CRYSTAL FIELD EFFECTS IN SQUARE PLANAR COMPLEXES:

The square planar arrangement of ligands may be considered to be one derived from the octahedral field by removing two trans-ligands located along the Z-axis. In the process, the eg and t₂g sets of orbitals is lifted i.e., these orbitals will no longer be degenerate.

The four ligands in square planar arrangement around the central metal ion are shown in Fig. As the ligands approach through the axes, they would have greatest influence on dx²-y² orbital, so the energy of this orbital, will be raised most. The d_xy orbital, lying in the same plane, but between the ligands will also have a greater energy though the effect will be less than that on the dx²-y² orbitals. On the other hand, due to absence of ligands along Z-axis, the z²d orbital becomes stable and has energy lower than that of d_xy orbital.

Similarly d_yz and d_xz become more stable. The energy level diagram may be represented as Fig. along with tetrahedral and octahedral fields.

The value of del.sp has been found larger than del.oct because of the reason that d_xz and d_yz orbitals interact with only two ligands in the square planar complexes, while in octahedral complexes the interaction takes place only with four ligands. del.sp has been found equal to 1.3 del.oct

FACTORS FAVOURING SQUARE PLANAR:

(1) Metals (atom/ion) with configuration 4d⁸ or 5d⁸ always form square planar complexes irrespective of natureof the liqand. Such metal atom or ions are as

[PtCl₄ ]^1-although Cl^1- are W.L.yet is is square planar complex

(2) But with the metal atom or the ion with 3d⁸ configuration, for example Ni(II)) complex will be square planar only with the strong field ligands. (tetrahedral with weak ligand).

Others Examples: