Conformational Changes in Six-member Metallacycles

Huttner et al. have studied complexes [Rh(COD)(PR2CH2CH(OH) CH2PR2)](PF6) with enantiomerically pure phosphines [37]. With bulky R and 

In Rh(I) complexes with (—)-(25, 45)-2,4-bis(diphenylphosphino)pentane and diolefins as ancillary ligands the metallacycle is conformationally very flexible in solution [39]. The X-ray diffraction solid state structures show a chair conformation for the complex with NBD, and a skew conformation for the complex with COD, but in solution both complexes show fast conformational exchange and behave as having C2 symmetry. 

Six-member P,N donor chelating ligands have been profusely used in coordination chemistry and catalysis. The simplest ligands of these series are the derivatives P(o-phosphino-iV,iV-dimetylbenzylamine) Ph3 (labeled PNi, PN2 and PN3). The successive substitution of phenyl by benzylamine groups at the phosphorus slows down noticeably the conformational inversion. 

Other comparable ligands, but chiral, are those bearing o-substituted N,N-dimethylbenzylamine (P N = P(a-methyl-A,A-dimethylbenzylamine-o) Phi-n [33,43,44]. Solid state structures of transition metal complexes (e.g. of 

Recent reviews on pyrazolylborate complexes are available [52-55]. Their solution study is complicated by the occurrence of equilibria between pentacoor-dinated and square-planar species, and by fast pyrazolyl exchange in the square-planar complexes. These fast phenomena are difficult to distinguish as they lead to spectroscopically equivalent results when followed by H NMR only. 

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