[gavinn]

Could you tell me what are "unpaired spin-states "? Why we say iron does not have such things?
thks?

[charco]

when there are singly occupies 'd' orbitals (or other orbitals for that matter) or even singly occupied molecular orbitals...

Orbital fits two electrons

it's only got one = unpaired spin

With transition metals the 'd' orbital arrangement energetically speaking depends upon the crystal (electrostatic) field produced by the ligands.

In some cases this means that the electrons can pair up and in others not.

In FeII compounds for example there are 6 d electrons. In som,e compounds these occupy three of the d orbitals and are all spin paired. In other compounds the d orbitals are split in a different way and there are two paired and two unpaired d electrons. This second one is an unpaired spin state.

[GCT]

Could you tell me what are "unpaired spin-states "? Why we say iron does not have such things?
thks?

try observing the quantum spin states of iron, what do you see?