Clusters have been the subject of intense investigations since their famous definition launched by Cotton in 1963, and the area has expanded ever since. One obvious development addresses the widening of the definition of what to call a cluster: from purely (transition) metal−metal linked assemblies, as per Cotton's early denomination, to nonmetal/metal clusters or purely nonmetal cages, like fullerenes, and even noncovalent aggregates such as water clusters. The other extension concerns the broadened spectrum of compositions within the aforementioned cluster types and their corresponding structures that range from trinuclear motifs to clusters with sizes in the range of the hemoglobin unit. This review article reports on one cluster family that has its origins in traditional Zintl anion chemistry but has undergone rapid development in recent years, namely, ligand-free clusters that combine main group and transition metal atoms. Depending on the position of the transition metal atom(s), one refers to such clusters as intermetalloid (endohedral) clusters or as a special type of heterometallic clusters. The predominant synthetic access makes use of soluble Zintl anions. Other pathways for their preparation include traditional solid state reactions of according element combinations or bottom-up syntheses employing low valent organo-main group element sources. This survey will shed light on all of these approaches, with an emphasis on the syntheses that employ soluble Zintl anion compounds. The article will give a comprehensive overview of the currently known compounds, their different synthesis protocols, and analytic techniques for determination of their compositions, structures, and further properties. Additionally, this survey will report peculiarities of bonding situations found within some of the cluster molecules, which were studied by means of sophisticated quantum chemical investigations.

The encapsulation of actinide ions in intermetalloid clusters has long been proposed but was never realized synthetically. We report the isolation and experimental, as well as quantum chemical, characterization of the uranium-centered clusters [U@Bi12](3-), [U@Tl2Bi11](3-), [U@Pb7Bi7](3-), and [U@Pb4Bi9](3-), upon reaction of (EE'Bi2)(2-) (E = Ga, Tl, E' = Bi; E = E' = Pb) and [U(C5Me4H)3] or [U(C5Me4H)3Cl] in 1,2-diaminoethane. For [U@Bi12](3-), magnetic susceptibility measurements rationalize an unprecedented antiferromagnetic coupling between a magnetic U(4+) site and a unique radical Bi12(7-) shell.

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Zintl anions have been known for more than a century and were studied systematically by Eduard Zintl in the 1930s. Since then, they have been investigated for their interesting structures, bonding, and physical properties - in solid Zintl phases, in solvate salts, and in solution. While their popularity remained limited for several decades, Zintl ion chemistry has recently experienced a renaissance as a result of breakthroughs regarding their modifications into multinary anions that include transition metal atoms, their organic derivatization, and their oxidative linkage. A plethora of reports from the past two decades - demonstrating the ever growing variety of Zintl ion chemistry - have been since summarized in several review articles. Herein, we intend to present the most recent developments, which also shed light on Zintl anions and clusters as useful precursors for materials development, as illustrated by one recent example.

Extraction of a solid with the nominal composition "K GeBi" with 1,2-diaminoethane (en) in the presence of 4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane (crypt-222) afforded the salt [K(crypt-222)] (Ge Bi ). The 18-atom Zintl anion (Ge Bi ) has a heretofore unknown molecular topology, which can be thought of as the formal condensation product of two E cages along a shared Ge waist. In this way, (Ge Bi ) represents the largest and most structurally complex Bi-containing polyanion. We describe its stepwise formation, its geometric and electronic structure, and comment on relative stabilities of isomers with different distributions of the four Ge atoms on the 18 positions that were investigated using DFT calculations.

Reaction of the binary Zintl anion (Sn2 Sb2 )(2-) with the β-diketiminato complex [LCu(NCMe)] (L=nacnac=[(N(C6 H3 (i) Pr2 -2,6)C(Me))2 CH](-) ) in ethylenediamine or DMF affords the ternary cluster dimer {[CuSn5 Sb3 ](2-) }2 (1) as its [K(crypt-222)](+) salt. The chemical formulation of 1 is supported by energy-dispersive X-ray spectroscopy (EDX) and quantum chemical calculations. Each monomeric part of the dimer represents a trimetallic "[CuSn5 Sb3 ](2-) " cluster, with an architecture in between a tricapped trigonal prism and a capped square antiprism. As shown by quantum chemical investigations, the presence of Sb atoms and, in particular, of Cu atoms in the cluster skeleton makes the monomeric unit behave like an inhomogeneous superatom, which clearly prefers to dimerize, thereby producing a relatively short, yet virtually non-bonding Cu⋅⋅⋅Cu distance.

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A new complex comprising [Co@Ge12]3– cluster core was synthesized through the reaction of CoMe(PMe3)4 and K4Ge9 in ethylenediamine solution. The pseudo‐D5d geometry of this cluster can be viewed as structurally derived from an icosahedral cage via Jahn‐Teller effect, leading to notable bonding differences from Ih‐[M@E12]q− clusters (E = Sn, Pb with q = 2, 3). The [Co@Ge12]3– cluster represents a structural conundrum. On the basis of the geometric considerations it could be viewed as a sandwich complex. However, chemical bonding analysis revealed that there is a direct covalent multicenter bonding between the two pentagons, thus indicating that the structure should be viewed as an elongated icosahedron. Further theoretical calculations on [M@Ge12]3− (M = Rh, Ir, Mt) indicate that similar compounds of larger‐size metal atoms can drive the complete transformation of [M@Ge12]3− clusters from icosahedron structure to a sandwich one.