Five-coordinate species

The stability, structure, and behavior of five-coordinate species is greatly influenced by solvents, the ligands, and the nature of the metal. It must be kept in mind that many of the studies reported in this section were performed in solvents such as chloroform. In more polar methanol or water, more commonly used in mechanistic studies of square-planar compounds, the lifetime of any five-coordinate adducts may be too small to allow detection, or for changes in geometry to occur. In a paper describing the 

A variety of trigonal bipyramidal Pt(II) complexes [PtCl2(L)(biL)] have been isolated and examined (L is tj -olefin biL is r-N,N -a-diimine, N,A -disubstituted-1,2-diaminoethane, or 6-substituted-pyridine-2- 

A similar series of five-coordinate rhodium(I) molecules has been examined. [RhCl(CO)(i7 -C2H4)(biL)] (biL = diimine) are trigonal bipyramidal with axial Cl and CO in CDCI3. The ethylene (which rotates on the nmr time scale) is readily lost. The related compounds [RhCl(CO)2(biL)] and [RhCl(PF3)2(biL)] also contain trans-axial Cl and CO or Cl and PF3. Intramolecular exchange of the CO sites is rapid, however, suggesting an easier pseudorotation type of motion for these complexes. 

An interesting variation of four versus five coordination is afforded by a series of di-2-pyridyl ketone complexes of Pd(II), Pt(II), and Au(III). The ligand coordinates in chelate fashion via the nitrogens, but protic molecules HX (X = OH, OMe, or OEt) essentially solvate the complex by converting the ketone to a diol form which can then also occupy the apical coordination site. Structure 14 depicts such a hydrated gold(III) species. 

Rates of the reversible hydrolysis, and the stability of the hydrated species, increased along the series Pt(II) Pd(II) Au(III), parallel to the electron-withdrawing properties of the metal cations, and their ability to form square-pyramidal molecules. 

This has been a fruitful area of study. Both kinetic and structural data have continued to illuminate the reaction pathways of stable five-coordinate species themselves, and the nature of five-coordinate intermediates or transition states encountered from associative reactions of four-coordinate molecules. 

Complexes of the type (8), though four-coordinate in their ground state, are fluxional in solution, presumably via rapid intramolecular exchange of coordinated free nitrogen atoms by way of five-coordinate intermediates. The complexes are 

Another adduct of a four-coordinate d species, which can be regarded as a trapped intermediate en route to oxidative addition, has been structurally charac- 

The structure of Tl2Pt(CN)4 also reveals expanded coordination, (14). The octahedral units are not unlike the local geometry in the so-called one-dimensional 

The link between nucleophilic attack at square-planar complexes en route to ligand exchange, and electrophilic attack en route to oxidative additions, is nowhere 

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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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