Synthesis, Crystal Structure, and Bonding Analysis of the Hypoelectronic Cubic Phase Ca₅Pd₆Ge₆

Abstract

The title compound, Ca5Pd6Ge6, was obtained during a systematic investigation of the Ca-Pd-Ge ternary phase diagram. The crystal structure was determined and refined from single-crystal X-ray diffraction data. It crystallizes in a new structure variant of the Y4PdGa12-type structure (Im3̅m, a = 8.7764(4) Å) that features an arrangement of vertex-sharing body-centered cubes of calcium, Ca@Ca8, with a hierarchical bcc network, interpenetrating a second (Pd6Ge6) network consisting of Ge2 dumbbells surrounded by Pd in a strongly flattened octahedron with Pd(μ(2)-η(2),η(4)-Ge2)-like motifs. These octahedra are condensed through the Pd to form a 3D open fcc network. Theoretical band structure calculations suggested that the compound is hypoelectronic with predominantly multicenter-type interatomic interactions involving all three elements and essentially a Hume-Rothery-like regime of electronic stabilization. The similar electronegativity between germanium and palladium atoms has a decisive impact on the bonding picture of the system.

Exo-bonded six-membered heterocycle in the crystal structures of RE7Co2Ge4(RE = La–Nd)

“…In fact, delocalized multicenter bonding is known to allow significant electronic flexibility to the compound and, is often preferred over the localized 2c-2e bond when the electron transfer from the active metal (or charge separation) is not effective enough. 23 For the series Ca 2+n Ag 2+x Ge 2+n-x , [9][10][11][12] it was recently established that the antibonding nature of the HOMO levels of the Zintl anions [Ge n ] (2n+2)− which is ascribed to the planar shape, will foster significant Ge-to-Ca electron back donation, and mitigate the destabilizing effects of the incomplete charge transfer on the covalent bonding of the system. 12 Since rareearth cations are much more polarizing than heavier alkalineearth (Ca-Ba) cations, the incomplete charge separation issue should be even more dramatic in RE based systems and, in turn, an efficient enough compensation mechanism should exist as well.…”

“…22 A unique situation was found in the hypo-electronic phase Ca 6 Pd 5 Ge 5 in which electron poor multicentre deltahedral bonding occurs between all atoms of the structure. 23 Rare-earth (RE) based binary Zintl phases, RE-E, are also known like La 4 Ge 3 (cubic, anti-Th 3 P 4 type) 24 for which the ionic formulation (La 3+ ) 4 (Ge 4− ) 3 is supported by theoretical DFT calculations, predicting semiconducting properties despite the magnitude of the charge transfer. 25 Fredrickson and co-workers have been examining how the structural chemistry of the corresponding RE based ternary system RE-T-E prioritizes and reconciles various bonding schemes of the corresponding binary systems.…”

Designing, Describing and Disseminating New Materials by using the Network Topology Approach

“…A class of compounds where this method may be valuable is the borderline Zintl phases, which often contain more than two metals. A recent example is Ca 5 Pd 6 Ge 2 , with Ca 2+ ions in a Pd 6 Ge 2 anionic network, described as “3D open fcc”, in our terms an nbo net (Figure ). Sometimes the bonding in these borderline compounds is not trivial to work out, therefore one may, in analogy with the discussion on hydrogen bonding, think that if the anionic part can be described by a simple, well‐known network topology, such as the srs or nbo net, then it may be that the bonding also is described well by an anionic and a cationic part.…”

Optimization of Chemical Bonding through Defect Formation and Ordering─The Case of Mg7Pt4Ge4

“…In this context, PIC Mg 3 Pt was recently identified in the Mg-based core–shell Mg@Pt nano-composite via in situ TEM (transmission electron microscopy), showing a remarkable “hydrogen pump” effect, as it can solubilize H atoms and transfer them, expediting the desorption rate of MgH 2 . However, very little is known regarding the structural or electronic factors behind the extraordinary properties of Mg 3 Pt. , In this context, the relativistic effects on the chemical bonding are of interest − and seem to be important for the excellent performance of Pt alloys as catalysts and hydrogen storage materials, and it is important to gain a deeper knowledge on the electronic structure of such materials. Interestingly, Mg 2 PtSi (Na 3 As-type, or more precisely the related ternary Li 2 CuAs type) is structurally very close to Mg 3 Pt (Cu 3 P type), and the former can be derived from the latter by the substitution of one Mg site by a Si atom.…”