Synthesis, Spectral, And Structural Studies Of Tetrahalodimolybdenum(Ii) Complexes With Alkyl Substituted Pyridines

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Description: The purpose of this research was to study the effects various bulky ligands would have on the geometry of complexes of the type Mo(,2)X(,4)L(,4), where X = Cl, Br and L is a neutral donor ligand. To this end, complexes of this type with L = pyridine (Py), 4-methylpyridine (4PIC), 3-methylpyridine (3PIC), 3,4-dimethylpyridine (34LUT), 3,5-dimethylpyridine (35LUT), and 4-t-butylpyridine (4TBP) were prepared. The (delta) (--->) (delta)* transition energy in the solid state was found to vary in a linear fashion with the ligand pK(,b) for complexes with Py, 3PIC, and 35LUT. This indicates the possibility of (delta) to ligand (pi)* backbonding in these complexes. Complexes with 4PIC, and 34LUT have significantly lower than expected (delta) (--->) (delta)* transition energies, while the complex with 4TBP has a slightly lower than expected (delta) (--->) (delta)* transition energy. Raman spectra were taken for the complexes with 4PIC, 3PIC, 34LUT, and 35LUT in order to verify the presence of the quadruple bond in these complexes. Room temperature nmr studies indicate a significant deshielding of all ligand protons ortho to the ring nitrogen which is brought about by the diamagnetic anisotropy of the quadruple bond. A variable temperature nmr study of the 4TBP complex (X = Br) indicates the downfield chemical shift of the ortho protons in the complex is indeed due to the diamagnetic anisotropy of the quadruple bond and not to second order paramagnetic effects. Infrared spectra of the complexes in the Mo-Cl stretching region were used in conjunction with group theoretical arguments to assign probably structures to the complexes. An x-ray crystallographic study of the Mo(,2)Br(,4)(4TBP)(,4) complex indicates it to have the usual eclipsed structure (D(,2H)) and Mo-Mo bond length, 2.148(2) (Angstrom), of most dimetallic, quadruply bonded complexes. The Mo(,2)Br(,4)(4TBP)(,4) complex is isolable as either green or blue crystals whose visible spectra differ with respect to the (delta) (--->) (delta)* transition energy. The blue form was shown to have an eclipsed geometry via x-ray crystallography, as described above. It is postulated that the green form, which has the lower energy (delta) (--->) (delta)* transition energy, has a unique staggered geometry making it the first dimolybdenum complex with a monodentate neutral donor ligand to have this geometry. (Abstract shortened with permission of author.)
Language: English
Format: Degree Work