Ligand Exchange or Reduction at Multiply Bonded Dimetal Units of Molybdenum and Rhenium by 2,6-Bis(diphenylphosphino)pyridine

Reaction of 2,6-bis(diphenylphosphino)pyridine (bdppp or (Ph2P)2Py)) with K4Mo2Cl8 in refluxing methanol gives
Mo2Cl4(bdppp)2 (1) with retention of the quadruple Mo−Mo bond. However, the quadruply bonded octachlorodirhenate(III) anion, Re2Cl82-, reacts with bdppp under similar conditions to afford Re2Cl4(bdppp)2 (2), which
has a triply bonded dirhenium(II) core, Re24+. An intermediate species, [Bun4N][Re2Cl7(bdppp)] (3), containing
an Re26+ core and only one bridging bdppp ligand has also been isolated. The crystal structures of complexes
1−3 have been investigated by X-ray crystallography. In all cases the potentially tridentate bdppp acts as a
bidentate ligand using one N and one of the P atoms, leaving the second phosphorus atom noncoordinated. The
crystallographic parameters for these structures are as follows:  Mo2Cl4(bdppp)2·2CH2Cl2 (1·2CH2Cl2), monoclinic
space group P21/c with a = 15.475(2) Å, b = 11.5958(7) Å, c = 16.7637 (6) Å, β = 98.178(5)°, Z = 2; Re2Cl4(bdppp)2 (2), monoclinic space group P21/n with a = 16.173(2) Å, b = 11.285(1) Å, c = 28.771 (4) Å, β =
96.17(1)°, Z = 4; [Bun4N][Re2Cl7(bdppp)]·CH2Cl2 (3·CH2Cl2), triclinic space group P1̄ with a = 16.054(2) Å,
b = 18.562(3) Å, c = 20.205 (6) Å, α = 89.71(2)°, β = 73.21(1)°, γ = 73.47(2)°, Z = 4.