Draw all of the isomers of an octahedral complex which has six unidentate ligands, two of type $\mathrm{A}$ and four of type $\mathrm{B}$.

Draw the energy level diagrams and indicate the occupancy of the orbitals in the following complexes.

(a) ${\mathrm{d}}^6$, octahedral, low spin
(b) ${\mathrm{d}}^{\text{9}}$, octahedral with tetragonal elongation
(c) ${\mathrm{d}}^8$, square planar
(d) ${\mathrm{d}}^6$, tetrahedral

Calculate the difference in crystal field stabilization energy (in unit of ${\mathrm{\Delta }}_{\mathrm{o}}$) between the complexes (a) and (d), assuming that the ligands are strong field ligands.

(Answer: octahedral $-2.4 {\mathrm{\Delta }}_{\mathrm{o}}$, tetrahedral $-0.27 {\mathrm{\Delta }}_{\mathrm{o}}$, difference $-2.13 {\mathrm{\Delta }}_{\mathrm{o}}$)