Multi-dentate ligands

Fig. 15-3 Mixed ligand and multi-dentate complexes (a) a hypothetical Me(OH)Cl° complex (b) nitrilotriacetate chelate of a divalent metal ion in a tetrahedral configuration.

Fig. 2. Trivial closed trimers (a,b), tetramers (c), and pentamers (d) assembled on multi-dentate 4-pyridyl porphyrin ligands.

Replacing one or more water molecules from the first coordination shell of a di- or a trivalent transition metal ion by a kinetically inert mono-or multi-dentate ligand can have a strong effect on the exchange rate constant of the remaining water molecules. In general the remaining water molecules become more labile (Tables VII and VIII) the acceleration can... 

Similar solutions have been worked out for bis- (bidentate) and multi-dentate systems, and the procedure has the merit of describing either the ligand conformations or the configuration of the complex, or both, for many systems. The octant method is rather cumbersome to use at the moment, but further examination may resolve this difficulty and lead to a general method for describing the stereochemical detail in these systems. In... 

This section of the coverage of palladium(II) coordination chemistry will concentrate on complexes of the more common, usually unidentate ligands containing sulfur, selenium or tellurium donor atoms coverage of the vast number of chelate complexes of bi- or multi-dentate ligands will be limited but the reader will find references to most classes of palladium(II) chelate complexes and these will provide an entry into the literature. 

The number and variety of coordination complexes of palladium(II) with sulfur-containing bidentate or multidentate ligands is legion, and it is possible here only to summarize the different kinds of complexes formed and to guide the reader into the literature. Table 2 contains a selection of typical palladium(II) complexes of bi- or multi-dentate sulfur or selenium donor ligands, and comments on pertinent aspects of particular complexes. Chelate... 

Table 2 Some Typical Palladium(II) Complexes of Bi- or Multi-dentate Sulfur or Selenium Donor Ligands...

Vogtle, F., Weber, E., Multi-dentate acyclic neutral ligands and their complexation. Angew. Chem., Int. Ed. Engl. 1979, 18, 753-776. 

The reaction of the transition-metal fragments with main group 15 elements directly has proven a very fruitful field for exploration. The methodology has been successful for a wide range of metal complexes. These fall generally into three basic types (1) reactions with cyclopentadienyl metal carbonyls, (2) reactions with homoleptic metal carbonyls and substituted derivatives, and (3) reactions with metal cations in the presence of a multi-dentate chelating ligand. 

The protonation equilibrium between ammonia and the ammonium ion is shifted to the right as a result of coordination of unprotonated ammonia to copper. The drop in pH decelerates the complexation reaction, as there is comparatively less free ammonia available. Such protonation equilibria are much more complex if multi-dentate ligands (bases) are involved, but the effect is generally similar a drop in pH is the immediate result of coordination, and this drop increases the protonation of the ligand and thus decreases its reactivity toward the metal ion. 

E. Vogtle, E. Weber, Multi-Dentate Acyclic Neutral Ligands and Their Complexation , Angew. Chem. int. Ed., 18, 753 (1979)... 

Of course, this approach may be extended to multi-dentate ligands such as amino acids, dicarboxylic acids... 

Unidentate ligands invariably add in a series of steps, as shown here. With multi-dentate ligands, the maximum coordination number of the cation may be satisfied with only one ligand or a few added ligands. For example, Cu(II), with a maximum coordination number of 4, can form complexes with ammonia that have the formulas Cu(NH3)2+, Cu(NH3)i+, Cu(NH3)i+, and Cu(NH3)i+.With the bidentate ligand glycine (gly), the only complexes that form are Cu(gly) and Cufgly) . ... 

Note Inorganic ligands are chiefly monodentate, whereas organic ligands are most often multi-dentate in complexes. 

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