Abstract
Exploratory synthesis in the area of polar intermetallics has yielded a rich variety of structures that offer clues into the transition in bonding between Zintl and Hume-Rothery phases. In this article, we present a bonding analysis of one such compound, Ca10Pt7Si3, whose large Ca content offers the potential for negative formal oxidation states on the Pt. The structure can be divided into a sublattice of Ca cations and a Pt–Si polyanionic network built from Pt7Si3 trefoil units linked through Pt–Pt contacts of 3.14 Å. DFT-calibrated Hückel models reveal that the compound adheres well to a Zintl-like electron counting scheme, in which the Pt–Si and Pt–Pt contacts are equated with two-center two-electron bonds. The experimental electron count is in excess of that predicted by 2%, a discrepancy which is attributed to the electron transfer from the Ca to the Pt–Si network being incomplete. For the Pt–Pt contacts, the occupancy of the bonding orbitals is dependent on the participation of the surrounding Ca atoms in bridging interactions. This use of multi-center interactions isolobal to classical two-center two-electron bonds may illustrate one path by which the bonds delocalize as one moves from the Zintl phases toward the Hume-Rothery domain.
Highlights
Intermetallic phases comprise a realm of staggering structural diversity
A rich structural chemistry of polar intermetallic phases has resulted, which offers opportunities to chart how the close-shell electron configurations of the Zintl phases map to the delocalized bonding of the Hume-Rothery phases
We have carried out a bonding analysis of the intermetallic Ca10Pt7Si3, using
Summary
Intermetallic phases comprise a realm of staggering structural diversity. One of the best compasses for navigating this realm is the grouping of compounds according to the electronegativity differences (Δχ) of their component elements. A rich structural chemistry of polar intermetallic phases has resulted, which offers opportunities to chart how the close-shell electron configurations of the Zintl phases map to the delocalized bonding of the Hume-Rothery phases. The Pt/Si sublattice has a large anionic charge that nearly guarantees a negative oxidation state on the Pt. Even if we fill the octets of the Si atoms to Si4−, we are still left with eight extra electrons for the seven Pt atoms per formula unit. Primitive hexagonal lattice, but with an important difference: neighboring clusters along c are rotated by 60°relative to each other about the stacking axis, so that the six outer Pt atoms of each Pt7Si3 unit make a close contact (3.14 Å; Figure 1c) with a counterpart in a neighboring layer. The resulting picture illustrates how the isolobal analogy [27] can serve to extend the Zintl concept deeper into the range of polar intermetallics
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