The sign of the spin-orbit coupling constant in (t2)52t2 states of mx4+ ions

It is shown that the sign of the spin-orbit coupling constant in 2T2 states of MX+4 molecular ions (M = C, Si, Ge; X = F, Cl, Br, I) arising from the (t2)5 electron configuration can be positive, negative or even zero. Predominantly, the sign depends on the orientation of the p-orbitals on the four halogen atoms with respect to the M-X bonds. Experimentally observed splittings show that it is incorrect to describe the two highest t2 valence orbitals of MX4 as pure pπ non-bonding and pure pσ bonding, since both exhibit pσ/pπ mixing. This is confirmed by semi-empirical SCF MNDO molecular-orbital calculations.