Has a low spin tetrahedral Ni(II) complex been synthesised?

Question

How can $\ce{Ni(II)}$ form a low spin tetrahedral compound? I was not able to find such compounds.

$\ce{Ni(II)}$ will have $\mathrm {4s^03d^8}$ and tetrahedral needs $\mathrm{sp^3}$. So $2$ electrons will always be unpaired. Is a rare case possible?

I am only aware of hybridisation theory and basic CFT theory for complexes.

Answer 1

You asked two different questions in your title and the text body of the post. I wasn't sure which one is you meant to know. However, orthocresol has explained the question about low spin tetrahedral compounds: Why are low spin tetrahedral complexes so rare?. He also explain "Why it is wrong to use the concept of hybridization for transition metal complexes."

About your question in the title: "Has a tetrahedral $\ce{Ni(II)}$ complex been synthesized?"

Answer is "yes." Orthocresol has given common example here, which is $\ce{NiCl4^2-}$. There are also non-common tetrahedral $\ce{Ni(II)}$ complexes have been synthesized. For instance, a series of $\ce{[(C2H5)3PO]2NiX2}$ where $\ce{X = Cl, Br, I}$ has been synthesized by Cotton and Goodgame (Ref.1; yes, that is the same Cotton in Cotton and Wilkinson):

The $\ce{Ni(II)}$ complexes $\ce{[(C2H5)3PO]2NiX2}$ where $\ce{X = Cl, Br,}$ and $\ce{I}$ have been prepared. Their physical properties, especially their electronic spectra and high magnetic moments, leave little doubt that the nickel ion in each case surrounded by two oxygen atom and two halogen atoms in an essentially tetrahedral array.

In addition, some distorted-tetrahedral nickel(II) complexes (Ref.2) and some pseudo-tetrahedral nickel(II) complexes (Ref.3) have been prepared as well.

References:

1. F. Albert Cotton, David M. L. Goodgame, "New Tetrahedral Complexes of Nickel(II)," J. Am. Chem. Soc. 1960, 82(22), 5771–5774 (https://doi.org/10.1021/ja01507a001).
2. S. Trofimenko, J. C. Calabrese, J. K. Kochi, S. Wolowiec, F. B. Hulsbergen, J. Reedijk, "Spectroscopic analysis, coordination geometry, and X-ray structures of nickel(II) compounds with sterically demanding tris(pyrazolyl)borate ligands and azide or (thio)cyanate anions. Crystal and molecular structures of bis[($\mu$-thiocyanato-N,S)(hydrotris(3-isopropyl-4-bromopyrazol-1-yl)borato)nickel(II)]-3-heptane and (thiocyanato-N)(hydrotris(3-tert-butyl-5-methylpyrazol-1-yl)borato)nickel(II)," Inorg. Chem. 1992, 31(19), 3943–3950 (https://doi.org/10.1021/ic00045a017).
3. John T. Donoghue, Russell S. Drago, "Non-aqueous Coördination Phenomena-Complexes of Hexamethyl-phosphoramide. II. Pseudo-tetrahedral Complexes of Nickel(II) and Cobalt(II)," Inorg. Chem. 1963, 2(3), 572–576 (https://doi.org/10.1021/ic50007a036).