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Ligands that bind via more than one atom are often termed chelating. A ligand that binds through two sites is classified as bidentate, and three sites as tridentate. The "bite angle" refers to the angle between the two bonds of a bidentate chelate. Chelating ligands are commonly formed by linking donor groups via organic linkers.
Ethylenediamine ligand chelating to a metal with two bonds Cu 2+ complexes with nonchelating methylamine (left) and chelating ethylenediamine (right) ligands. The chelate effect is the greater affinity of chelating ligands for a metal ion than that of similar nonchelating (monodentate) ligands for the same metal.
Diphosphines are a class of chelating ligands that contain two phosphine groups connected by a bridge (also referred to as a backbone). The bridge, for instance, might consist of one or more methylene groups or multiple aromatic rings with heteroatoms attached. Examples of common diphosphines are dppe, dcpm (Figure 1), and DPEphos (Figure 2
Two tridentate ligands may form a complex with such an atom. There are two possible arrangements for such a complex: fac where the coordination is in a triangle on one face of the octahedron, and mer where the coordinating atoms are in an arc around the central atom, with two atoms of the ligand opposite each other.
This early attempt was followed by ligands with more rigid backbones. "TRANSPHOS" was the first trans-spanning diphosphane ligand that usually coordinates to palladium(II) and platinum(II) in a trans manner. TRANSPHOS features benzo[c]phenanthrene substituted by diphenylphosphinomethyl (Ph 2 PCH 2) groups at the 1 and 11 positions.
Tetradentate ligands can be neutral so that the charge of the whole complex is the same as the central atom. A tetradentate monoanionic (TMDA) ligand has one donor atom with a negative charge. [8] A tetradentate dianionic ligand has a double negative charge, and tetradentate trianionic ligands have a triple negative charge.
In coordination chemistry, a macrocyclic ligand is a macrocyclic ring having at least nine atoms (including all hetero atoms) and three or more donor sites that serve as ligands. [1] Crown ethers and porphyrins are prominent examples. Macrocyclic ligands often exhibit high affinity for metal ions, the macrocyclic effect.
In coordination chemistry, clathrochelates are ligands that encapsulate metal ions. Chelating ligands bind to metals more strongly than related monodentate ligands, and macrocyclic ligands bind more strongly than typical chelating ligands. It follows that bi- or polymacrocyclic ligands would bind to metals particularly strongly.