Evidence of metal-ligand covalent bonding in complexes

1.Cr(CO)6 is a volatile compound and Ni(CO)4 is a liquid. This indicates that there is a covalent bonding between metal and ligand instead of the ionic. If there would be an ionic bond, then Cr(CO)should be non-volatile and  Ni(CO)4 is solid.

2. Electron Spin Resonance(ESR):

  • The ESR spectrum of [IrCl6]2- suggests that the single unpaired electron is only 70% localized on the metal atom, and 30% is localized on the chloride ion. This indicates that there is sharing of electron and hence some covalency between metal and ligands.

3. The Nuclear Magnetic Resonance(NMR):

  • The fluoride NMR has detected the delocalization of electron in the fluoro complexes, of the paramagnetic metal ion. It is possible only when an unpaired electron spends more than negligible time on  19F nucleus.

4. The Nephelauxetic Effect:

  • The electronic repulsion in d-orbitals of transition metal cations gives rise to a number of energy levels depending upon the arrangement of electrons in d-orbitals.
  • The energy difference between two energy states can be expressed in terms of interelectronic repulsion parameters, called Racah parameters B and C. The difference in energy between two levels having same spin multiplicity can be expressed in terms of  only B, and the difference in energy between  two  energy levels having different spin multiplicities can be expressed in terms of B and C.
  • It is observed experimentally(i.e. from electronic spectra of complexes)that the magnitude of B and C decreases when the complex is formed. The reduced value of B and C indicates that electron density is reduced on metal cation i.e. electron cloud is delocalized over both the metal cation and the ligands. This suggests that there is some covalency between metal cation and ligands.
  • The more the value of B and C is reduced, the greater the delocalization of electron cloud and greater the covalency. The delocalization of the electron cloud over the metal cation and the ligand is called the nephelauxetic effect.

5. Nuclear Quadrupole Resonance(NQR);

  • The NQR spectrum of some of the complexes containing halide ions as ligands like [PtCl4]2- ,  [PdCl4]2-suggests that metal-ligand bond is partly ionic and partly covalent.

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