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For example in PbCl 2, the coordination number of Pb 2+ could be said to be seven or nine, depending on which chlorides are assigned as ligands. Seven chloride ligands have Pb-Cl distances of 280–309 pm. Two chloride ligands are more distant, with a Pb-Cl distances of 370 pm. [13]
Many ligands are capable of binding metal ions through multiple sites, usually because the ligands have lone pairs on more than one atom. Such ligands are polydentate. [12] 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 coordination geometry depends on the number, not the type, of ligands bonded to the metal centre as well as their locations. The number of atoms bonded is the coordination number. The geometrical pattern can be described as a polyhedron where the vertices of the polyhedron are the centres of the coordinating atoms in the ligands. [1]
The number of ligands coordinating is indicated by the prefixes di-, tri-, tetra- penta- etc. for simple ligands or bis-, tris-, tetrakis-, etc. for complex ligands. For example: For example: [CoCl(NH 3 ) 5 ]Cl 2 pentaamminechloridocobalt(3+) chloride where ammine (NH 3 )precedes chloride.
Well known complexes such as Ferrocene and Uranocene also serve as examples where LBN and coordination number differ. Ferrocene has two η 5 cyclopentadienyl ligands while Uranocene has two η 8 cyclooctatetraene ligands; however, by covalent bond classification the complexes are found to be ML 4 X 2 and ML 6 X 4. [4]
Polyfunctional ligands can attach to metals in many ways and thus can bridge metals in diverse ways, including sharing of one atom or using several atoms. Examples of such polyatomic ligands are the oxoanions CO 2− 3 and the related carboxylates , PO 3− 4 , and the polyoxometalates .
The application of phosphoramidites as effective monodentate ligands for transition metal catalysis was first reported by Dutch chemist Ben Feringa. The introduction of phosphoramidite ligands challenged the notion that high flexibility in the metal–ligand complex is detrimental for high stereocontrol. General Structure of Phosphoramidite Ligand
The Tp ligands are usually prepared from the reaction of pyrazole with potassium borohydride: [3] KBH 4 + 3 C 3 H 3 N 2 H → K[HB(C 3 N 2 H 3 ) 3 ] + 3H 2 Intermediates include the monopyrazolylborate ([H 3 B(C 3 N 2 H 3 )] − ) and the bispyrazolylborate ([H 2 B(C 3 N 2 H 3 ) 2 ] − ).