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I have rarely seen any chromium $(\ce{Cr})$ square planar complexes and I have been told that $\ce{[Cr(CN)4]-}$ is tetrahedral.

So, if the statement is actually correct, are there any other known examples of square planar complexes of chromium?

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This doesn't appear to be a rare case and there are plenty $\ce{Cr^2+}$ square planar complexes. I found 15 square planar complexes with well-determined crystal structures $(R_\mathrm{int} < 10\%)$ by applying the following geometrical restrains:

CrEl4 fragmant

  • Chromium must have C.N. exactly 4 (T4).
  • Chromium is allowed to be coordinated by virtually any element of periodic table $\ce{El}$.
  • Dashed lines signify any bond type.
  • All four angles between neighbors should lead to nearly ideal square planar geometry: $∠(\ce{El-Cr-El}) = \pu{(90.0 ± 0.5)^\circ}$ (by allowing higher tolerance the number of structures grows in geometrical progression: for $∠(\ce{El-Cr-El}) = \pu{(90.0 ± 5.0)^\circ}$ there were nearly 100 structures).

List of compounds

  1. CCDC ACACCS [1] bis(2,4-Pentanedionato)-chromium(II)

  2. CCDC BHPZCR [2] bis(Dihydro-bis(1-pyrazolyl)borato) chromium(II)

  3. CCDC CRPORT [3] α,β,γ,δ-Tetraphenylporphinato-chromium(II) toluene solvate

  4. CCDC DAQDUV [4] hexakis(Tetrahydrofuran)-magnesium tetrakis(perfluorophenyl)-chromium(II) tetrahydrofuran solvate

  5. CCDC DAYNEW [5] bis(Pyridinium) diaqua-dibromo-chromium(II) dibromide

  6. CCDC EGINEN [6] bis((μ2-Chloro)-hexakis(tetrahydrofuran)-di-lithium) tetrakis(pentachlorophenyl)-chromium(II)

  7. CCDC EQIVOP [7] trans-Dichloro-bis(2,6-dimethylpyridine)-chromium(II)

  8. CCDC HARKIU [8] trans-bis(tris(t-Butoxy)siloxy)-bis(diethylammonio)-chromium(II)

  9. CCDC KOMRAF [9] Tetrapyridine-chromium(II) bis(hexafluorophosphate) dipyridine clathrate

  10. CCDC PITVAP [10] bis(Tetra-n-butylammonium) tetrakis(isothiocyanato)-chromium(II)

  11. CCDC PUQPEY [11] bis(1-t-butyl-3-(di-t-butylphosphino)imidazol-2-ylidene)-dichloro-chromium

  12. CCDC PUQPIC [11] dichloro-bis(1-(di-t-butylphosphino)-3-mesitylimidazol-2-ylidene)-chromium tetrahydrofuran solvate

  13. CCDC PUQPIC [12] bis((Phenothiazinyl-N)-tetrahydrofuran)-chromium(II)

  14. CCDC XEBZAF [13] trans-bis(4-t-Butylpyridine)-dichloro-chromium(II)

  15. CCDC ZOYCUL [14] Dichloro-bis((trimethylphosphine)trimethylsilylamido)-chromium(II)

References

  1. Cotton, F. A.; Rice, C. E.; Rice, G. W. The Crystal and Molecular Structures of Bis(2,4-Pentanedionato)Chromium. Inorganica Chimica Acta 1977, 24, 231–234. https://doi.org/10/bhjhh5.
  2. Dapporto, P.; Mani, F.; Mealli, C. Vanadium(II) and chromium(II) Complexes with Poly(1-Pyrazolyl)Borate Ligands. Crystal and Molecular Structures of Potassium Tris[Dihydrobis(1-Pyrazolyl)Borato]Vanadium(II) Monoalcoholate and Bis[Dihydrobis(1-Pyrazolyl)Borato]chromium(II). Inorg. Chem. 1978, 17 (5), 1323–1329. https://doi.org/10/dzfdh8.
  3. Scheidt, W. R.; Reed, C. A. Stereochemistry of the Toluene Solvate of α,β,γ,δ-Tetraphenylporphinatochromium(II). Inorg. Chem. 1978, 17 (3), 710–714. https://doi.org/10/cvqg73.
  4. Forniés, J.; Martín, A.; Martín, L. F.; Menjón, B.; Zhen, H.; Bell, A.; Rhodes, L. F. The First Structurally Characterized Homoleptic Aryl-Manganese(III) Compound and the Corresponding Isoleptic and Isoelectronic chromium(II) Derivative. Organometallics 2005, 24 (13), 3266–3271. https://doi.org/10/bz3r8w.
  5. Ladd, M. F. C.; Larkworthy, L. F.; Leonard, G. A.; Povey, D. C.; Tandon, S. S. Crystal and Molecular Structure of a Complex Bromide Containing Planar Diaquadibromochromium(II) Units. J. Chem. Soc., Dalton Trans. 1984, No. 10, 2351. https://doi.org/10/cj5z95.
  6. Alonso, P. J.; Forniés, J.; García‐Monforte, M. A.; Martín, A.; Menjón, B.; Rillo, C. A New Series of Homoleptic, Paramagnetic Organochromium Derivatives: Synthesis, Characterization, and Study of Their Magnetic Properties. Chemistry – A European Journal 2002, 8 (17), 4056–4065. https://doi.org/10/cx9r27.
