of bonding in such species is evaded by the noncommittal term “hypercoordinate” species apparently first coined by Schleyer [4]

Methods

Results

Conclusion