I am looking for inorganic crystal structures that exhibit tetrahedrally coordinated cations arranged in a vertex-sharing dimer arrangement, where the cation - shared vertex - cation angle is close to 180°:

Feature of ionic arrangement I am looking for. The cations form a vertex-sharing dimeric structure

Are there any known instances of such crystal structures? If there are, then any number of examples would be welcome. If there are examples of compounds with just similar features (dimers with different coordination, for instance), then this would also be appreciated.

  • $\begingroup$ A starting point would be to search for molecules with the same symmetry as the structure you show. $\endgroup$
    – porphyrin
    Commented Feb 23, 2023 at 15:48
  • $\begingroup$ I did. I checked every database i have access to, and found nothing. I have repeatedly expanded the search range, and found nothing. Further searching would entail just checking every compound with a cation that might assume tetrahedral arrangement. Which would take a very long time. $\endgroup$ Commented Feb 23, 2023 at 16:49
  • $\begingroup$ Why aren't the silicates known to yield all kinds of "tetrahedra" polyhedra connected via corners, ridges, and surfaces suitable here? $\endgroup$
    – Buttonwood
    Commented Feb 26, 2023 at 16:22
  • $\begingroup$ @Buttonwood I am not just looking for compounds with tetrahedral coordinations in general. If you know of silicate compounds that exhibits isolated, vertex-sharing dimers, then please inform me. $\endgroup$ Commented Feb 26, 2023 at 18:22

2 Answers 2


Given the unit must be isolated and should not be the subunit of an ionic network, the following graph model for CCDC ConQuest's (ver. 2022.3.0) query with restraints based on connectivity $(\mathrm T_n)$ and fixed the central bond angle within 170° to 180° range gave 119 hits.

CCDC 2022 ConQuest's query

The unique found moieties are as follows with exemplary references from CSD database:

Formula          Name Bridging angle           Conf. CSD ID
$\ce{[Fe2OCl6]^2-}$ (μ-Oxo)bis[trichloroferrate(III)] $\angle\ce{Fe-O-Fe} = 180°$ Staggered ADETIM
$\ce{[Fe2OBr6]^2-}$ (μ-Oxo)bis[tribromoferrate(III)] $\angle\ce{Fe-O-Fe} = 180°$ Staggered AWIDEQ
$\ce{[Be2F7]^3-}$ (μ-Fluoro)bis[trifluoroberyllate(II)] $\angle\ce{Be-O-Be} = 180°$ Staggered SEYHOT
$\ce{[Hg2I7]^3-}$ (μ-Iodo)bis[triiodomercurate(II)] $\angle\ce{Hg-O-Hg} = 180°$ Staggered GODDUY
$\ce{Zn(CH3)2}$ Dimethylzinc $\angle\ce{C-Zn-C} = 180°$ Staggered SAJDEO
$\ce{Cd(CH3)2}$ Dimethylcadmium $\angle\ce{C-Cd-C} = 180°$ Eclipsed AROPAA
$\ce{[Tl(CH3)2]^+}$ Dimethylthallium(III) $\angle\ce{C-Tl-C} = 179°$ Eclipsed DOMHAO
$\ce{[Cu(CH3)2]^-}$ Dimethylcuprate(I) $\angle\ce{C-Cu-C} = 180°$ Staggered DAZWIK
$\ce{[Cu(CF3)2]^-}$ Bis(trifluoromethyl)cuprate(I) $\angle\ce{C-Cu-C} = 180°$ Staggered NONVIW
$\ce{Hg(CF3)2}$ Bis(trifluoromethyl)mercury $\angle\ce{C-Hg-C} = 180°$ Staggered DTFMHG
$\ce{Hg(CHCl)2}$ Bis(dichloromethyl)mercury $\angle\ce{C-Hg-C} = 180°$ Staggered PIPFEZ
$\ce{[Mg(NH3)2]^2+}$ Diamminemagnesium(II) $\angle\ce{N-Mg-N} = 180°$ Eclipsed QUQQEB
$\ce{[Ag(CF3)2]^-}$ Bis(trifluoromethyl)argentate(III) $\angle\ce{C-Ag-C} = 180°$ Staggered ATABAB
$\ce{[Ag(NH3)2]^+}$ Diamminesilver(I) $\angle\ce{N-Ag-N} = 178°$ Staggered AWEGOZ
$\ce{[Au(CF3)2]^-}$ Bis(trifluoromethyl)aurate(I) $\angle\ce{N-Au-N} = 178°$ Staggered QOCPAC
$\ce{[Au(CH3)2]^-}$ Dimethylaurate(I) $\angle\ce{C-Au-C} = 180°$ Eclipsed WEMGOK
$\ce{[Au(NH3)2]^+}$ Diamminegold(I) $\angle\ce{N-Au-N} = 180°$ Staggered DIWZOB
$\ce{[Au(GeCl3)2]^-}$ Bis(trichlorogermyl)aurate(I) $\angle\ce{N-Au-N} = 180°$ Staggered RAGGEL


