Question

1. Add 0.5 mL of 0.10 M NiCl2 solution to each of 7 test tubes using dropper provided. Label these tubes 1 through 7. 2. Add the following to the respective tubes containing the NiCl2 solution,

Tube 1: 5 drops of water (used here as a control experiment for dilution)

Tube 2: 1 mL of conc. HCl

Tube 3: 5 drops of conc. ammonia

Tube 4: 5 drops of dilute (5% v/v) ethylenediamine solution

Tube 5: 5 drops of 0.25 M C2O4 2- (oxalate) solution

Tube 6: 5 drops of 0.1 M SCN- (thiocyanate) solution

Tube 7: 5 drops of 0.1 M EDTA solution Mix each solution thoroughly and be sure to note the colour of each tube.

3. Check the stability of each complex by adding 3-5 drops of 1 M NaOH solution. Note any changes (e.g., formation of precipitate, colour change, etc.)

4. Add 0.5 mL of 0.10 M CuSO4 solution to each of 7 new test tubes labeled 8 through 14 using dropper provided. Repeat steps 2 and 3.

The “field strength” of a ligand describes how strong the coordination bond is, for a given metal and the ligand that is considered.  The field strength is evaluated by considering how much it causes the d orbitals to split (DE), which of the ligands is strongest?  Which is the weakest?  Explain your choices in each case.

Answer 1

Among the given ligands, 'ethylene diamine' is the strongest one and 'thiocyanate' is the weakest one.

stronger ligands form inner orbital octahedral complexes after forcible pairing of electrons within 3d level of the metal.

weaker ligands cannot cause forcible pairing of electrons within 3d level and thus form outer orbital octahedral complexes.

strong ligands like ethylene diamine in the present case are σ bond donors only, with no orbitals of appropriate symmetry for π bonding interactions. Bonding by these ligands to metals is relatively simple, using only the σ bonds to create relatively weak interactions generating a larger ligand field split, Δ. Lower the metal ligand interaction, high will be the ligand field split.

Ligands that have occupied p orbitals are potentially π donors. These types of weak ligands for example thiocyamte , tend to donate these electrons to the metal along with the σ bonding electrons, exhibiting stronger metal-ligand interactions and an effective decrease of Δ.