Q.What are the high-spin and low-spin complexes ?

Step 1. Recall CFT's splitting picture (section 9.9.6). Ligands split a metal ion's d orbitals into a lower t2g set and an upper eg set, separated by Delta-o.

Step 2. Define high spin. With a WEAK-field ligand, Delta-o is small (smaller than the electron-pairing energy), so electrons occupy all five d orbitals SINGLY first (Hund's rule) before any pairing occurs -- giving the maximum possible number of unpaired electrons for that electron count. This is a high-spin complex.

Step 3. Define low spin. With a STRONG-field ligand, Delta-o is large (larger than the pairing energy), so electrons instead PAIR UP completely within the lower t2g set before any electron occupies the higher eg set -- giving the minimum possible number of unpaired electrons. This is a low-spin complex. …

• Defining high/low spin in terms of the metal's oxidation state rather than the ligand's field strength. …