CLASSICAL AND NONCLASSICAL POLYHYDRIDE RUTHENIUM(II) COMPLEXES STABILIZED BY THE TETRAPHOSPHINE P(CH2CH2PPH2)3

The dichloride [(PP3)RuCl2] (1) is selectively transformed into [(PP3)Ru(H)(eta-1-BH4)] (2), [(PP3)Ru(H)Cl] (3), and [(PP3)-Ru(H)2] (4) by reaction with NaBH4, LiHBEt3, and LiAlH4, respectively [PP3 = P(CH2CH2PPh2)3]. Complex 2 is fluxional on the NMR time scale in ambient-temperature solutions due to equilibration of the BH4 - hydrogens through cleavage of the Ru-H(b)-B bridge and interconversion of H(t) by rotation. The hydride-tetrahydroborate complex exhibits amphoteric nature: it reacts with protic acids, yielding the cis-hydride-dihydrogen complex [(PP3)Ru(H)(eta-2-H2)]Y (Y = PF6, 6a; BPh4, 6b), and with bases such as PEt3 or KO(t)Bu, converting to the dihydride 4. The latter compound can be obtained also by thermal decomposition of 2 in refluxing toluene or benzene. Complexes 6a,b can be prepared by reaction of the dihydride 4 with protic acids. The nonclassical structure of 6a,b is established by variable-temperature H-1 and P-31 NMR data, T1 measurements and J(HD) values. In the solid state and in solution at low temperature [(PP3)Ru(H)(eta-2-H2)]+ is octahedral, and the hydride and dihydrogen ligands occupy mutually cis positions. At ambient temperature in solution the complex is trigonal-bipyramidal and the equilibration of the hydrogens might proceed through an "H-3" unit occupying an axial position trans to the bridgehead phosphorus atom of PP3. Octahedral monohydride complexes of the type [(PP3)Ru(H)(L)]BPh4 are obtained from 6b by substitution of dihydrogen with the appropriate ligand (L = N2, CO, PEt3, CH3CN,SO2). In all cases, the hydride ligand is located trans to a terminal phosphorus of the ligand. The role played by the tripodal tetradentate phosphine PP3 in the stabilization, in the fluxionality, and in the structure of all of the present Ru complexes is amply discussed.