Five-coordinated species

The mono- and bidentate complexes (425) and (426), respectively, form from the reaction of [Ir(cod)Cl]2 with mercaptopyrimidine.6 2 (426) reacts with CO, yielding the dicarbonyl analogue through displacement of cod. One CO ligand may be replaced by PPh3, but further addition of PPh3 results in a five-coordinate species. 

Transmetalation, though, requires enhanced electrophilicity of the Pd. Additionally a free coordination site may be required, which may be freed by dissociation of either a neutral or an anionic ligand. The involvement of five-coordinate species and association-dissociation ligand-exchange mechanisms in the individual steps of Pd-catalyzed reactions also cannot be neglected (Scheme 3).384... 

When a large excess of ligand is used, as in the actual process, the system becomes more complicated and species such as (C2H4)L3Ni, HNi(CN)L3, and ML4 are also observed. The activation parameters of the reaction are AG (-40 °C)= 17 kcal/mol, AH = 9 kcaLmoF1, AS = -34 cal.mol. K 1. The negative entropy of activation is consistent with the formation of a five-coordinate species. The reason for the associative character of the reductive elimination is two-fold ... 

The ruthenium(III) complex of edta in which the ligand acts only as a five-coordinate species and in which an acetate arm remains free, exists in three pH-related forms ... 

Fig. 7.3 Free energy profiles for pianar-octahedral equiiibria. In A, ligand or solvent exchange is more rapid than the planar, octahedral equilibrium as established in (7.7). In B, the formation and dissociation of the octahedral complex is rate-determining and the interconversion of the planar and five-coordinated species is more rapid.

The five-coordinated species A may pseudorotate to form B directly without the intermediacy of the ionic species (Schemes (7.31) and (7.32)). 

The aqua ion Au(H20)4+ has not been characterized either in solution or in the solid state. Most of the substitution studies have involved the halide complexes AuXj and Au(NH3) (Ref. 319). A number of earUer generalizations have been confirmed. Rates are very sensitive to the nature of both entering and leaving ligands and bond formation and breaking are nearly synchronous. The double-humped energy profiles witnessed with Pd(II) and Pt(II) are not invoked the five-coordinate species resulting from an associative mechanism is the transition state ... 

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