Synthesis and Structure of a Molybdenum Dimer Illustrating d? Orbital Participation in ? Donation, ? Acceptance, and Metal-Metal Bond Formation

Article abstract of DOI:10.1021/ja00347a014

The synthesis of a series of molybdenum dimers with bridging sulfur and alkyne ligands has been realized from monomeric Mo(R¹C<*>CR²)2(S2CNR2)2 reagents (R¹=H, R²=Ph; R¹ = CH3, R² = Ph; R¹ = H, R² = n-Bu; R¹ = R² = Et).The molecular structure of the dimeric 3-hexyne derivative Mo2(μ-S)(μ-EtC<*>CEt)(S2CNMe2)3(SCNMe2) has been determined by a single-crystal X-ray diffraction study.The crystals were found to be monoclinic of space group P2₁/c with a = 12.105(3) Angstroem, b = 16.998(4) Angstroem, c = 17.998(5) Angstroem, and β = 94.57(2) deg.The structure was refined to R = 0.044 and R_W = 0.038 using 3649 reflections with I > 3?(I).The unsymmetrical dimer can be viewed as a conlateral bioctahedron with one sulfur atom bridge and one alkyne bridge linking the two metal centers.The coordination sphere of one molybdenum is completed by two chelating dithiocarbamate ligands while the second molybdenum binds one chelating dithiocarbamate and one thiocarboxamido ligand through both the sulfur and the carbon, in the terminal positions.The formation of a bridging sulfur ligand and a thiocarboxamido ligand (SCNR2) from one of the original dithiocarbamates (S2CNR2) corresponds to oxidative addition of one C-S bond to the molybdenum(II) centers.The Mo-C bond length of 2.056(6) Angstroem and the low-field carbon-13 chemical shift of this thiocarboxamido carbon (263.7 ppm) reflect considerable carbene character.Infrared ν(CN) frequencies and spin saturation transfer experiments probing C-N bond rotation processes are consistent with enhanced C-N multiple-bond character in the η²-SCNR2 ligand compared to the S2CNR2 ligands.Proton NMR spectra of the dimeric products indicate the presence of at least two isomers for symmetrical alkyne derivatives (RC<*>CR) and at least four isomers for unsymmetrical alkyne derivatives (RC<*>CR') in solution.The Mo-Mo distance of 2.647(1) Angstroem observed in the 3-hexyne-bridged dimer suggests an attractive metal-metal interaction.A qualitative molecular orbital description has been formulated which accounts for the diamagnetism of these dimers and the observed structural parameters.The two nests of three d? orbitals remaining on the metals following formation of six ? bonds to each center are used to generate Mo-Mo ? and ?^* orbitals, two alkyne ?^*-stabilized molecular orbitals, one combination destabilized by ? donation from a filled bridging sulfur atom p orbital, and one d? combination destabilized by ? donation from terminal dithiocarbamate sulfur atoms.The orientation of the d? orbital combination available for ? donation from chelating terminal ligands limits this interaction to only two of the seven terminal sulfurs, and the structure reveals an average Mo-S_t distance of 2.44 Angstroem to these two atoms while the remaining five Mo-S_t distances average 2.57 Angstroem.