By using which of the following whether a given ligand is strong field or weak field can be determined.

Octahedral $C{r}^{2+}$ complexes should be used. High spin or weak field  $C{r}^{2+}$ complexes have 4 unpaired electrons but low spin (strong field)  $C{r}^{2+}$ complexes have 2 unpaired electrons. Octahedral  $N{i}^{2+}$ complexes cannot be used to distinguish weak – from stron – field ligands by examining magnetic properties. By using the visible spectra of the complexes, the strength of ligand can be decided depending on splitting energy.