why cobalt forms diamagnetic complexes with some weak ligands also as oxalato etc.

Spin

A complex can be classified as high spin or low spin. When talking about all the molecular geometries, we compare the crystal field splitting energy (?) and the pairing energy (p). Normally, these two quantities determine whether a certain field is low spin or high spin.

When the crystal field splitting energy is greater than the pairing energy, electrons will fill up all the lower energy orbitals first and only then pair with electrons in these orbitals before moving to the higher energy orbitals. Electrons tend to fall in the lowest possible energy state, and since the pairing energy is lower than the crystal field splitting energy, it is more energetically favorable for the electrons to pair up and completely fill up the low energy orbitals until there is no room left at all, and only then begin to fill the high energy orbitals. On the other hand, when the pairing energy is greater than the crystal field energy, the electrons will occupy all the orbitals first and then pair up, without regard to the energy of the orbitals.If every orbital of a lower energy had one electron, and the orbitals of the hext higher energy had none, an electron in this case would occupy the higher energy orbital. This followsHund's rulethat says all orbitals must be occupied before pairing begins. Remember, this situation only occurs whenthe pairing energy is greater than the crystal field energy. These phenomena occur because of the electron's tendency to fall into the lowest available energy state.

Another method to determine the spin of a complex is to look at its field strength and the wavelength of color it absorbs. If the field is strong, it will have few unpaired electrons and thus low spin. If the field is weak, it will have more unpaired electrons and thus high spin. In terms of wavelength, a field that absorbs high energy photons (in other words, low wavelength light) has low spin and a field that absorbs low energy photons (high wavelength light) has high spin.

Once again, whether a complex is high spin or low spin depends on two main factors: the crystal field splitting energy and the pairing energy. The electrons will take the path of least resistance--the path that requires the least amount of energy. If the paring energy is greater than ?, then electrons will move to a higher energy orbital because it takes less energy. If the pairing energy is less than ?, then the electrons will pair up rather than moving singly to a higher energy orbital. Below, tips and examples are given to help figure out whether a certain molecule is high spin or low spin.

Thank You
Ruchi
Askiitians Faculty