A certain complex of metal M is formulated as MCl3 ∙ 3 H2O . The coordination number of the complex is not known but is expected to be 4 or 6.
(a) Would conductivity measurements
provide information about the coordination number?
(b) In using conductivity measurements to test which ligands are bound to the metal ion, what assumption is made about the rate at which ligands enter or leave the coordination sphere of the metal?
(c) Suppose you experimentally determine that this complex exists in aqueous solution as a single species. Suggest
a likely coordination number and the number and type of each ligand.
a. yes
b. Ligands in the coordination sphere don't leave at all (or extremely slow) or leave quickly.
c. I would think the coordination number would be six (6) and the formula might be M[H2O)6]
(a) Conductivity measurements can provide some information about the coordination number of the complex. Conductivity is a measure of the ability of a substance to conduct electric current. In the case of metal complexes, conductivity is influenced by the mobility of the charged particles, such as ions. If the complex has a higher coordination number, it typically has more ions present in solution, which can increase the conductivity. Conversely, a lower coordination number may result in lower conductivity. So, conductivity measurements can offer some insights into the coordination number, but they cannot provide a definitive answer.
To determine the coordination number more accurately, other techniques such as spectroscopic methods (e.g., UV-Vis, infrared spectroscopy) or crystallography can be employed.
(b) When using conductivity measurements to infer the bound ligands in a metal complex, an assumption is made regarding the rate at which ligands enter or leave the coordination sphere of the metal. This assumption is known as the "rapid equilibrium" assumption. It assumes that ligand exchange occurs rapidly compared to the timescale of the conductivity measurement. In other words, it assumes that the ligand exchange process is fast enough that it can be considered instantaneous, allowing the equilibrium to be established quickly.
(c) When a complex exists in aqueous solution as a single species, it suggests that there is only one dominant coordination number and type of ligands. Based on the given formula MCl3 ∙ 3 H2O, it indicates that there are three water molecules coordinated to the metal ion M. Since the coordination number is expected to be 4 or 6, and we already have three water molecules, it is likely that the coordination number is 6.
Therefore, a likely coordination number for this complex in aqueous solution is 6, and the ligands involved are chloride ions (Cl-) and water molecules (H2O).