  7. Gibson, V. C.; Newton, C.; Redshaw, C.; Solan, G. A.; White, A. J. P.; Williams, D. J. Low valent chromium complexes bearing N,O-chelating pyridyl-enolate ligands $\ce{[OC(Bu^t)(=2-CHN5H3Me-x)]^-}$ $(x = 3–6)$. Dalton Trans. 2003, No. 24, 4612–4617. https://doi.org/10/ffdkvr.
  8. Terry, K. W.; Gantzel, P. K.; Tilley, T. D. chromium(II) and Chromium(III) Tri-Tert-Butoxysiloxy Complexes. Inorg. Chem. 1993, 32 (23), 5402–5404. https://doi.org/10/dfw68w.
  9. Fochi, Giovanni.; Straehle, Joachim.; Gingl, Franz. Disproportionation of the Bis(Benzene)Chromium Cation Induced by Pyridine. Crystal and Molecular Structure of Two Tetrapyridine Complexes Containing chromium(II) and Chromium(III). Inorg. Chem. 1991, 30 (24), 4669–4671. https://doi.org/10/c3ffnj.
  10. Larkworthy, L. F.; Leonard, G. A.; Povey, D. C.; Tandon, S. S.; Tucker, B. J.; Smith, G. W. Crystal Structures and Magnetic Behaviour of the Two Forms of Tetra-n-Butylammonium Tetraisothiocyanatochromate(II). J. Chem. Soc., Dalton Trans. 1994, No. 9, 1425. https://doi.org/10/cgtgbm.
  11. Ai, P.; Danopoulos, A. A.; Braunstein, P. N-Phosphanyl- and N,N′-Diphosphanyl-Substituted N-Heterocyclic Carbene Chromium Complexes: Synthesis, Structures, and Catalytic Ethylene Oligomerization. Organometallics 2015, 34 (16), 4109–4116. https://doi.org/10/ggc5mx.
  12. Edema, J. J. H.; Gambarotta, S.; Meetsma, A.; Spek, A. L.; Smeets, W. J. J.; Chiang, M. Y. chromium(II) Amides: Synthesis and Structures. J. Chem. Soc., Dalton Trans. 1993, No. 5, 789. https://doi.org/10/fcfcdg.
  13. Cotton, F. A.; Dikarev, E. V.; Gu, J.; Herrero, S.; Modec, B. Alkylpyridine Complexes of Tungsten(II) and chromium(II). First Rotational Isomers of $\ce{W2X4L4}$ Molecules with $D_\mathrm{2h}$ and $D_2$ Symmetries. Inorg. Chem. 2000, 39 (23), 5407–5411. https://doi.org/10/c5cfr4.
  14. Miekisch, T.; Mai, H. J.; zu Köcker, R. M.; Dehnicke, K.; Magull, J.; Goesmann, H. Silylierte Phosphanimin‐Komplexe von Chrom(II), Palladium(II) und Kupfer(II). Die Kristallstrukturen von $\ce{[CrCl2(Me3SiNPMe3)2]},$ $\ce{[PdCl2(Me3SiNPEt3)2]}$ und $\ce{[CuCl2(Me3SiNPMe3)]2}.$ Z. Anorg. Allg. Chem. 1996, 622 (3), 583–588. https://doi.org/10/bm3fq8.
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    $\begingroup$ Clearly the porphyrin complex is cheating as that could never be tetrahedral ;) $\endgroup$ – Jan Nov 19 '19 at 4:56
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    $\begingroup$ @Jan No (DABDA), your zinc is cheating, my chromium is adapting:D Could be worse, and sit way above the plane, like naughty lanthanoids do:) $\endgroup$ – andselisk♦ Nov 19 '19 at 7:01
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An article by Bradley et al. [1] documents $\ce{[Cr(N(SiMe3)2)2(THF)2]}$ as a square planar complex. However, the $\ce{[Cr(N(SiMe3)2)3NO]}$ complex is pseudotetrahedral $(C_\mathrm{3v}).$

Figures below are provided by andselisk, the are from the reference above.

References

  1. Bradley, D. C.; Hursthouse, M. B.; Newing, C. W.; Welch, A. J. Square Planar and Tetrahedral Chromium(II) Complexes; Crystal Structure Determinations. J. Chem. Soc., Chem. Commun. 1972, No. 9, 567–568. https://doi.org/10/br8qtf. enter image description hereenter image description here
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  • $\begingroup$ I think this one can really benefit from adding an illustration. Besides, this crystal structure is gorgeous:) $\endgroup$ – andselisk♦ Nov 18 '19 at 17:58
  • $\begingroup$ @andselisk I believe this one can benefit from getting past the paywall in order to reach said illustration (in my reference only the abstract is available up front). Must we pay for an improved answer? $\endgroup$ – Oscar Lanzi Nov 18 '19 at 18:19
  • $\begingroup$ Sorry, my bad, I assumed people actually read or at least flip through the paper before referring to it. Anyway, here are figure 1 and figure 2; feel free to use those as you see fit, or just ignore:) $\endgroup$ – andselisk♦ Nov 18 '19 at 18:40
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    $\begingroup$ My bad also, I assumed that papers would be readily available to read through. Hard to read behind a paywall, thanks @andselisk. $\endgroup$ – Oscar Lanzi Nov 18 '19 at 19:10

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