Coordination compounds bidentate ligands

The CK" ion can act either as a monodentate or bidentate ligand. Because of the similarity of electron density at C and N it is not usually possible to decide from X-ray data whether C or N is the donor atom in monodentate complexes, but in those cases where the matter has been established by neutron diffraction C is always found to be the donor atom (as with CO). Very frequently CK acts as a bridging ligand - CN- as in AgCN, and AuCN (both of which are infinite linear chain polymers), and in Prussian-blue type compounds (p. 1094). The same tendency for a coordinated M CN group to form a further donor-aceeptor bond using the lone-pair of electrons on the N atom is illustrated by the mononuclear BF3 complexes... 

Chiral P-heterocycles as P-mono- and P,N-bidentate ligands in the synthesis of coordination compounds and homogeneous asymmetric catalysis 98KK883. 

Coordination compounds containing bidentate ligands are often thermally more stable than those comprised of related monodentate ligands, e.g. ethylenediamine (en) complexes dissociate at a higher temperature than those of ammonia or pyridine. Compounds containing a ring structure, such as coordinated salicylaldehyde (sal) and acetyl-acetonate (acac), are particularly stable, and may often be sublimed... 

Coordination to the central P atom of two different types of symmetrical bidentate ligands leads to structures of type P(aa)2(bb), which are this time C2-symmetric as detailed in Fig. 16. The same chiral descriptors A and A apply to these compounds. Derivatives like 3,4,14-17 and 19-22 fit this description and have only been reported in racemic form so far. If the ligand bb is itself chiral, then diastereomers are generated. This will be described in the next section. 

NMR measurements also provide information on the coordination of the ligands in the uranyl polymers. Solid-state I c-NMR confirms the coordination modes of the carboxylate ligands to the uranyl ion that is, both monodentate and bidentate carboxylate coordination modes are evident. The uranyl dicarboxyl ate polymers which possess two moles of coordinated DMSO exhibit two carbon-13 carbonyl resonances, one at about 175 ppm downfield from tetramethylsilane (TMS) and one at about 185 ppm. The polymers which possess only one mole of coordinated DMSO exhibit only the carbonyl peak near 185 ppm. Based on other known coordination compounds, the 175 ppm peak can be assigned to monodentate carboxylate and the 185 ppm peak to bidentate carboxylate. Thus, 7-coordination predominates in the polymers with either one or two moles of solvent coordinated to the uranyl ion, which is consistent with the infrared results reported elsewhere (5). 

Pyrazines and substituted pyrazines have long been known to act as exo-bidentate ligands to linearly bridge metal ions, and then have also been employed for the self-assembly of polymeric materials. With pyrazine (prz) varying the reaction conditions and stoichiometry, ID, 2D, and 3D structures containing silver(I) atoms with several coordination modes have been prepared.606-609 A 3D racemate is present in [Ag2(2,3-Me2prz)3](SbF6)2.610 A sawhorse connection has been achieved in the compound [Ag(N02)(pyz)] .611 With 2,2 -bipyrazine an infinite loop has been obtained.612... 

Amides of phosphorous acid (e.g., P(NMe2)3 and the bicyclic compound (L16)) are useful ligands for cross-coupling of arylboronic acids with aryl bromides and chlorides (system Pd(OAc)2/2L, Cs2C03, toluene, 80 °C), the bicyclic amide being markedly more effective than simple acyclic ones.421 Amides of phosphinous acids are also excellent bidentate ligands with PN coordination mode ((L6), Section 9.6.3.4.2). 

Polysulfides of several metals can be prepared by reaction of the metals with excess sulfur in liquid NH3 (group IA metals) or by heating sulfur with the molten metal sulfide. The polysulfide ion binds to metals to form coordination compounds in which it is attached to the metal by both sulfur atoms (as a so-called bidentate ligand). One example is an unusual titanium complex containing the S52-ion that is produced by the following reaction (the use of h to denote the bonding mode of the cyclo-pentadienyl ion is explained in Chapter 16) ... 

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