Computational exploration of hypercoordinate atoms in molecules
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This dissertation presents a computational exploration of unconventional molecules containing hypercoordinate atoms (i.e., those having a higher number of nearest neighboring atoms than expected by Lewis-based predictions). The four chapters in this dissertation discuss two different structural types: (a) those containing three-dimensional hypercoordinate environment (Chapters 2 and 3) and, (b) those having atoms exhibiting planar hypercoordination (Chapters 4 and 5). While di-coordinate bridging hydrogens are well known, higher hydrogen coordinations are rare. Chapter 2, entitled “Hypercoordinate Hydrogen in Scandium Hydride Clusters” demonstrates unusually high hydrogen coordinations (up to eight) in scandium hydride clusters. In contrast to known molecules containing hexacoordinate carbons, which have asymmetric environments, Chapter 3 (Synergistic Bonding in Three Dimensional Hypercoordinate Carbon) presents molecules with symmetrical three-dimensional hexacoordinate carbons (surrounded by six equivalent carbons). Achieving hypercoordination in planar geometries is especially challenging due to the reduction of out-of-plane bonding opportunities and increased steric repulsion between the ligands. Chapter 4, entitled “Myriad Planar Hexacoordinate Carbon Molecules Inviting Synthesis”, focuses on the elaboration of planar hexacoordinate carbon cluster (CB62–) based on a hydrocarbon grafting strategy. Chapter 5 (Cyclic Boron Clusters Enclosing Planar Hypercoordinate Cobalt, Iron and Nickel) illustrates cyclic boron clusters containing octa- or nona- planar hypercoordinate iron, cobalt, and nickel atoms. Although transition metals often have high coordination numbers (above six) in three-dimensional environments, such coordination numbers are unusual and difficult to achieve in planar geometries. The computational findings presented in this work contribute to the development of structural chemistry by proposing unconventional bonding environments.