[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 (∼1:8 Ni/Sb molar ratio) to give dark-brown crystals of the paramagnetic cluster anion [Ni ₅Sb₁₇]^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 Pd ₂AS₁₂ bowl. Density functional theory and bond valence analyses suggest delocalized charge distributions and intermetallic-like properties.