Symbiosis
Symbiosis
This biological term was adopted for chemistry by Jørgensen in 1964, who applied it to explain the process by which a hard ligand on a metal encourages the metal to receive another hard ligand rather than a soft ligand while a soft ligand on a metal encourages the metal to receive another soft ligand rather than a hard ligand.Hard ligands have a tendency to combine with a metal ion already having hard ligands and soft ligands have a tendency to combine with a metal ion already having soft ligands. This tendency is known as symbiosis.
The formation of [BF3NH3] by the combination of BF3 (in which F atoms are hard ligands) and NH3 ion (hard ligands).
Similarly, the formation of BH4− ion by the combination of BH3 (in which H atoms are soft ligands) and H− ion (soft ligands) are an example of symbiosis.
The ambidentate ligand SCN- (thiocyanate) can interact through either its S or N atom with Lewis acids. Interaction will be through the S-atom with a soft acid and through the N-atom with hard acids.
The isolated Co+3 is a hard acid and is expected to make the bond with SCN- ion through N atom as observed in [Co(NH3)5(NCS)]-3.
However, when bound to five soft base ligands like CN- ions, the hardness of cobalt ion (Co+3) is reduced. Thus [Co(CN)5]-2 behaves as a soft acid and prefers to bind with SCN- ion through S atom to form [Co(CN)5(SCN)]-3.
Another example is-
Cr(III) interacts as Cr-NCS, while Pt(II) does so as Pt-SCN.
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