Ni5Sb17]4- transition-metal Zintl ion complex: crossing the Zintl border in molecular intermetalloid clusters

K3Sb7 and Ni(COD)2 react in the presence of 2,2,2-cryptand in ethylenediamine solutions ( approximately 1:8 Ni/Sb molar ratio) to give dark-brown crystals of the paramagnetic cluster anion [Ni5Sb17]4- as the [K(2,2,2-crypt)]+ salt. The cluster has a Ni(cyclo-Ni4Sb4) ring unit that sits inside a Sb13 bowl. The structure is similar to that of the previously reported [Pd7As16]4- ion containing a Pd(cyclo-Pd4As4) ring unit that sits inside a Pd2As12 bowl. Density functional theory and bond valence analyses suggest delocalized charge distributions and intermetallic-like properties.