Five-coordinated complexes Isomerism

Azomethines bear a formal resemblance to azo compounds and many parallels exist in the coordination chemistry of the two series of compounds. Thus the bidentate azomethines (191) behave in a strictly comparable manner to the bidentate azo compounds (20) (Section 58.2.2.1). The isomeric, bidentate azomethines (192), however, form metal complexes which undergo very facile hydrolysis as a result of polarization of the azomethine linkage. The difference between the two types of complex is dramatically illustrated by the results of a study135 of metal complex formation by the bis(azomethine) (193). This cannot function as a tetradentate ligand for steric reasons and reacts with copper, nickel and cobalt halides in cold ethanolic solution to form the five-coordinate complexes (194). Crystallization of these products from ethanol gives the five-... 

This is an example of geometrical isomerism in five coordinate complex, having square pyramid structure. 

The ethyl group and the hydride in 5c are cis to each other and thus reductive elimination might take place directly without isomerization. However, reductive elimination from a d five-coordinate complex would favor a transition state where three ligands but and H are in the same plane, as shown below. 

A five-coordinated complex [CoCl2(NO)(PCH3Ph2)2], shown in Figure 6, is known to exist in two isomeric forms. The trigonal bipyramidal isomer exhibits N—O stretching frequency at 1750 cm while the square pyramidal isomer absorbs at 1650 cm . The bond angle and nitrosyl-stretching frequencies of some mononuclear nitrosyl complexes of the first transition series is shown in Table 1. 

Five-coordinate complexes of d metals are believed to be involved as intermediates in ligand substitution and some cis-trans isomerization reactions, but they have scarcely been isolated. The reactions of Pd(hfac)2 with an equivalent amount of P(o-tolyl)3 in n-hexane at room temperature gave Pd(hfac)2 P(o-tolyl)3 ] (31a)vf diO. 85% yield. Similarly, Pt(hfac)2 P(o-tolyl)3 (Sib) and Pt(hfac)2(PCy3) (Sic) were also obtained in 89 and 93% yields, respectively. 

The formation of the complexes shown in Scheme 14 and Eq. (5) has been rationalized according to Scheme 15. Thus, it has been proposed that the insertion of the Os—H bond of OsHCl(CO)(P Pr3)2 into the carbon-carbon triple bond of the alkynol initially gives five-coordinate (E )-alkenyl intermediates, which subsequently isomerize into the Os CH=CHC(OH)R1R2 derivatives. The key to this isomerization is probably the fact that the five-coordinated ( )-alkenyl intermediates are 16-electron species, while the Os CH=CHC(OH)R1R2 derivatives are... 

If one end of a chelate ring on an octahedral complex is detached from the metal, the five-coordinate transition state can be considered as a fluxional molecule in which there is some interchange of positions. When the chelate ring reforms, it may be with a different orientation that could lead to racemization. If the chelate ring is not symmetrical (such as 1,2-diaminopropane rather than ethyl-enediamine), isomerization may also result. For reactions carried out in solvents that coordinate well, a solvent molecule may attach to the metal where one end of the chelating agent vacated. Reactions of this type are similar to those in which dissociation and substitution occur. 

---