Parallels in Structural Chemistry between the Molecular and Metallic Realms Revealed by Complex Intermetallic Phases

The structural diversity of intermetallic phases poses a great challenge to chemical theory and materials design. In this Account, two examples are used to illustrate how a focus on the most complex of these structures (and their relationships to simpler ones) can reveal how chemical principles underlie structure for broad families of compounds. First, we show how experimental investigations into the Fe-Al-Si system, inspired by host-guest like features in the structure of Fe₂₅Al₇₈Si₂₀, led to a theoretical approach to deriving isolobal analogies between molecular and intermetallic compounds and a more general electron counting rule. Specifically, the Fe₈Al_17.6Si_7.4 compound obtained in these syntheses was traced to a fragmentation of the fluorite-type structure (as adopted by NiSi₂), driven by the maintenance of 18-electron configurations on the transition metal centers. The desire to quickly generalize these conclusions to a broader range of phases motivated the formulation of the reversed approximation Molecular Orbital (raMO) approach. The application of raMO to a diverse series of compounds allowed us recognize the prevalence of electron pair sharing in multicenter functions isolobal to classical covalent bonds and to propose the 18 - n electron rule for transition metal-main group (T-E) intermetallic compounds. These approaches provided a framework for understanding the 14-electron rule of the Nowotny Chimney Ladder phases, a temperature-driven phase transition in GdCoSi₂, and the bcc-structure of group VI transition metals. In the second story, we recount the development of the chemical pressure approach to analyzing atomic size and packing effects in intermetallic structures. We begin with how the stability of the Yb₂Ag₇-type structure of Ca₂Ag₇ over the more common CaCu₅ type highlights the pressing need for approaches to assessing the role of atomic size in crystal structures, and inspired the development of the DFT-Chemical Pressure (CP) method. Examples of structural phenomena elucidated by this approach are then given, including the Y/Co₂ dumbbell substitution in the Th₂Zn₁₇-type phase Y₂Co₁₇, and local icosahedral order in the Tsai-type quasicrystal approximant CaCd₆. We next discuss how deriving relationships between the CP features of a structure and its phonon modes provided a way of both validating the method and visualizing how local arrangements can give rise to soft vibrational modes. The themes of structural mechanisms for CP relief and soft atomic motions merge in the discovery and elucidation of the incommensurately modulated phase CaPd₅. In the conclusion of this Account, we propose combining raMO and CP methods for focused predictions of structural phenomena.