From a chemical point of view, bismuth is a pretty remarkable element, which in a textbook sense, has only a couple of stable oxidation states. However, in the real world, bismuth has incredibly rich subvalent chemistry, for which it was coined “The Wonder Metal” by J.D. Corbett [1
], one of the pioneers of bismuth cluster studies. And that was before the true diversity of bismuth subvalent compounds was known! Among particularly interesting objects in the field are bismuth cluster polycations, i.e., electron-deficient polyatomic clusters of varying geometry. The first solid compound containing bismuth cluster polycation was bismuth “monochloride” Bi6
], discovered in the early 60s, and the cation was Bi95+
. In the next six decades, other binary and ternary compounds featuring this cation followed [3
]. A number of other species were also discovered, such as ‘classical’ Bi53+
] and Bi82+
], and more recent additions Bi5+
], and Bi104+
, although the latter has not been obtained as a true homoatomic unity and was mostly centered by palladium or platinum, or on rare occasion even by gold [28
] (we leave its capped forms outside the scope of this paper as their homoatomic nature is questionable, as well as other heteroatomic bismuth clusters). The compounds that contain these polycations are essentially complex salts, with counterions for bismuth clusters being metal halide anions. They are typically synthesized either from melts or non-aqueous solutions. A comprehensive review of synthetic approaches to polycations of group 15–16 elements is given in [32
], with additional information on the use of ionic liquids provided in [33
]. From a topological point of view, these bismuth clusters are fairly unique, however, not completely without relatives. Bismuth’s lighter group 15 neighbor, antimony, features far less abundant homopolyatomic cation chemistry, although Sb53+
] and Sb82+
] have been reported, both having matching polyhedral bismuth counterparts. And then there are also naked clusters of group 14 elements, Zintl ions E52−
(E = Si, Ge, Sn, Pb) [35
], appearing in various intermetallic and coordination compounds, which are isoelectronic and (more or less) isostructural to Bi53+
, although to maintain the same electron count they obviously need to be anions.
The compounds that feature bismuth cluster polycations turned out to be rather difficult to work with, due to them often being air- and moisture-sensitive, and particularly complex objects for the X-ray crystallography, for both technical and fundamental reasons (e.g., a tendency to show pseudo-symmetry or form disordered arrangements) [22
]. As a consequence, some of the reported structures, particularly those from earlier times, were plagued by errors. The quality of structure determination was much improved in more recent times. Some of the older structures, like Bi6
], were re-determined with improved quality, which in the case of Bi6
, led to a slight reinterpretation of its anionic arrangement. More recently, in 2013, these two structures were reinvestigated again, based on the XRD data obtained from crystals synthesized in ionic liquids [37
was also recently synthesized using the reaction of bismuth with its tribromide in [BMIm]Cl·2AlCl3
]. Its crystal structure was reinvestigated by low-temperature single-crystal XRD, however, the authors provided no description of structural details.
In our studies, initially focused on optimizing crystal growth technique for another bismuth subbromide, Bi4Br4, we have come up with an alternative way of producing crystals of Bi6Br7. Having obtained good quality single crystals, we investigated them by means of XRD, primarily to see if there were differences with what was already reported on the compound. And since the structure description still stood as it was given in 1978, we decided to update it based on our data. Improved structure description and atom localization gave us an opportunity to reliably study the band structure of the compound and the topology and bonding in polycations.
To put electronic structure and bonding information in proper context, we also reinvestigated bonding in bismuth polycations and related isoelectronic homoatomic main-group clusters within the concept of R. Bader’s Quantum theory of atoms in molecules (QTAIM) [38
] via topological analysis of the electron localization function (ELF) [39
]. Almost exactly twenty years ago, bonding analysis, based on a combination of natural bond orbital analysis and ELF topology for bismuth polycations that were known at the time, was reported [42
], which was the first comprehensive study of this type. However, with bonding not being the main focus of the study, and with ELF being in its relatively early days of practical use, some of the conclusions made within that paper were not entirely solid. Since then, with the rapid progress of the density functional theory (DFT) applications in chemistry and growing number of bonding analysis tools, along with the discovery of new clusters and emergence of more in-depth approaches to the analysis of bonding indicators, the need to revisit and update that material has been long overdue (although selected cations have received better treatment in more recent publications [10
]). With this research, we offer a sufficiently expanded and amended picture of homoatomic bonding in bismuth cluster polycations and their isoelectronic analogues of group 14 and 15 from the point of direct-space analysis.