Suppose you are working in a laboratory synthesizing complexes of Mn and Fe. You have created a new ligand and need to know how strong- or weak-field ligands might affect the properties of your new complex. Predict the electron configuration of an octahedral d⁵ complex with

(a) Strong-field ligands and

(b) Weak-field ligands, and state the number of unpaired electrons in each case.

ANTICIPATE (a) For strong-field ligands, the d-orbitals are widely separated in energy, and so you should expect the electrons to pair in the lower-energy orbitals, resulting in fewer unpaired electrons. (b) For weak-field ligands, the d-orbitals are all close together in energy, so you should expect the electrons to be distributed across all of them with the maximum number of unpaired electrons.

PLAN Add electrons to the orbitals in accordance with the building-up principle, to obtain the lowest energy configuration.