Structural and Spectroscopic Studies of Rhodium-(1) and -(M) Nitrato Complexes t

Refluxing Rh(NO3),=2H,O with an excess of PPh, in MeOH or EtOH for over 48 h gives trans[Rh(CO)(ONO,) ( PPh3),] 1 whereas the same reaction at room temperature gives predominantly [Rh(ONO,)(PPh,),] 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 NO, groups in 1 precludes accurate determination of bond lengths and angles. There is no reaction of trans-[Rh(CO)X(PPh,),] (X = CI, F or ONO,) with H, at room temperature and 1 atm pressure, whereas there is an immediate reaction of [Rh(ONO,)(PPh,),] under the same conditions to give [RhH,(ONO,)(PPh,),] 3, which has been characterised by multinuclear NMR spectroscopy. Compound 3 loses PPh, to give [RhH,(O,NO) (PPh,),] 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. The reaction of rhodium trichloride with excess of triphenylphosphine in ethanol to give Wilkinson's compound, [RhCl(PPh,),], has long been known' and it has been successfully used commercially as an efficient hydrogenation catalystn2 Multinuclear NMR measurements in solution have proved to be very useful in characterising the stereochemistries, reactions and exchange processes of both [RhCI(PPh,),] 3,4 and the product resulting on oxidative addition with H2, viz. [RhH2Cl(PPh3)3].4 This information is extremely important in elucidating the involvement of each of these species and the associated equilibria in the catalytic hydrogenation of alkene~.~.' Many other analogues of [RhCl(PPh,),], obtained by replacing chloride with other monovalent groups, have been reported but NMR measurements on these complexes and their related hydrogenated derivatives, [RhH,X(PPh,),], have been much less extensive, and there are no NMR measurements on either rhodium-(I) or -(HI) complexes when X = NO,. We now report the preparation, chemical reactivity, structural and spectroscopic characterisation of a series of rhodium nitrato complexes which, although related to Wilkinson's compound and analogous derivatives, do show significant differences.