Subshells, atomic half-filled, stability

Because of stability of the half-filled and filled 4f subshell, the electronic configuration of the Ln atom is either [Xe]4f"5d°6s2 or [Xe]4f 5d16s2. 

In general, the octet rule works for representative metals (Groups lA, IIA) and nonmetals, but not for the transition, inner-transition or post-transition elements. These elements seek additional stability by having filled half-filled or filled orbitals d or/subshell orbitals. The octet rale does not, however, accurately predict the electron configurations of all molecules and compounds. Not all nonmetals, nor metals, can form compounds that satisfy the octet rale. As a result, the octet rale must be used with caution when predicting the electron configurations of molecules and compounds. For example, some atoms violate the octet rale and are surrounded with more than four electron pairs. 

Both the first and the second ionization processes remove electrons from the 4s orbital of these atoms, with the exception of Cr. In general, the 4s electrons are poorly shielded by the 3d electrons, so Zen(4s) increases from left to right and 4 also increases from left to right. While the 7) process removes the sole 4s electron for Cr, the 72 process must remove a 3d electron. The higher value of h for Cr relative to Mn is a consequence of the special stability of half-filled subshell configurations and the higher Zeff of a 3d electron vs. a 4s electron. 

The non half-filled open subshell is the l hg, a d-like molecular orbital. It has 3 electrons. Adding two electrons will make it half filled. Subtracting three will make it empty. This suggests looking at di- and tri-valent atoms to put in the center position to stabilize this icosahedral shell. (Other valences could also work by filling or half-filling other orbitals. Table 1 give the Jahn-Teller stable clusters found in a study of 13 different candidates for the central atom [55]. 

Table 1. Stability, size, and magnetic moments for selected central atoms in icosahedral Fe12X clusters, that have all electrons in either filled or half-filled and maximally spin-polarized icosahedral electronic subshells. The binding energy (relative to Fe12 + X) is in Hartrees and radial bond distance in Bohr.

---