Square planar coordination vibrational modes

This molecule has a square-planar structure with >4h symmetry. With reference to the coordinate system displayed in Fig. 7.3.6, the symmetry of the vibrational modes may be derived in the following manner ... 

Figure 3.20 Vibrational modes, u, to u7, of atoms in a square planar site such as the [FeOJ coordination environment in gillespite (from Hitchman, 1985). Note the acentric a2u, p2u ar>d eu modes that facilitate electronic transitions within Fe2+ ions by vibronic coupling.

Figure 2.21 shows the seven normal modes of vibration of square-planar XY4 molecules. Vibrations V3, V6, and V7 are infrared-active, whereas Vi V2, and V4 are Raman-active. However, V5 is inactive both in infrared and Raman spectra. In the case of the [PdCl4] ion, the V5 was observed at 136 cm by inelastic neutron scattering (INS) spectroscopy [1088]. Table 2.6j lists the vibrational frequencies of some ions belonging to this group. Chen et al. [1088a] also reported the IR and Raman spectra of K2[MX4] (M = Pt or Pd and X = Cl or Br) and band assignments on the basis of normal coordinate analysis. 

Figure 5.5 Normal modes of vibration of (a) tetrahedral and (b) square-planar XY4 molecules. From Nakamoto, K., Infrared and Raman Spectra of Inorganic and Coordination Compounds , in Handbook of Vibrational Spectroscopy, Vol. 3, Chalmers, J. M. and Griffiths, P. R. (Eds), pp. 1872-1892. Cop)uight 2002. John " Afiley Sons Limited. Reproduced with permission.

In order to illustrate the vibrational motions of a molecule belonging to a non-commutative symmetry point group, we return to the considerations of Section 2.3.2 and once more use as our example the square-planar complex, NiFj. A non-linear penta-atomic molecule has nine independent vibrational coordinates, distributed among the symmetry species of T>4h. These can be fully specified by standard methods [7], but the following simple qualitative considerations allow us to conclude that there are seven in-plane and two out-of-plane vibrations. Fig. 4.10 depicts several of the in-plane modes the motion of the nickel atom to conserve the center of mass is implied. 

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