  1. There are different conformations as well as several entries that do not follow your cation-anion assignment exactly.
  2. There is a bias for protons placement for the structures refined with Shelx due to the limitations of the riding model, so the confirmations for H-containing (methyl, ammonia) ligands might be incorrect.
  3. The database contains about four structures with linear diethyl ether moieties, which I omitted as outliers.
  • 1
    $\begingroup$ Unrelated, but I'd be grateful if someone could suggest the way for preventing automatic line breaking for MathJax in Markdown tables so that chemical formulas don't split across the lines. $\endgroup$
    – andselisk
    Commented Feb 25, 2023 at 15:11
  • $\begingroup$ This is very close to exactly what i was looking for, but I appear to have made my phrasing of the question insufficiently clear. You made your search on the assumption it "should not be the subunit of an ionic network". This is the opposite of my intention. It should be a subunit of a crystalline structure, with the An positions bonding into a wider structure, but dimeric. Not sharing futher vertecies/edges/faces with other TME coords. As an example, the structure of La3FeO6, but with the isolated Fe tetrahedra arranged as isolated dimer instead, in the configuration i showed in the question. $\endgroup$ Commented Feb 26, 2023 at 13:39
  • $\begingroup$ @AChemicalEntity The moieties here are the units of the structure. I stressed out the fact they are not arbitrarily selected from the infinite chains or sheets, i.e. they are not covalently bound to other atoms, i.e. fulfilling "not sharing further vertices/edges/faces with other TMEs" criteria. $\endgroup$
    – andselisk
    Commented Feb 26, 2023 at 14:47
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    $\begingroup$ @andselisk The answer by @pawel provided here with multiple instances of   apparently does «the trick». Indeed a bit ugly, I didn't identify a way to state something in line of "do this x times" to keep the underlying code easier to read and maintain. Eh bien ... it is an attempt. The edit affected both the chemical, as well as the mathematical formulae (first and third column). $\endgroup$
    – Buttonwood
    Commented Feb 26, 2023 at 16:15
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    $\begingroup$ @andselisk While «filling up» one column, I noticed «the other» required some of these special characters; an iteration over multiple steps. And this was at the rim of «surprise», and «annoying» (which is a biased comparison between LaTeX and markdown / JavaScript) because there is trailing space in columns 1, 2, 3 as if there were trailing(!) padding. Or, markdown simply assumes non-proportional characters all on the same base line (no horizontal shrinkage for the sub- and superscripts, either) and hence has a particular difficult task. Maybe something for the site's engineers? $\endgroup$
    – Buttonwood
    Commented Feb 26, 2023 at 17:44

Sodium sulfide has an anti-fluoritecstructure, in which the sodium ions are tetrahedrally coordinated and the sulfide ions are cubically coordinated. If you look along any body diagonal of the cube of sodium ions coordinating the sulfur, you see your dimeric structure. Each sulfide ion is shared by four such dimeric arrangements oriented in different directions.

Magnesium silicide has a similar structure, but with less ionic bonding and more electron delocalization leading to a narrow indirect gap for semiconduction [2]. This electronic property has led to potential thermoelectric applications.


  1. Dr. Hiroshi Mizoguchi, Dr. Yoshinori Muraba, Prof. Daniel C. Fredrickson, Dr. Satoru Matsuishi, Prof. Toshio Kamiya, Prof. Hideo Hosono (2017). "The Unique Electronic Structure of Mg2Si: Shaping the Conduction Bands of Semiconductors with Multicenter Bonding". Angewandte Chemie 129(34), 10269-10273. https://doi.org/10.1002/ange.201701681
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    $\begingroup$ Hmm... This isn't really what I was after. I am not looking for a three-dimensional structure where you could rotate and choose the axis to pigeonhole the arrangement, but rather a structure where the dimer is a distinct structural feature within a separate matrix. $\endgroup$ Commented Feb 23, 2023 at 14:44
  • $\begingroup$ @AChemicalEntity How would the matrix be attached to the dimer? Via ionic interactions? The dimer shown in the question has 7 anions and 2 cations, so it is likely that there is a net charge. $\endgroup$
    – Karsten
    Commented Feb 24, 2023 at 19:09
  • $\begingroup$ The dimeric units share a common center. So your count multiple-counts an anion. $\endgroup$ Commented Feb 24, 2023 at 19:27
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    $\begingroup$ @Karsten The dimer is part of the unit cell of a larger inorganic crystal structure. Essentially, just a feature in that full crystal structure. Yes, it does have a net negative charge. $\endgroup$ Commented Feb 24, 2023 at 20:07
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    $\begingroup$ @Karsten To add on the previous comment, the entire crystal structure is ionic. $\endgroup$ Commented Feb 24, 2023 at 20:27

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