Homo- and heterobimetallic trihydride complexes stabilized by the tripodal phosphine ligand MeC(CH2PPh2)3: experimental and theoretical studies

The trihydride (triphos)RhH3 [triphos = MeC(CH2PPh2)3] reacts with coordinatively unsaturated metal fragments to form a family of bimetallic polyhydride complexes of general formula [(triphos)Rh(μ-H)3M(L)]n+ [M = Rh, Co, L = triphos, n = 3, 2, 1; M = Ni, L = triphos, n = 2, 1; M = Fe, L = MeC(CH2PEt2)3, n = 2]. All of the compounds were isolated as BPh4-, ClO4-, or BF4- crystalline salts and their spectroscopic properties studied in detail by IR, NMR, and ESR techniques. The crystal structure of the paramagnetic homonuclear derivative [(triphos)Rh(μ-H)3Rh(triphos)](BPh4) 2·DMF was established by X-ray methods. Crystal data: triclinic, P1, a = 16.868 (4) Å, b = 15.863 (4) Å, c = 14.026 (3) Å, α = 111.74 (2)°, β = 91.01 (2)°, γ = 116.25(2)°, Z = 1. The structure was refined to an R factor of 0.059 Rw = 0.064) by using 9149 unique reflections with I > 3σ(I). The complex cation consists of two pyramidal L3Rh fragments (L = phosphine) held together by three bridging hydride ligands. The relative orientation of the L3Rh fragments is halfway between eclipsed and staggered, the overall symmetry of the molecule being approximately D3. The Rh-Rh separation measures 2.644 (1) Å. The electrochemical behavior in nonaqueous solvents shows that all of the compounds reversibly undergo multisequential one-electron-redox exchanges with no major variation of the primary geometry. NMR and ESR spectroscopies provide useful information concerning the geometric and electronic nature of diamagnetic and paramagnetic compounds, respectively. EHMO studies, not novel for this class of compounds, allow a new interpretation of the metal-metal bond in the dimers. In particular, the bond cannot be as strong as that predicted by simple electron-counting rules. Other comments are devoted to the potential existence of dimers with a bridging polyhydrogen unit [i.e. μ-H3 vs (μ-H)3]. Finally, the MO picture is used, on a very qualitative scale, to correlate structural, electrochemical, and spectroscopic parameters, in particular those derived from the ESR spectra.