Bidentate chelates

A typical value for the N O distance in nitro complexes is 124 pm whereas in nitrito complexes the terminal N-O (121pm) is shorter than the internal N-O(M) 129 pm. In the bidentate chelating mode (iii) the 2 M O distances may be fairly similar as in [Cu(bipy)2(02N)]N03 or quite different as in [Cu(bipy)(02N)2] ... 

Periodates form numerous complexes with transition metals in which the octahedral unit acts as a bidentate chelate. Examples are ... 

Possible modes of regulation of filament assembly may be anticipated from the basic properties of actin. We have shown that the tightly bound divalent metal ion (Ca or Mg ) interacts with the P- and y-phosphates of ATP bound to actin, and that the Me-ATP bidentate chelate is bound to G-actin in the A configuration. The nature of the bound metal ion affects the conformation of actin, the binding kinetics of ATP and ADP, and the rate of ATP hydrolysis. 

Both types of complex are extremely air-sensitive and are paramagnetic, with one unpaired electron per titanium atom. Their formulation as monomeric, symmetrical, bidentate, chelate complexes, (IV) and (V), has been established from spectral, magnetic, and molecular-weight data. 

As reaction proceeds, the polymer chain (which is in random coil form) unwinds as the charge on it grows as a result of neutralization and ionization. This contributes to thickening of the cement paste. Cations released become bound to the polymer chain. Countercations can either be bound to a polyanionic chain by general electrostatic forces or be site-bound at specific centres. More than one type of site binding is possible. Complex formation and, if the ligand is bidentate, chelate formation enhance the effect. 

Acetate remains the classical carboxylate ligand, with the capacity to bind in monodentate bidentate chelate (/i2), or bridging modes. Whereas early chemistry featured simple fii... 

The dithioacid family of ligands includes dithiocarbamates (R2N CS2 ), xanthates (RO-CS2 ), thioxanthates (RS CS2 ) and dithiocarboxylates (R CS2 ), which have been described in CCC(1987, Section 47.8.10).1 The ligands are prepared by addition of a suitable nucleophile to the carbon center in CS2, usually in the presence of a base. It has been established for some decades that they bind to Co almost exclusively as bidentate chelates, including S-donors acting as bridging ligands in dinuclear systems. 

Mg11 complexes are also effective for controlling asymmetric radical reactions.33,34 Moreover, enantioselective radical reactions using chiral Mg11 complexes have been studied, and high enantioselectivities have been realized in the presence of stoichiometric or catalytic amounts of chiral auxiliaries such as bis-oxazolines (Scheme 8).35-39 In most cases, substrates having bidentate chelating moieties are required. 

In stereoselective reactions, Zn11 Lewis acids work well to achieve high selectivities (Scheme 54). Chiral complexes of Zn11 with chiral bis(oxazoline) ligands act as effective catalysts in Diels-Alder reactions of reactive dienes with dienophiles having bidentate chelating moieties such as... 

Despite the importance of organozinc catalysts bearing /Taminoalkoxides in asymmetric catalysis, there has been no comprehensive computational screening of suitable monoprotic and aprotic bidentate chelating ligands for this... 

Although diphosphine disulfides of formula R2P(S)P(S)R2 take up a trans conformation in the solid state (see Section 5.2.3), upon coordination to a metal centre they can rearrange to adopt a cis conformation, hence facilitating bidentate chelation of the metal (e.g., Equation 78).117 In addition, a cis-bridging mode has also been proposed for these ligands in bi-metallic complexes.117... 

---