Dear Parul Sood many thanks for sharing this very interesting technical question with the RG community. Although the most common coordination number for nickel are 4 and 6, a coordination number of 8 is not that unusual. High coordination numbers are often the result of the presenec of multidentate (chelating) ligands in which the donor atoms are in close proximity, e.g. chelating nitrate or sulfate ligands. If, in your case, the coordination number 8 has been determined by X-ray crystallography, it needs no particular explanation or "justification". For example, vanadate, molybdate and tungstate anions coordinated to other transition metals also have several oxygen donor atoms in close proximity and can be the cause of higher than usual coordination mumbers.

Good luck with your research and best wishes, Frank Edelmann

The unusual oxidation states of Ni(II) have been extensively reported while the unusual coordination chemistry is not well established not only for Ni(II) but also other transition metal ions. The higher coordination number may be possible in cluster compounds, polynuclear complexes as well as non-stochiometric systems. However, the exact number can be arrived at only if there is a single crystal x-ray diffraction analysis.

Though uncommon but it can happen in polyoxometalate based coordination polymers. Along with single crystal XRD, you can also do other experiments to confirm the 8 coordination number of Nickel.