Chemical bonding trends in Y2M3Si5 polar intermetallics (M = Mn–Cu, Tc–Pd, Re–Pt)

Polar R2M3X5 intermetallics (R = rare earth metal/actinide, M = transition metal, X = p-block ele-ment) represent a large family of inorganic compounds that have been extensively studied due to their intriguing physical properties, offering a valuable platform to explore structural and physical changes as a function of composition[1]. The interest in the physics of these compounds has not been accompanied by an equally strong focus on their chemistry, particularly in terms of under-standing their chemical bonding and properties, for example as heterogeneous catalysts. To fill this gap, the Y2M3Si5 series (M = Mn–Cu, Tc–Pd, Re–Pt) was selected for a detailed bonding analysis aimed at uncovering the chemical factors favoring their crystallization in tetragonal (tP40-Sc2Fe3Si5), monoclinic (mS40-Lu2Co3Si5), or orthorhombic (oI40-U2Co3Si5) structures. The quantum-chemical tools required for this study were extracted from the wavefunction by means of the LOBSTER[2] code. In particular, the (projected) Crystal Orbital Hamilton Population (pCOHP), Crystal Orbital Bond Index (COBI) and their integrated values (IpCOHP and ICO-BI)[3] were selected. In addition to covalently bonded Si polyanions, expectable based on intera-tomic distances analysis, covalent Si–M and Si–Y interactions were identified. The overall bond-ing scenario is completed by the occurrence of metal–metal interactions whose covalency changes noticeably with M, revealing a more complex picture than that predicted by the Zintl model. The distribution of covalency within the unit cell, expressed as IpCOHP%/cell, was found to behave like a periodic property with monotonic trends across periods and down groups (Figure 1). The maximization of the IpCOHP%/cell associated with M–Si interactions was found to play a crucial role in stabilizing the experimentally observed crystal structure.