Question

Most transition metal complex ions with octahedral geometry follow 18-electron rule. explain why it is so using the molecular orbital diagram.

some transition metal with d8 electron configuration ten to form square planar geometry rattan than octahedral geometry. explain the reason using the molecular orbital diagram.

Answer 1

according to 18 electron rule thermodynamically more stable compounds of transition metal must contain 18 valence electrons in their shell which comprises of the metal d electrons and the electrons which is given by the metal bound ligands

The driving force for this is that the complexes which obeys 18 e rule , the Δ_o splitting will be the highest and is generally applicable to ligands which are good σ donor and π acceptor like CO, PF₃, olefins and arenes which is placed at the higher position of the spectrochemical series.

The t_2gorbital now called sa bonding due to interactions with these ligand orbitals and therefore be occupied by 6 electrons. On the other hand e_g* orbital is antibonding and hence remains unoccupied.

THE MOT diagram is given below

For square planar complexes , think of this way that removal of a ligands along the z-axis of regular octahedron will only leaves four ligands directed mainly in the x-y plane. Hence , the CFT diagram for such kind of square planar complexes can be redrawn or restructured from the regular geometry of octahedral .

Hence removal of two ligands mainly stabilizes the d_z2 level, which then leaves the d_x2-y₂ level as highly destabilized. As a result of this , the d_x2-y₂ remains vacant in such square planar complexes for those metals which is having d⁸ configuration