Which of the following sequences of hybridisation, geometry and magnetic nature are correct for the given coordination compounds?
A. $$[NiCl_4]^{2-}$$ -- sp$$^3$$, tetrahedral, paramagnetic
B. $$[Ni(NH_3)_6]^{2+}$$ -- sp$$^3$$d$$^2$$, octahedral, paramagnetic
C. $$[Ni(CO)_4]$$ -- sp$$^3$$, tetrahedral, paramagnetic
D. $$[Ni(CN)_4]^{2-}$$ -- dsp$$^2$$, square planar, diamagnetic
Choose the correct answer from the options given below :

To determine the correct sequences for the given nickel coordination compounds, we analyze the hybridization, geometry, and magnetic nature of each complex using Valence Bond Theory.

For ([NiCl_4]^{2-}), nickel is in the (+2) oxidation state with the electronic configuration (3d^8). Since (Cl^-) is a weak field ligand, it does not cause pairing of electrons. The complex undergoes (sp^3) hybridization, giving a tetrahedral geometry with two unpaired electrons. Hence, it is paramagnetic, making statement A correct.

For ([Ni(NH_3)_6]^{2+}), nickel is again in the (+2) oxidation state ((3d^8)). The complex uses outer orbital (sp^3d^2) hybridization to form an octahedral structure. It contains two unpaired electrons and is therefore paramagnetic. Hence, statement B is correct.

For ([Ni(CO)_4]), nickel is in the zero oxidation state with electronic configuration (3d^8 4s^2). Carbon monoxide is a strong field ligand and causes complete pairing of electrons before bonding. The complex undergoes (sp^3) hybridization and forms a tetrahedral structure. Since all electrons are paired, the complex is diamagnetic. Therefore, any statement describing it as paramagnetic is incorrect, making statement C incorrect.

For ([Ni(CN)_4]^{2-}), nickel is in the (+2) oxidation state ((3d^8)). Cyanide is a strong field ligand and forces pairing of the (3d) electrons, leaving one vacant (3d) orbital available for (dsp^2) hybridization. The complex adopts a square planar geometry and is diamagnetic. Hence, statement D is correct.

Therefore, the correct statements are A, B, and D only, while statement C is incorrect because ([Ni(CO)_4]) is diamagnetic.