Structural and spectroscopic studies of rhodium-(I) and -(III) nitrato complexes

Abstract

Refluxing Rh(NO₃)₃·2H₂O with an excess of PPh₃ in MeOH or EtOH for over 48 h gives trans-[Rh(CO)(ONO₂)(PPh₃)₂]1 whereas the same reaction at room temperature gives predominantly [Rh(ONO₂)(PPh₃)₃]2. X-Ray structure determinations of 1 and 2 show the nitrate groups to be monodentate in each case although disorder of the CO and NO₃ groups in 1 precludes accurate determination of bond lengths and angles. There is no reaction of trans-[Rh(CO)X(PPh₃)₂](X = Cl, F or ONO₂) with H₂ at room temperature and 1 atm pressure, whereas there is an immediate reaction of [Rh(ONO₂)(PPh₃)₃] under the same conditions to give [RhH₂(ONO₂)(PPh₃)₃]3, which has been characterised by multinuclear NMR spectroscopy. Compound 3 loses PPh₃ to give [RhH₂(O₂NO)(PPh₃)₂]6a which has been characterised similarly and by X-ray crystallography; although the metal-bound hydrogens were not located the structure is clearly based on an octahedral geometry with trans phosphines and a bidentate nitrate.