Chromium cyanide complexes
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 $(R_\mathrm{int} &lt; 10\%)$ by applying the following geometrical restrains:




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


CCDC ACACCS
[1]
bis(2,4-Pentanedionato)-chromium(II)
CCDC BHPZCR
[2]
bis(Dihydro-bis(1-pyrazolyl)borato) chromium(II)
CCDC CRPORT
[3]
α,β,γ,δ-Tetraphenylporphinato-chromium(II) toluene solvate
CCDC DAQDUV
[4]
hexakis(Tetrahydrofuran)-magnesium tetrakis(perfluorophenyl)-chromium(II) tetrahydrofuran solvate
CCDC DAYNEW
[5]
bis(Pyridinium) diaqua-dibromo-chromium(II) dibromide
CCDC EGINEN
[6]
bis((μ2-Chloro)-hexakis(tetrahydrofuran)-di-lithium) tetrakis(pentachlorophenyl)-chromium(II)
CCDC EQIVOP
[7]
trans-Dichloro-bis(2,6-dimethylpyridine)-chromium(II)
CCDC HARKIU
[8]
trans-bis(tris(t-Butoxy)siloxy)-bis(diethylammonio)-chromium(II)
CCDC KOMRAF
[9]
Tetrapyridine-chromium(II) bis(hexafluorophosphate) dipyridine clathrate
CCDC PITVAP
[10]
bis(Tetra-n-butylammonium) tetrakis(isothiocyanato)-chromium(II)
CCDC PUQPEY
[11]
bis(1-t-butyl-3-(di-t-butylphosphino)imidazol-2-ylidene)-dichloro-chromium
CCDC PUQPIC
[11]
dichloro-bis(1-(di-t-butylphosphino)-3-mesitylimidazol-2-ylidene)-chromium tetrahydrofuran solvate
CCDC PUQPIC
[12]
bis((Phenothiazinyl-N)-tetrahydrofuran)-chromium(II)
CCDC XEBZAF
[13]
trans-bis(4-t-Butylpyridine)-dichloro-chromium(II)
CCDC ZOYCUL
[14]
Dichloro-bis((trimethylphosphine)trimethylsilylamido)-chromium(II)


References


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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.