The complex [Ru(EDTA)(H2O)]- undergoes substitution reactions with several ligands, replacing the water molecule with the ligand. In all cases the ruthenium stays in the +3 oxidation state and the ligands use a nitrogen donor atom to bind to the metal.
[Ru(EDTA)(H2O)]- + L → [Ru(EDTA)L]- + H2O)
The rate constants for several ligands are as follow:
| Ligand, L |
k(M-1s-1) |
| Pyridine |
6.3 x 103 |
| SCN- |
2.7 x 102 |
| CH3CN |
3.0 x 10 |
(a) One possible mechanism for this substitution reaction is that the water molecule dissociates from the Ru(III) in the rate-determining step, and then the ligand L binds to Ru(III) in a rapid second step. A second possible mechanism is that L approaches the complex, begins to form a new bond to the Ru(III), and displaces the water molecule, all in a single concerted step. Which of these two mechanisms is more consistent with the data? Explain.
(b)What do the results suggest about the relative donor ability of the nitrogens of the three ligands toward Ru(III)?
(c) Assuming that the complexes are all low spin, how many unpaired electrons are in each?