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Symbiosis (chemical)
The biological term symbiosis was adopted for chemistry by Jørgensen in 1964,〔C. K. Jørgensen; ''Inorg. Chem.''; 1971, 10, 1097.〕 who applied it to the process by which, say, a hard ligand on a metal predisposes the metal to receive another hard ligand rather than a soft ligand. In fact, 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.〔R. G. Pearson, quoted in Anthony Nicholl Rail; Some reactions of a ditertiary arsine ligand; Ph.D. thesis; University College London, 1973.〕 The effect is also found with borderline metals in the presence of high trans effect ligands. As illustration, 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.〔J. L. Burmeister & N. J. DeStefano; ''Chem. Comm.''; 1970, 1698.〕 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. 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.”〔Anthony Nicholl Rail; ''op. cit.''〕
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