K 10 Ga 3 Bi 6.65 – The First Compound in the Ternary A -Ga-Bi System Comprising Cyclic Tris-meta Borate-Analogous [Ga 3 Bi 6 ] 9– Units and Bi 2 Dumbbells

. K 10 Ga 3 Bi 6.65 is the first representative in the ternary system A -Ga-Bi ( A = alkali metal). It contains [Ga 3 Bi 6 ] 9– anions with planar triangular-coordinated Ga atoms as the main structural feature, ac-companied by isolated Bi–Bi dumbbells. Alkali metal cations are


Introduction
Heteroatomic anions [Tr x Pn y ] zas they occur in Zintl phases with different compositions, charges and bonding situations comprise a large number of representatives (Tr = Ga, In, Tl, Pn = P, As, Sb, Bi). The Tr atoms in these species are either in a trigonal planar environment as monomeric, dimeric or trimeric [TrPn 3 ] fragments, or they are arranged in condensed five-membered rings (Table S1, Supporting Information) and [Tr@Pn 4 ] tetrahedra, which then can be interconnected in different ways. Edge, face and corner sharing units lead to structures that can extend in various dimensions. The nature of the anionic framework of these structures is collectively related to the number, the size, and the charge of the cations and, in turn, to the atomic ratio in the [Tr x Pn y ] zanion and its valence electron count. Consequently, the less reduced general anionic compositions [Tr x Pn y ] zexhibit one-, two-or three-dimensional anionic frameworks. [1] A large number of A-Tr-Pn systems, which contain isolated fragments, are located in the alkali metal-rich area of the ternary phase diagrams. The most simple unit is the trigonal planar [TrPn 3 ] 6anion which is isostructural to the trioxoborate(III) anion unit BO 3 3-, and in which the Ga or In atoms are covalently bound to three Pn atoms. We obtained the highly charged carbonate analogue [SnBi 3 ] 5even from solution. [2] Phases with the highly charged [TrPn 3 ] 6anions consequently have a high alkali metal content for charge compensation, such as in Na 6 GaPn 3 (Pn = P, As) [3,4]  As), [5,6] and Cs 6 GaSb 3 [7] (Figure 1a). Phases with the heavier alkali metals like Cs 6 Ga 2 P 4 , [8] Rb 3 GaP 2 , [9] and Cs 3 GaAs 2 [10] contain isolated dimeric anions [Ga 2 Pn 4 ] 6-, which share two Pn atoms in an almost planar Ga 2 Pn 2 butterfly-type ring structure with two Ga-Pn exo bonds (Figure 1b). The Ga-containing ternary K 20 Ga 6 As 12.66 and K 20 Ga 6 Sb 12.66 [11] as well as quaternary phases RbNa 8 Ga 3 P 6 and RbNa 8 Ga 3 As 6 [12] comprise cyclic trimers [Ga 3 Pn 6 ] 9-(Pn = P, As and Sb) in which three monomers formally share two Pn atoms each. The planar and slightly deformed Ga 3 Pn 3 hexagon in the anion [Ga 3 Pn 6 ] 9shows Ga atoms in a trigonal planar environment with three Ga-Pn exo bonds (Figure 1c).
Linking of the building units TrPn 3 also allows for the construction of one-dimensional infinite chains in ternary compounds. Almost planar five-membered rings consisting of two Ga and three Pn atoms, which are inter-connected by the terminal Ρn atoms with a Ga-Pn exo bond, build a linear chain (Figure 2a) in K 2 GaP 2 , [13] A 2 GaAs 2 (A = K, Rb), [14,15] and A 2 GaSb 2 (A = K, Rb, Cs). [16][17][18] In case of the heavier homologue Tl, two compounds in the A/Tl/Pn systems are known with more complex heteroatomic chains including Tl-Tl bonds in Na 6 TlSb 4 [19] (Figure 2b) and Tl-Tl bonds beside Pn-Pn bonds in K 6 Tl 2 Sb 3 [1] (Figure 2c), that are part of one-dimensionally linked six-membered rings.
Herein we report on a novel ternary compound that contains two different independent anionic units. Single crystals of K 10 Ga 3 Bi 6.65 were, along with copper gallides, first discovered as one of the products, obtained in an attempted synthesis of the K analogue of Na 12 Cu 12 Sn 21 [20] and iso-valence electronic replacement of the tetrel Sn by the triel Ga and Bi. Later, K 10 Ga 3 Bi 6.65 was synthesized as the main product together with some residual binary KBi 2 in a high temperature reaction of the elements K, Bi and Ga with the ratio 10:3:6.

