Valence Bond Theory of Complexes

As an example of complex formation in a transition metal ion, let s look at the formation of the yellow Cr(NH3)6 ion. This ion is known from experiment to be paramagnetic. We can think of this ion as formed by complexing NH3 ligands with the free chromiumflll) ion. First, we need the configuration of the free ion. The chromium atom configuration is [Ar]3d 4s. In the formation of a transition-metal ion, the outer s electrons (here, 4s ) are lost first, then the outer d electrons (here, two of the 3d electrons). Therefore, Cr has the configuration [Ar]3d. The orbital diagram is 

Note that the 3d electrons are placed in separate orbitals with their spins in the same direction, following Hund s rule. Two empty 3d orbitals, in addition to orbitals of the n = 4 shell, can be used for bonding to ligands. 

To bond electron pairs from six NH3 ligands to forming six equivalent 

---