The biological term

symbiosis Symbiosis (from Greek , , "living together", from , , "together", and , bíōsis, "living") is any type of a close and long-term biological interaction between two different biological organisms, be it mutualistic, commensalistic, or parasit ...

was first used in chemistry by C. K. Jørgensen in 1964, to refer to the process by which a

hard Hard may refer to: * Hardness, resistance of physical materials to deformation or fracture * Hard water, water with high mineral content Arts and entertainment * ''Hard'' (TV series), a French TV series * Hard (band), a Hungarian hard rock super ...

ligand on a metal predisposes the metal to receive another

ligand rather than a soft one. Two superficially antithetical phenomena occur: symbiosis and antisymbiosis.

Chemical antisymbiosis

This is found principally with soft metals. Two soft ligands in mutual trans position will have a destabilizing effect on each other. The effect is also found with borderline metals in the presence of high trans effect ligands. For example the selenocyanate ion trans to the soft carbon dioxide in trans-Rh(PPh₃)₂(CO)(NCSe) bonds via the nitrogen, the harder of its two donors. The phenomenon may be explained in terms of a trans influence:

“With two π-acid ligands in mutual trans positions at a class-b metal, there would be a destabilizing competition for the dπ electrons on the metal. A π-acid bonded to a soft metal thus makes a metal a harder
Lewis acid A Lewis acid (named for the American physical chemist Gilbert N. Lewis) is a chemical species that contains an empty orbital which is capable of accepting an electron pair from a Lewis base to form a Lewis adduct. A Lewis base, then, is any sp ...
. Similarly a soft σ-donor will tend to polarize the electron density on a soft metal, causing it to favour an electrovalently bonded ligand in the trans position.”

Chemical symbiosis

This effect occurs with class-a metals such as iron(II). The

Cyclopentadienyl Cyclopentadienyl can refer to *Cyclopentadienyl anion, or cyclopentadienide, **Cyclopentadienyl ligand *Cyclopentadienyl radical, • *Cyclopentadienyl cation, See also *Pentadienyl In organic chemistry, pentadienyl refers to the organic radic ...

complex (C₅H₅)Fe(CO)₂(SCN) is an example of chemical symbiosis. The cyclopentadienyl directs the thiocyanate to bond through its softer Sulphur donor. A more definitive example are the halopentamminocobalt(III) ions, Co(NH₃)₅X²⁺, which are more stable when the

halogen The halogens () are a group in the periodic table consisting of five or six chemically related elements: fluorine (F), chlorine (Cl), bromine (Br), iodine (I), astatine (At), and tennessine (Ts). In the modern IUPAC nomenclature, this group is ...

, X, is

fluoride Fluoride (). According to this source, is a possible pronunciation in British English. is an inorganic, monatomic anion of fluorine, with the chemical formula (also written ), whose salts are typically white or colorless. Fluoride salts typ ...

than with iodide, and the halopentcyanocobalt(III) ions, Co(CN)₅X³⁻, which are most stable when the halogen is iodine.

“
Hard Hard may refer to: * Hardness, resistance of physical materials to deformation or fracture * Hard water, water with high mineral content Arts and entertainment * ''Hard'' (TV series), a French TV series * Hard (band), a Hungarian hard rock super ...
bases (electronegative donor atoms) retain their valence (outer shell) electrons when attached to a given central metal ion, thus enabling the metal ion to retain more of its positive charge, making it a
hard Hard may refer to: * Hardness, resistance of physical materials to deformation or fracture * Hard water, water with high mineral content Arts and entertainment * ''Hard'' (TV series), a French TV series * Hard (band), a Hungarian hard rock super ...

Lewis acid A Lewis acid (named for the American physical chemist Gilbert N. Lewis) is a chemical species that contains an empty orbital which is capable of accepting an electron pair from a Lewis base to form a Lewis adduct. A Lewis base, then, is any sp ...
. With soft bases the central metal atom is made a softer
Lewis acid A Lewis acid (named for the American physical chemist Gilbert N. Lewis) is a chemical species that contains an empty orbital which is capable of accepting an electron pair from a Lewis base to form a Lewis adduct. A Lewis base, then, is any sp ...
, because the metal’s positive charge is reduced by delocalization of electron density from the ligand into the ligand-metal bond. But we have the distinction that with a class-a metal there is little concomitant polarization of the electron density away from the trans position of the metal. In addition, symbiosis, unlike antisymbiosis, is probably not specifically trans directional, and is just as effective in, say, tetrahedral complexes.” Anthony Nicholl Rail; ''op. cit.''

References

{{reflist Coordination chemistry