Term Ligand

In the context of ion channel biochemistry, and indeed in other areas of biochemistry, the term ligand has a different, and more 

AMELI DREHER, GERALD STEPHAN and FELIX TUCZEK 

Institut fur anorganische Chemie der Christian Albrechts Universitat zu Kiel, Max-Eyth-Strasse 2, D-24098 Kiel, Germany 

Molybdenum Dinitrogen Complexes with Polydentate Phosphine 

In Nature, nitrogen fixation is mediated by the enzyme nitrogenase according to Eq. (1) (6) 

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It is also noteworthy that Alfred Stock, who is universally acclaimed as the discoverer of the boron hydrides (1912). " was also the first to propose the use of the term "ligand (in a lecture in Berlin on 27 November 1916). Both events essentially predate the formulation by G. N. Lewis of the electronic theory of valency (1916). It is therefore felicitous that, albeit some 20 years after Stock s death in 1946, two such apparently disparate aspects of his work should be connected in the emerging concept of boranes as ligands . 

See the Appendix for a discussion of the different meanings of the term ligand in relation to ion channels and in coordination chemistry. 

Note In VSEPR theory the term bond pair is used for a single bond, a double bond, or a triple bond, even though a single bond consists of one pair of electrons, a double bond two pairs of electrons, and a triple bond three pairs of electrons. To avoid any confusion between the number of electron pairs actually involved in the bonding to a central atom, and the number of atoms bonded to that central atom, we shall occasionally use the term ligand" to indicate an atom or a group of atoms attached to the central atom. 

As a result of the catalytic center-chiral entity interaction the reaction rate accelerates substantially. This phenomenon was described for the first time by Sharpless [6], who coined the term ligand accelerated catalysis. Unfortunately, the reasons for this phenomenon are still not well... 

To stress the localized character of chemisorption (a term surface pseudomolecules was introduced at that time), Sachtler introduced for the alloying effects discussed in paragraph (2) a term ligand effect (5). It was then a task for an experimentalist to establish how important—relatively—the effects (1) and (2) were. A general consensus now is that effect (1) is more essential than (2) in any case, but the discussion is still going on, on the reliability of some pieces of evidence which have been presented in the literature in favor of a role for effect (2). 

The term ligand is used to describe any atom or group of atoms which is chemically bonded to a central atom, i.e. an atom at the coordinate origin. In the example under discussion the oxygen atom is at the centre of the point group and the two hydrogen atoms may be described as ligands. 

We shall use the term ligand (L) in its broadest sense to refer to those atoms that are attached to the central atom. What we mean by the shape of a molecule or ion is the geometrical arrangement of the ligands about the central atom. 

Whereas in the older literature (78, 80,116) no clear distinction was made between surface properties and bulk properties, it is now generally accepted that the catalytic properties of the surface atoms are primarily determined by their immediate environment (3). The term ligand effect (41) stresses that the influence on an adsorbing atom by its neighbors in and below the surface decreases steeply with their distance, the greatest contribution coming from the direct neighbors of the metal atom considered. 

The subsequent discussion will focus on two different repulsion mechanisms which may lift the pseudo-spherical degeneracy of the 2D term ligand delocalization and configuration interaction. 

We use the term ligand to comprise both atoms and groups. 

From the Latin ligare, to bind. For an interesting discussion of the use and spread of this term see Brock, W. H, Jensen, K. A, Jorgensen, C. K, Kauffman, G. B, The origin and dissemination of the term ligand in chemistry. Polyhedron 1983, 2, 1-7. 

W. H. Brock, Searching the literature to learn how the term ligand became part of the English language , J. Chem. Inf. Comp. Sci., 1982, 22, 125-129. 

Inner sphere complexation involves interactions between metal ions and other species in solution which possess lone pairs of electrons. Inner sphere complexation involves the transfer of at least one lone pair of electrons. Those species which possess electron lone pairs are termed ligands and reactions may involve inorganic or organic ligands. 

Crystal field splitting The removal of a degeneracy of the energy levels of molecular entities or ions due to the lower site symmetry created by a crystalline environment. This term is sometimes incorrectly used synonymously with the term ligand field splitting. 

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