Experimental Section
Synthesis: The sample was prepared from the elements using 210.9 mg K, 124.4 mg Ga and 676.2 mg Bi, corresponding to an  atomic ratio K:Ga:Bi of 10:3:6. The reaction mixture was weighted in an Ar-filled glove box and then packed in a niobium ampoule, which was sealed afterwards. The ampoule was transferred into a silica tube and heated to 500°C with a rate of 4°·min -1 , held at this temperature for 72 h, then slowly cooled (with a rate of 0.1°·min -1 ) to 400°C and finally quenched to room temperature. The PXRD analysis showed the presence of K 10 Ga 3 Bi 6.65 as the main phase with some residual binary KBi 2 (see Figure S1, Supporting Information) Because of the high absorption of the sample due to the presence of Bi, the sample had to be significantly diluted by diamond powder.
Structure Determination: Single crystals were picked from the reaction product in a glove box, and data collection was performed on a Stoe StadiVari diffractometer with an exposure time of 50 seconds and a detector distance of 60 mm. The structure was solved by Direct Methods in the hexagonal space group P6 3 /m. A numerical absorption correction was carried out using the software packages X-Red and Z. Anorg. Allg. Chem. 2020, 659-664 www.zaac.wiley-vch.de X-Shape, [21,22] showing an expected high absorption coefficient of 41.729 mm -1 . The refinement revealed three crystallographically independent Bi sites (two in Wyckoff position 6h and one in 2b), one Ga (6h) and three K atoms (all in Wyckoff position 6h). The electron deficiency in the Bi3 position is most likely due to a defect, resulting in an occupancy of 64.7 %. Partial defects were tested for all K positions, which led to lower occupation factors for the Bi3 position, but no sufficient difference was observed, suggesting no significant correlation between the K content and the defect in the Bi position. Crystallographic data and selected details of the structure refinement for the compound with the refined composition K 10 Ga 3 Bi 6.647 (4) are listed in Table 1, atomic parameters and anisotropic displacement parameters in Table 2 and Table 3, respectively.
Further details of the crystal structure investigations may be obtained from from the Cambridge Crystallographic Data Centre, CCDC, 12 Union Road, Cambridge CB21EZ, UK (Fax: +44-1223-336-033;       atoms. The Ga-Bi distances are 2.710 and 2.647 Å within the hexagonal ring and the exo bond, respectively (Table 4) and are thus shorter than the sum of the covalent radii of 2.75 Å typical for Ga-Bi single bonds (1.24 and 1.51 Å for Ga and Bi, respectively [23] ), but longer than the value calculated for a Ga-Bi double bond (2.58 Å [23] ), indicative of a certain degree of electron delocalization within the anion (Scheme 1).  (Figure 4b). This distance is shorter than in the singly bonded [Bi 2 ] 2unit (1.51 ϫ 2 = 3.02 Å [23] ) and closer to range of doubly bonded [Bi 2 ] -(1.41 ϫ 2 = 2.82 Å [23] ) and even triplybonded Bi 2 units (1.35 ϫ 2 = 2.70 Å [23] ). Bi-Bi distances in Zintl phases are generally longer due to the anionic charge located at the Bi atoms. Two-bonded Bi atoms in KBi [24] reveal Bi-Bi distances longer than 3 Å. The smallest anion has been described as a doubly-bonded Bi dumbbell [Bi=Bi] 2- [25] in K 3 Bi 2 , while the tetrameric planar zigzag [Bi 4 ] 4unit [26] in K 5 Bi 4 features a delocalized double bond. According to the 8-N rule both compounds contain an additional free electron which most likely is delocalized. [Bi 2 ] dumbbells are found in several binary A-Bi (A = K, Cs) phases and show a Bi-Bi distance of 2.975 Å in Cs 3 Bi 2 . [27] K 5 Bi 4 [28] contains a planar zigzag [Bi 4 ] 4tetramer with six delocalized electrons. Both compounds contain according to the 8-N rule one additional electron which most likely is fully delocalized over the structure (metallic). Consequently, the rather short Bi-Bi distance of 2.734 Å in K 10 Ga 3 Bi 6.65 reveals a higher bond order than a single bond.
Despite the different atoms or the different connection of the Bi atoms in the [Ga 3 Bi 6 ] 9subunits the potassium atoms form almost regular trigonal prismatic coordination polyhedra around the Ga and Bi atoms (Figure 5a). The prisms of one unit are linked within the ab plane by sharing rectangular faces. Further, the remaining Bi atoms (Figure 5b) are encapsulated by trigonal antiprisms of K atoms. In the unit cell, [Ga 3 Bi 6 ] 9units are shifted by ½ in [001] direction with respect to each other. As a consequence the vertices of the prisms of one unit become caps of the outside rectangular faces of the prisms surrounding neighboring [Ga 3 Bi 6 ] 9units ( Figure 5c).

Discussion
According to the Zintl-Klemm concept and the (8-N) rule, the [Ga 3 Bi 6 ] unit contains three three-bonded (3b-Ga 0 ), three two-bonded (2b-Bi 1-) and three singly-bonded (1b-Bi 2-) atoms giving the polyanion [Ga 3 Bi 6 ] 9-, and its charge is balanced by nine K cations. There are in average almost 2/3 Bi3 atoms per one [Ga 3   (K + ) 10 [Ga 3 Bi 6 ] 9-([Bi-Bi] 4-) 1/6 ([Bi=Bi] 2-) 1/6 . Since the value obtained from the single crystal structure determination is slightly smaller (0.647(4) instead of 0.667), statistically a small amount of Bi 2 dumbbells might be replaced by isolated Bi 3as anticipated for Pn atoms before in K 10 Ga 3 As 6.33 and K 10 Ga 3 Sb 6.33 . Note added in Proof: During the revision of this manuscript, it came to our attention that a compound with similar composition K 10 Ga 3 Bi 6.33 has been described in a PhD Thesis (S. Klos, Dissertation Thesis, Rheinische Friedrich-Wilhelms-Universität Bonn, Germany, 2018). The lower Bi content does however not hint for Bi-Bi dumbbell formation.