Ligands stepwise coordination

Table 1. Thermodynamics of the stepwise coordination of hard ligands at 25 °C to... 

FIGURE 2. Chemical structures of the ligands and stepwise coordination methods to prepare linear and branched oligomer and polymer wires on an Au/mica or Au/ITO plate. (Reprinted with permission from Ref. 13.)... 

FIGURE 11. Chemical structure of the ligands used in this chapter, and stepwise coordination methods for the preparation of modified ITO electrodes (i) immobilization of tpy-BzA, (ii) complexation with a metal ion, and (iii) complexation with tpy-ZnTPP. (Reprinted with permission from Ref. 14.)... 

Metal ions such as Cu2+ are Lewis acids that combine with Lewis bases such as H20 to form complex ions, e.g., CuCOH ". The number of ligands, the coordination number, varies from one metal ion to another the most common coordination numbers are 4 and 6, but 2, 3, 5, 7, and 8 are also known. In the presence of another Lewis base such as NH3, the H20 ligands can be replaced in stepwise equilibria ... 

The step-by-step process of chelation described above is believed to be the usual way all polydentate ligands go about coordinating to metal ions in general - knitting themselves onto the metal ion stitch by stitch (or, more correctly, undergoing stepwise coordination). It also provides a way whereby compounds can achieve only partial coordination of a potential donor set, when there are insufficient coordination sites for the full suite of potential donor groups available on a ligand. 

The stepwise coordination of a ligand L to a metal M to form a complex ML , can... 

Asymmetric 1,3-dipolar cycloadditions of azomethine imines with terminal alkynes have been catalysed by 11 chiral ligand (8) coordinated metal amides to form N,N-bicyclic pyrazolidinone derivatives. Mechanistic studies have established the factors that determine the regioselectivity of the stepwise reaction. Novel phosphoramidite ligands (9) coordinated with palladium have been used to effect enantioselective synthesis of pyrrolidines by 3-P 2-cycloaddition of trimethylenemethane (from 2-trimethylsilylmethyl allyl acetate) to a wide range of imine acceptors (Scheme 11). ... 

The thermodynamic stability of a species is a measure of the extent to which this species will be formed from other species under certain conditions, provided that the system is allowed to reach equilibrium. Consider a metal ion M in solution together with a monodentate ligand L, then the system may be described by the following stepwise equilibria, in which, for convenience, coordinated water molecules are not shown ... 

In summary, the fragmentary evidence at hand suggests that sulfur was the primordial iron complexing material and that this ligand system was selectively modified by stepwise transition to a mixed sulfur-nitrogen and sulfur-oxygen and, finally, to an all-oxygen type of coordination. 

Having established structural and electronic analogies between metal oxides and alkoxides of molybdenum and tungsten, the key remaining feature to be examined is the reactivity patterns of the metal-alkoxides. Metal-metal bonds provide both a source and a returning place for electrons in oxidative-addition and reductive elimination reactions. Stepwise transformations of M-M bond order, from 3 to 4 (37,38), 3 to 2 and 1 (39) have now been documented. The alkoxides M2(0R)6 (MiM) are coordinatively unsaturated, as is evident from their facile reversible reactions with donor ligands, eq. 1, and are readily oxidized in addition reactions of the type shown in equations 2 (39) and 3 (39). 

The need for multiple desolvation of the metal ion in some systems may provide a barrier to complex formation which is reflected by lower formation rates - especially for inflexible macrocycles such as the porphyrins. Because of the high energies involved, multiple desolvation will be unlikely to occur before metal-ion insertion occurs rather, for flexible ligands, solvent loss will follow a stepwise pattern reflecting the successive binding of the donor atoms. However, because of the additional constraints in cyclic systems (relative to open-chain ones), there may be no alternative to simultaneous (multiple) desolvation during the coordination process. 

It has been proposed that the first stage of alkali metal complex formation by dibenzo-30-crown-10 in methanol involves a fast ligand conformational change which is then followed by a stepwise substitution of the coordinated solvent by the ligand (Chock, 1972) ... 

Complexes formed by tetradentate siderophores involve stepwise complex formation and therefore, have somewhat different equilibria from their hexadentate analogs. Initial chelation will occur with a tetracoordinate FeL complex forming. A subsequent equilibrium then occurs, where the FeL complexes will react in a 2 1 stoichiometry with free ligands in solution to form a single Fe2L3 complex (coordinated water and charges not shown for clarity). 

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