Square Planar Complexes
In square planar complexes the central metal cation is either dsp 2 or sp2d hybridised. Examples are shown below:
[Ni(CN)4]2– ion:-In this complex the valence shell electronic configuration of Ni (o.s. +2) is 3d8. Since CN– is a strong ligand, therefore, pairing of two unpaired electrons of 3d orbitals takes place resultin g in a vacant d orbital. This vacant 3d orbital gets hybridised with the vacant 4s and two of 4p orbitals to give four dsp2 hybrid orbitals. This lead to the formation of square planar geometry and the magnetic moment is zero. Hence the complex is diamagnetic.
n = 0 , μ = 0 (diamagnetic)
[PdCl4]–: In this complex the o.s. of Pd is +2. Hence the outermost electronic configuration is 3d8. Alt hough Cl– is a weak ligand, pairing of d electrons takes place because Pd is a transit ion metal of 4d series and according to VBT assumptions, the pairing always takes place in the 4d and 5d metals irrespective of the ligand type. Hence, 3d electrons get paired here. Consequently, dsp2 hybridisation takes place and the magnetic moment is zero. Thus, the geometry of this complex is square planar in contrast to corresponding Ni complex NiCl42– which is tetrahedral.
n = 0, μ = 0 (diamagnetic)
[Cu(NH3)4]2+: In this complex ion, the o.s. of copper is +2 and its valence shell electronic configuration is 3d9. Magnetic moment observations indicate that the complex is paramagnetic. This observation is in correspondence with the tetrahedral geometry of this complex ion having sp3 hybridisation as shown below:
n = 1, = 1.73 B.M. (paramagnetic)
In this compound the expected hybridisation was sp3 but according to ESR (Electron Spin Resonance Spectroscop y) and X-ray crystallography studies, the structure of [Cu(NH3)4]2+ ion is square planar. But this is only possible with either dsp2 or sp2d hybrid orbitals.
To form dsp2 hybrid orbitals, it is considered that the unpaired electrons in 3d orbital are promoted to the 4p orbital as shown below:
In the above electronic configuration the single unpaired electron is present in the higher level 4p orbital. Hence it can be oxidised ver y easily. But the Cu 3+ ion does not exist i.e. oxidation of Cu2+ to Cu3+ is not possible.
This observation led to the failure of above theory of dsp2 hybridisation also.
Finally, it was suggested that in square planar [Cu(NH3)4]2+ complex, Cu2+ ion is sp2d hybridised as shown below.
In this configuration, one of the three 4p orbitals, 4pz orbital does not participate in hybridisation because pz orbital lies above and below the plane of the ion. [Cu(py)2]2+, [Cu(en)2]2+, [Cu(CN)4]2– all complexes are square planar and Cu2+ is sp2d hybridised.
Limitations of VBT
Although, VBT is useful to visualize the bonding in complexes but it fails to explain certain observations in coordination complexes:
Crystal Field Theory (CFT)
To explain the above properties which did not explain by VBT, Bethe and Van Vleck proposed another theory which is known as Crystal Field Theory or CFT.
This theory is based on following assumptions:
The shapes of five d orbitals are shown below:
All the three orbitals dxy, dyz and dzx lie in between the axes. These orbitals lie in xy, yz and zx-planes respectively. The dx2 – y2 orbital lie on x and y axes and the dz2 orbital on z-axis. All the five d orbitals are gerade because they have centre of symmetry exist between their axis. The (+) and (-) signs indicate the different phase of the lobes of orbitals.