Nuclear quadrupole coupling in transition metal compounds

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Virginia Polytechnic Institute and State University


The work described in this dissertation represents an effort to extend the application of Nuclear Quadrupole Resonance spectroscopy to the study of transition element compounds. Using a conventional noise controlled superregenerative spectrometer compounds of cobalt, copper and molybdenum have been investigated.

Three biscobalt(tetracarbonyl) tin(II) compounds were investigated and the ⁵⁹Co resonances measured in each. Each compound exhibited a doublet indicative of two crystallographic inequivalent sites. The asymmetry parameters were all between 0.005 and 0.10 indicating little distortion of the cobalt environments from the expected C₃v symmetry. The coupling constants as obtained by use of a series approximation for the transition frequencies and confirmed by a frequency ratio plot were Cl₂ Sn [Co(CO)₄]₂ -146.9MHz, (C₆H₅)ClSn [Co(CO)₄]₂ -137.7 MHz, (C₆H₅)₂Sn[Co(CO)₄]₂ -112.9 MHz. The observed coupling constants correlate with the inductive effects of the substituents in the tin.

The study of several Copper(I) coordination compounds represents the first known attempt at using Cu nuclear quadrupole coupling constants to study bonding in a situation other than an isolated compound. Assuming zero asymmetry parameters the following ⁶³Cu coupling constants were observed; Cu(tu)₂NO₃ -50.18 MHz, Cu(tu)₂Cl - 41.41 MHz, Cu(tu)₂Br -32.62 MHz, Cu(etu)₂ClO₄ - 45.76 MHz, [Cu(etu)₄]₂ SO₄ - 63.12 MHz, Cu(etu)₂C1 - 55.72 MHz, Cu(etu)₂Br - 64.02 MHz, Cu(dmtu)₃C1 - 77.60 MHz. The ligands used were tu-thiourea, etu-ethylene thiourea, and dmtu -N,N’ dimethylthiourea. The crystal structures of only Cu(tu)₂C1 and Cu(dmtu)₃Cl are known making direct comparison difficult. The general increase of the coupling constants with ligand substitution correlates with the partial charge on the sulfur atom of the free ligand. The reversal of the order of the coupling constants between the thiourea and ethylene thiourea halides indicates an appreciable ion contribution to the coupling constant from the halogen. The observation of ⁷⁹Br resonance at 38.83 MHz in Cu(etu)₂Br also confirms this point. By using sp² and sp³ hybridization schemes for Cu(tu)₂C1 and Cu(dmtu)₃Cl the coupling constants were calculated to be 36.0 MHz and 17.27 MHz respectively. This represents reasonable agreement in view of the uncertainties in the Sternheimer factor used and the approximate nature of the model. The allowance for dπ - dπ bonding between the Cu and S atoms decreases the calculated constants indicating that such bonding probably is of little importance.

Resonance were observed for Mo isotope in both MoOCl₄ and MoO₂Cl₂. Both possible Mo resonances as well as the Cl resonances were observed. The particular isotope to which the resonances belong is as yet undetermined since those belonging to the other Mo isotopes with. I= 1 will be outside the range of available instrumentation.