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 La3Ni2Ga2 type of the hypothetical “Eu3Pd2Ge2” and the YPd2Si type of the EuPd2Ge parent structures. 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 structures form a unique class of electron precise ‘transition metal Zintl phases’ featuring metal–metal bonding according to the 18-n rule. Therefore, Pd phase with divalent Europium could be anticipated to comply with the 18-n rule as well with formally cationic Pd+ (d9), according to the formulation (Eu2+)4{(Pd2)2+}2(Ge4–)3 in close analogy with the prototype with neutral Rh pairs and trivalent rare-earth (Nd3+)4{(Rh2)0}2(Ge4–)3.