Question

A charge neutral macrocyclic ligand having four donor atoms froms a red, diamagnetic complex of Ni(II). The counter ions for this complex is the very weak Lewis base, ClO4-. When ClO4- is replaced by thiocyanate, violet complex forms.

a. Draw the structures of the red and violet complexes such that thier geometires can be decerned based only on your drawings.

b. Draw the d-orbital slitting diagrams of the red and violet complexes such that the relative energies of the orbitals in the two complexes can be compared.

c. Wse your orbital diagrams to show were the d electroms reside in each complex. Explain your electron configurations.

d. describe an experiment that you could do to test whether your predicted electron configurations are reasonable. Show what the results of this experiment would be for the d electron configureation that you showed in part c

Answer 1

A neutral macrocyclic ligand with four donor atoms produces a red diamagnetic low-spin d8 complex of Ni(II) if the anion is the weakly coordinating perchlorate ion. When perchlorate is replaced by two thiocyanate ions SCN–, the complex turns violet and is high-spin with two unpaired electrons. Interpret the change in terms of the structure.

The perchlorate ion either does not coordinate, or provides a sufficiently weak ligand field, that the complex with this anion is effectively square planar. In square planar d8 complexes (D4h symmetry), all 8 electrons are paired, and the complex will be diamagnetic. However, when strong axial ligands are introduced, the electronic structure approximates that of an octahedral complex, and octahedral d8 is paramagnetic with two unpaired electrons.

Thiocyanate is produced by the reaction of elemental sulfur or thiosulfate with cyanide:

8 CN⁻ + S₈ → 8 SCN⁻

CN⁻ + S₂O₃²⁻ → SCN⁻ + SO₃²⁻