d
The central metal ions in the complexes \(\left[ Fe ( CN )_6\right]^{3-},\left[ Fe ( CN )_6\right]^{4-}\) and \(\left[ Co \left( NO _2\right)_6\right]^{3-}\) undergo \(d ^2 sp ^3\) hybridization to form inner orbital complexes with octahedral geometry.