A Series of Edge-Sharing Bioctahedral, M-M Bonded Molecules: Non-Monotonic Bond Length Variation and Its Interpretation

Article abstract of DOI:10.1021/ja00265a022

Three new stoichiometrically homologous compounds, Ta2Cl6(dmpm)2 (1), Mo2Cl6(dppm)2 (2) Ru2Cl6(dmpm)2 (3) have been prepared and their structures determined.Crystal data are as follows. 1: P2₁/n, a = 7.263(3) Angstroem, b = 15.536(3) Angstroem, c = 10.827(2) Angstroem, β = 93.73(2) deg, V = 1219(1) Angstroem³, Z = 2. 2: C2/c, a = 23.083(4) Angstroem, b = 10.866(4) Angstroem, c = 23.254(5) Angstroem, β = 124.25(3) deg, V = 4821(4) Angstroem³, Z = 4. 3: I2/a, a = 12.479(5) Angstroem, b = 15.330(6) Angstroem, c = 13.212(4) Angstroem, β = 113.91(3) deg, V = 2310(1) Angstroem³, Z = 4.These three compounds, as well as the previously described Nb2Cl6(dmpm)2 and Re2Cl6(dppm)2,consist of molecules in which two octahedra share an edge via μ-Cl atoms, thus forming a central, planar Cl2M(μ-Cl)2MCl2 unit with dridging dmpm or dppm molecules connecting pairs of adjacent axial positions above and below the central plane.The M-M distances in these five molecules are the following: Nb-Nb, 2.711(3) Angstroem; Ta-Ta, 2.692(2) Angstroem; Mo-Mo, 2.789(1) Angstroem; Re-Re, 2.616(1) Angstroem; Ru-Ru, 2.933(1) Angstroem.The up, down, up variation on proceeding through the d²-d², d³-d³, d⁴-d⁴, d⁵-d⁵ series of metal atom pairs is consistent with the following electronic configurations: ?²?², ?²?²δ^*2, ?²?²δ^*2_δ2_?*2.The relationship of this structurally indicated ordering of the orbitals is discussed in the light of molecular quantum mechanical calculations on model systems, and it is shown that the overall experimental and theoretical picture is internally consistent.