A Study on the Coordinative Versatility of the Zwitterionic S,N,S Ligand EtNHC(S)Ph 2 P=NPPh 2 C(S)NEt in Its Anionic, Neutral and Cationic Forms – Determination of Absolute p K a Values in CH 2 Cl 2 of Rh I Complexes

The coordination properties of EtNHC(S)Ph 2 P=NPPh 2 C(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 H 2 EtSNS + 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°); 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)H 2 EtSNS] 2+ , and their p K 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 νCO infrared absorption frequency.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)