VSEPR model square planar structure

Section 8.13 molecular structure valence shell electron-pair repulsion (VSEPR) model linear structure trigonal planar structure tetrahedral structure trigonal pyramid trigonal bipyramid octahedral structure square planar structure... 

The VSEPR model for predicting structure does not work for complex ions. However, we can safely assume that a complex ion with a coordination number of 6 has an octahedral arrangement of ligands, and that complexes with two ligands are linear. On the other hand, complex ions with a coordination number of 4 can be either tetrahedral or square planar there is no reliable way to predict which will occur in a particular case. 

It is well at this stage to recall the basic premise of the VSEPR approach the electron pairs are similar in energy and repel by either simple electrostatic forces or by the Pauli exclusion principle (75). Within the d-block transition metals, this implies that the 4s, 4p, and 3d electrons should be of similar energy if the model is to work. We know that this is not true, and certainly the energy differences will be greater for metals like Sc, Ti, Zr, Zn, and Hg (27). Worse and different problems exist for the elements Rh, Ir, Pd, Pt, and Au, which exhibit the robust square planar 16-electron structure and which participate in oxidative addition and r uctive elimination reactions. Similarly, the geometry of the common low-spin square planar d compounds of Ni(II) (like Ni(DMG)2 and [Ni(CN)4] "), which do not obey the EAN rule, cannot be deduced from the VSEPR approach. 

There are nine normal modes of vibration for a square planar (Z>4h) XY4 molecule. These are illustrated for [PtCy in Fig. 3.17, along with their appropriate symmetry labels. In the character table (see Appendix 3), the A2 and E representations contain z and x,y) functions, respectively. Therefore, of the vibrational modes shown in Fig. 3.17, only the A2u and Eu modes are IR active. Since [PtCm contains an inversion centre, the rule of mutual exclusion applies, and the 2 and modes are Raman inactive. Similarly, the A g, B g and B2g modes that are Raman active, are IR inactive. Among compounds of the p-block elements, Z>4ii XY4 structures are rare the observation of absorptions at 586, 291 and 161 cm in the IR spectrum of Xep4 is consistent with the structure predicted by the VSEPR model. 

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