A study on the coordinative versatility of the zwitterionic S,N,S ligand EtNHC(S)Ph2P=NPPh2C(S)NEt in its anionic, neutral and cationic forms -: Determination of absolute pKa values in CH2Cl2 of RhI complexes

The coordination properties of EtNHC(S)Ph2P = NPPh2C(S)-NEt (HEtSNS) towards Rh-I species derived from [Rh(CO)(2)Cl](2) and [Rh(cod)Cl](2) (cod = 1,5-cyclooctadiene) were studied. This ligand is an amphoteric zwitterion, which forms the H2EtSNS+ cation upon protonation and the EtSNS- dianion-cation upon deprotonation. All three forms coordinate to metal centers. Their geometrical versatility allows many coordination fashions: S-monodentate; S,S-bidentate (with a bite angle spanning from 90 to 180 degrees); S,N,S-tridentate; N,N,N-tridentate; and S,S-bridging, as determined by X-ray diffraction methods. The [Rh(CO)EtSNS] complex is a biprotic base; its conjugated acids are [Rh(CO)HEtSNS](+) and [Rh(CO)H2EtSNS](2+), and their pK(a) values were determined in dichloromethane solutions. In this triad of compounds, which are geometrically similar, the Rh-I metal center features variable charge density as confirmed by the vCO infrared absorption frequency. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008).