Formation of Metal Complexes

For the addition of a metal ion, i.e., the formation of polynuclear complexes, the following notation is used, analogous to Eq.(II.6)  

The second index can be omitted if it is equal to 1, i.e., becomes if m=. The formation constants of mixed ligand complexes are not indexed. In this case, it is necessary to list the chemical reactions considered and to refer the constants to the corresponding reaction numbers. 

It has sometimes been customary to use negative values for the indices of the protons to indicate complexation with hydroxide ions. Off. This practice is not adopted in this review. If Off occttrs as a reactant in the notation of the eqttihbrium, it is treated like a normal hgand L, but in general formttlae the index variable n is used instead of q. If H O occttrs as a reactant to form hydroxide complexes, H O is considered as a protonated ligand, HL, so that the reaction is treated as described below in Eqs.(II.14) to (11.16) using n as the index variable. For convenience, no general form is used for the stepwise constants for the formation of the complex In many 

Note that an asterisk is only assigned to the formation constant if the protonated ligand that is added is deprotonated during the reaction. If a protonated ligand is added and co-ordinated as such to the metal ion, the asterisk is to be omitted, as shown inEq.(II.17). 

---

On the other hand, the formation of metal complexes of bothpyrido-[2,3-f ]- and -[3,4-f ]-pyrazines is widely reported (e.g. 52JCS4985, 54JCS505). 

Reactions. The reduction of arylazoquinoxalines to quinoxalinamines has been covered in Section 6.3.1. The only other reaction reported recently is the formation of metal complexes. Although 6,6, 7,7 -tetraoctyloxy-2,2 -biquinoxaline and palladium chloride formed a complex (306) that proved too insoluble for analytical... 

Synthesis of Block Copolymers by the Post-Polymerization Formation of Metal Complexes... 

Penicillamine is known to form complexes of varying stability with several metal ions. In neutral solution, penicillamine complexes with mercury, lead, nickel, and copper are relatively more stable than those of zinc, iron, and manganese. The three functional groups of penicillamine may be engaged in the formation of metal complex, and the resultant compounds may be polymeric in structure. 

As we have seen, the Lewis theory of acid-base interactions based on electron pair donation and acceptance applies to many types of species. As a result, the electronic theory of acids and bases pervades the whole of chemistry. Because the formation of metal complexes represents one type of Lewis acid-base interaction, it was in that area that evidence of the principle that species of similar electronic character interact best was first noted. As early as the 1950s, Ahrland, Chatt, and Davies had classified metals as belonging to class A if they formed more stable complexes with the first element in the periodic group or to class B if they formed more stable complexes with the heavier elements in that group. This means that metals are classified as A or B based on the electronic character of the donor atom they prefer to bond to. The donor strength of the ligands is determined by the stability of the complexes they form with metals. This behavior is summarized in the following table. 

The free ligand concentration, [A ], is an important parameter in the formation of metal complexes (see Chapter 3 and section 4.8). In a solvent extraction system with the volumes V and of the aqueous and organic phases, respectively, [A ] is calculated from the material balance ... 

Uf course, the enhancement of chromatographic selectivity by secondary chemical equilibria is neither new nor confined to reversed-phase systems. Most widespread probably has been the exploitation of protonic equilibria by appropriately ati usting the pH of the eluent so that the degree of ionization of the eluite is altered. Generally the ionized and neutral forms of an eluite are retained differently (2( 7. 208). Formation of metal complexes of certain eluites has also been utilized for modulating retention behavior for higher selectivity. 

As highly reactive heterocycles, the thietes are well suited for the formation of metal complexes. Takahashi and Dittmer both individually and in collaboration have been involved with the interaction of thietes and iron or cobalt carbonyls. During the thermally or photochemically induced complexation process, ring opening takes place so that the resultant thioacrolein is the actual ligand in the organometallic compound (Scheme i7)/ 3-203,204... 

Table I. Kinetic Data for the Formation of Metal Complexes in Aqueous...

There are numerous examples of the use of cyclophosphazenes with exocyclic dialkylamino or alkyl/aryloxy substituents on phosphorus as scaffolds for the formation of metal complexes. However, the number of complexes in which the endocyclic nitrogen atoms are involved in adduct formation with electrophilic... 

Although the noncatalytic thermal disproportionation of alkenes is symmetry-for-bidden, the formation of metal complex 1 with the four-centered cyclic intermediate makes the reaction symmetry-allowed.61... 

Concerning the question of phosphole aromaticity there are some indications of 6-rr-delocalization from NMR spectroscopy and other physical methods. The formation of metal complexes with derivatives of Mn, Fe and other metals, however, gives the best chemical proof of such 6ir- delocalization but there are still some doubts arising from the considerable differences in chemical behaviour compared with pyrroles, furans and thiophenes. This once more points to the very different properties of phosphorus, compared to nitrogen in particular. Therefore the problem is still open and much work, both practical and theoretical, remains. 

---