Polydentate ligands chelation

Theoretical and experimental studies on penta-coordinate silicon derivatives demonstrate that their existence is determined by a combination of factors electronegativity of the substituents and steric interactions between substituents. One should also emphasize the role and significance of polydentate ligand (chelate effect), the size and number of chelate rings involving the silicon atom, and strain reduction for five-membered ring system, wich can stabilize unusual structures, as well as to the role of medium effect. This is consistent with the results of NMR spectroscopic studies. 

TABLE 24.3 Some Common Polydentate Ligands (Chelating Agents) ... 

Titanium chelates are formed from tetraalkyl titanates or haUdes and bi- or polydentate ligands. One of the functional groups is usually alcohoHc or enoHc hydroxyl, which interchanges with an alkoxy group, RO, on titanium to Hberate ROH. If the second function is hydroxyl or carboxyl, it may react similarly. Diols and polyols, a-hydroxycarboxyflc acids and oxaUc acid are all examples of this type. P-Keto esters, P-diketones, and alkanolamines are also excellent chelating ligands for titanium. 

Complexes with coordination number 6 tend to be octahedral those with coordination number 4 are either tetrahedral or square planar. Polydentate ligands can form chelates. 

Charles s law The volume of a given sample of gas at constant pressure is directly proportional to its absolute temperature V T. chelate A complex containing at least one polydentate ligand that forms a ring of atoms including the central metal atom. Example [Co(en)3]3+. chemical analysis The determination of the chemical composition of a sample. See also qualitative, quantitative. 

When two or more donor atoms from the same ligand are coordinated to a single metal centre, the ligand is said to be chelating. It is a general observation that chelated complexes of polydentate ligands are always more thermodynamically stable than those of the same metal with an equivalent number of comparable monodentate ligands. That is to say, the equilibrium... 

The Chelate Effect and Polydentate Ligands 147 Table 8-1. Stability constants for some nickel(ii) complexes of ammonia and 1,2-diaminoethane. 

Chelate ring formation may be rate-limiting for polydentate (and especially macrocyclic) ligand complexes. Further, the rates of formation of macrocyclic complexes are sometimes somewhat slower than occur for related open-chain polydentate ligand systems. The additional steric constraints in the cyclic ligand case may restrict the mechanistic pathways available relative to the open-chain case and may even alter the location of the rate-determining step. Indeed, the rate-determining step is not necessarily restricted to the formation of the first or second metal-macrocycle bond but may occur later in the coordination sequence. 

Polydentate ligands, such as ethylenediamine (en) and ethylenediaminetetraacetato (edta), cause ring formation to occur in a complex. This phenomenon is called chelation, resulting from a polydentate ligand bonding to the central metal atom or ion through two or more donor atoms at the same time. 

It is also observed that chelating agents (polydentate ligands) form more stable complexes than a comparable number of donor atoms in monodentate ligands. Consider the following example of Ni+ complexes with four ammonia molecules (each monodentate) and two ethylenediamine... 

Suitably substituted diazines also form chelate complexes, e.g. 2,3,5,6-tetrakis(a-pyridyl)pyrazine yields red tridentate complexes with Fe11. In the fast development of metallosupramolecular chemistry, many other polydentate ligands, based on 2,2 -bipyridyl units, have been obtained and studied (94CI(L)56). Thus, two molecules of oligopyridine (58) interact with various metal ions (Fe2+, Co2+, Cu2+ etc.) to form a double-helical [M2L2]4+ complex in which each metal is bonded to a tridentate region from each ligand. 

Whatever theoretical explanation of the chelate effect is adopted, there is no doubt as to the extra stability conferred by polydentate ligands capable of forming one or more chelate rings. This is of especial importance in titrimetry (see Section 10.6) and, as we shall see shortly, bidentate ligands play a significant role in the formation of precipitates of value in gravimetry. 

Polydentate ligands do not necessarily need to bind a metal at all possible locations. In some cases, only one site is bound to the metal and the other one or more are left dangling. Such complexes are rare, and it has been necessary to develop special synthetic methods to prepare them. Explain in terms of thermodynamic arguments why polydentate ligands will almost always form chelating complexes rather than leave dangling arms. 

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