Infrared, Mössbauer, and NMR Studies of Some Stannous Chloride and Bromide Complexes of Thiourea and Tetramethylthiourea
Complexes having the formulas Sn(TU)Cl2, Sn(TU)2Cl2, Sn(TU)2Br2 (TU = thiourea), Sn(TMTU)Cl2, and Sn(TMTU)Br2 (TMTU = tetramethylthiourea) have been prepared in methanol solutions or by the solid-state reaction method. These pure, solid complexes and solutions of the complexes have been studied by IR, Mössbauer, and NMR spectroscopy. A discussion of the relative merits of each technique in determining properties such as the symmetry and the bonding of these complexes is presented.
Infrared data have shown these ligands to be coordinated to the tin through the sulfur with sulfur acting as a bridging atom only in the TMTU complexes. The IR data of the respective chloride and bromide complexes are not very different and do not point out in a clear-cut fashion the relative tin-to-sulfur bond strengths.
Complexation of the sulfur donors to tin causes a rearrangement of the tin bonding orbitals and the resulting isomer shifts of the complexes are less than the parent tin(II) halides. An increase in the shielding of the tin outer s electrons from the nucleus by the electrons of the coordinated sulfur ligands can also account, in part, for the above-mentioned observation. TMTU is observed to have a greater rearranging and shielding effect than TU and, therefore, forms a stronger tin-to-sulfur bond.
Quadrupole splittings in the range 0.61–1.15 ± 0.05 mm/sec have been observed for all of the complexes. These values can be interpreted in terms of steric factors of both the ligands and the halogens. A correlation plot of the isomer shifts as a function of the quadrupole splittings, similar to that of Lees and Flinn, has been made. The bromide complexes show an equatorial field gradient asymmetry. Unexpectedly, the electric field gradient asymmetry for the chlorides lies on a third line between that of the axial and equatorial field distortions. The slopes of the three lines are in the ratio of 2:3:4. A relationship between the electronegativity of the elements bonded to the tin and the three lines of the δ, Δ plot has been noted.
Mössbauer data have been obtained for these complexes in organic solvents frozen to liquid nitrogen temperatures. Spectra of the glassed alcohol solutions of the TU complexes indicate that the solvent molecules are coordinated along with the sulfur ligands to the tin halides. Tin(IV) species have also been found to be present in several of these solutions, resulting mainly from the air oxidation of the tin(II) halides. Methanol has the same effect on the isomer shift of both of the 2:1, thiourea complexes of SnCl2 and SnBr2 after a compensation for the effects of the thiourea has been made. Data for these complexes in dimethylsulfoxide (DMSO) are also discussed.
NMR data for these complexes in CH3OH and DMSO solutions show a direct correlation with the Mössbauer data. A correlation between the IR, Mössbauer, and NMR data for the Sn(TU)2Br2 and Sn(TU)2Cl2 complexes can be achieved if it is assumed that Sn(TU)2Br2 is only 3-coordinate in the solid state and the ion (Sn(TU)2Br)+ is being observed.
KeywordsIsomer Shift Quadrupole Splitting Stannous Chloride Freeze Solution Mossbauer Spectrum
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