Ligand strength refers to the ability of a molecule or ion, known as a ligand, to bind to a central metal atom or ion through the formation of coordinate bonds.
The stronger the ligand, the more stable the coordination compound will be. This is because stronger ligands form stronger coordinate bonds, which are more difficult to break.
The ligand strength increases from [Cu(H2O)6]2+ to [CuCl4]2- because chloride ions are stronger ligands than water molecules. This is due to their larger size and greater polarizability, which allows them to form stronger coordinate bonds with the central copper ion.
Ligand strength can affect the color of a coordination compound through the phenomenon of ligand field splitting. Stronger ligands cause a larger splitting of the d-orbitals in the central metal ion, resulting in a higher energy difference between the split orbitals and therefore a higher energy of light absorbed. This can lead to a color change in the compound.
The ligand strength of a coordination compound can be determined experimentally using a variety of techniques, such as UV-Vis spectroscopy, magnetic susceptibility measurements, and crystal field theory calculations. These methods allow us to measure the energy difference between the split d-orbitals and determine the strength of the ligand based on the resulting color and magnetic properties of the compound.