Eu4Pd4Ge3 with the Nd4Rh4Ge3 Structure type: A Class of Transition metal Zintl phase with metal–metal bonding following the 18-n rule?

The ternary phase Eu4Pd4Ge3 was obtained from the reaction of element mixtures at high temperature, using small silver additive and, its crystal structure was refined from single crystal diffraction data. It adopts the predicted Nd4Rh4Ge3 type structure (monoclinic space group C2/c, No 15) with unit cell parameters a = 21.474(7) Å; b = 5.869(2) Å, (β = 110.15(1)°), c = 8.100(3) Å and, V= 958.5(4) Å3 (Z = 4). This structure type is characterized by remarkable noble metal pairs that strongly interact with the surrounding tetrel elements (Si or Ge), and has been reported so far with the noble metals Rh or Pd only, in combination with either rare-earth or calcium as electropositive ‘active’ metals, respectively. Thus, the crystal structure of ternary phase Eu4Pd4Ge3 may be best viewed as a 1:1 intergrowth of fragments cut from the parent structures EuPd2Ge (YPd2Si type) and the imaginary “Eu3Pd2Ge2” (La3Ni2Ga2 type). All previously reported Pd (d10) based compounds are obtained with divalent Ca, while Rh (d9) based phases are with trivalent rare-earth. At first glance, the Nd4Rh4Ge3 type structural family forms a unique class of electron precise ‘transition metal Zintl phases’ featuring metal–metal bonding according to the 18-n rule. Therefore, the Pd phase with divalent Europium could be anticipated to comply with the 18-n rule as well, by assuming formally cationic Pd+ (d9), according to the formulation (Eu2+)4{(Pd2)2+}2(Ge4–)3 in close analogy with the prototype in which rather neutral Rh0 (d9) is assumed in combination with trivalent rare-earth (Nd3+)4{(Rh2)0}2(Ge4–)3.