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(B) $\left[\mathrm{Ni}(\mathrm{CO})_4\right]$

$\mathrm{Ni}:[\mathrm{Ar}] 3 \mathrm{~d}^8 4 \mathrm{~s}^2$

$CO$ is $S.F.L,$ so pairing occur

Pre hybridization state of $\mathrm{Ni}$

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$\begin{aligned} & \text { (C) }\left[\mathrm{Co}\left(\mathrm{NH}_3\right)_6\right] \mathrm{Cl}_3 \\ & \mathrm{Co}^{+3}:[\mathrm{Ar}] 3 \mathrm{~d}^6 4 \mathrm{~s}^0 \\ & \text { With } \mathrm{Co}^{3+}, \mathrm{NH}_3 \text { act as S.F.L }\end{aligned}$

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$\begin{aligned} & \text { (d) } \mathrm{Na}_3\left[\mathrm{CoF}_6\right] \\ & \mathrm{Co}^{3+}:[\mathrm{Ar}] 3 \mathrm{~d}^6\left(\mathrm{~F}^{\Theta}: \text { W.F.L }\right)\end{aligned}$

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