SOLID-GAS REACTIONS OF MOLECULAR ORGANOMETALLIC COMPLEXES - DETECTION OF A METASTABLE INTERMEDIATE IN THE CARBONYLATION OF A NICKEL(0) COMPLEX

The reaction of the trigonal pyramidal Ni(0) complex [Ni(NP3)] (1) with CO in tetrahydrofuran (THF) solution has been re-investigated [NP3 = N(CH2CH2PPh2)3]. Depending on the experimental conditions, the reaction gives either the monocarbonyl [Ni(eta3-NP3-P,P',P'') (CO)] (2) or the dicarbonyl [Ni(eta2-NP3-P,P')(CO)2] (3a). Both products exhibit a distorted tetrahedral coordination geometry; in the monocarbonyl complex, the NP3 ligand uses the three phosphorus atoms to bind the metal center, whereas it uses two phosphorus only in 3a. Compounds 2 and 3a have been characterized by IR and P-31 and C-13 NMR spectroscopies in both the solid state and solution. Compound 3a is thermally stable in the solid state under inert atmosphere even at high temperature whereas, in ambient temperature solutions, it readily converts to 2 via CO elimination. The solid-gas reaction between crystals of 1 and CO (1 atm) at room temperature gives a mixture of 2 and of a new species 3b, which has the same stoichiometry as 3a but exhibits a different P-31 CPMAS spectrum. On dissolution in THF saturated with CO, 3b converts to 3a whereas, under inert atmosphere, it dissolves to give initially 3a and later 2 and CO. In light of in situ DRIFT and P-31 CPMAS spectroscopies, 3b is suggested to be an ensemble of metastable species in which a phosphine arm from NP3, although unfastened, is located close to the metal center as a consequence of the constraining environment of the crystal lattice. The proximity of the free phosphine arm to nickel makes solid 3b capable of losing one CO ligand, thus converting to 2. Unlike in solution, the latter compound undergoes thermal dissociation of CO in the solid state to generate the starting complex 1.