Chelate trans-spanning

Compound B, in boiling hypochloric acid, was converted to trans-dichlorodiamine platinum(II). Efforts to place a trans-spanning chelate on each side of the platinum plane have not yet succeeded... 

In an effort to develop new systems able to combine both stability and activity, Siiss-Fink and co-workers reported a family of trans-spanning, but flexible diphosphines.These ligands form stable complexes, due to the chelate effect, which resemble the ones obtained with PEts, (/r . r-[RhI(CO)(PEt3)2]), with the two phosphorus atoms in a /r< //i -disposition. These ligands, as benzoic acid diphenylphosphine derivatives, are supposed to increase the electron density on the rhodium center due to a weak Rh-O interaction. 

Werner based some of his conclusions on the supposition that a chelate ring could span only the cis positions in a complex. While this is valid for the small rings with which he was familiar, it does not necessarily hold for larger rings. Several investigators have prepared compounds in which a single bidentate ligand occupies trans positions in a planar complex. 

AA represents a symmetrical bidentate (chelate) ligand. Such ligands coordinate at two adjacent positions. They can span cis positions but not trans positions. 

The C2-symmetric biferrocene 16 has been reported by Ito and co-workers to behave as a frans-spanning chelating ligand. The combination of C2-symmetry and trans coordination geometry is conceptually new in the field and bears promises for future development. TRAP (the abbreviation for this kind of compounds) affords high enantioselectivities in the Rh-catalyzed asymmetric Michael reaction of 2-cyanopropionates [17], the hydrosilylation of simple ketones [18], the hydrogenation of )3,/9-disubstituted A-acetylaminoacrylic acid... 

Cis and trans isomers of square-planar complexes are common many platmum(ll) examples are known. The isomers of [Pt(NH3)2012] are shown in Figure 9.5. The cis isomer is used in medicine as the antitumor agent cisplatin. Chelate rings can enforce a cis structure if the chelating ligand is too small to span the trans positions. The distance across the two... 

---