An atom in a molecule has electrons in $1 s, 2 s, 2 p, 3 s, 3 p, 3 d$ and $4 s$ -orbitals. Thus, the outermost most valence shell composed of $3 d$ -orbitals.
$3 d$ -orbitals can form inner and outer orbital complex depending upon the types of ligands present.
It can undergoes hybridisation of type $s p^{3}$
$\left(\right.$ e.g. $\left.\left[ NiCl _{4}\right]^{2-}\right), d s p^{2}\left(\right.$ e.g $\left.\left[ Ni ( CN )_{4}\right]^{2-}\right), s p^{3} d^{2}$
$\left(\right.$ e.g. $\left[ CoF _{6}\right]^{3-} d^{2} s p^{3}\left(\right.$ e. $\left.g \cdot\left[ Cr \left( NH _{3}\right)_{6}\right]^{3+}\right)$
$d s p^{3}\left(\right.$ e. $g \cdot\left[ Fe ( CO )_{5}\right] $