Spin—coupling constants in organometallic derivatives : I. A semi-empirical approach to the study of ( d- p)π contribution to bonds in organostannanes

Abstract

The contact term contributions to direct 1 J(SnH) and long-range 2 J(SnH) coupling constants in methylchlorostannanes and methylhydrides have been calculated in the one-electron MO approximation employing extended Hückel wave functions. The OA basis set for the different atoms is that given by Clementi, while for tin an analytical form of 5 s and 5 p orbitals was derived from Herman and Skillman digital tables. Calculatins were performed with and without d orbitals (Slater type) on the tin AO set, and the behaviour of calculated coupling constants in relation to d orbital energy and contraction was explored. The results show that for tetramethylstannane and tin hydride the coupling constants are unaffected by d orbital participation, while for mixed methyltinhydrides and methylchlorostannanes a dependence is found on the form of d orbital employed. The analysis of MO combinations and energy levels provides an indication of the mechanism involved in the participation of d orbitals of tin to bonding, but difficulties are encountered when flexibility criteria are introduced into the method in order both to compute the coupling constants resulting from the different redistribution of orbitals and their energies when different bonding situatins are present. A study of the dependence of calculated coupling constants on geometrical parameters shows that small changes in bond lengths and bond angles give large intervals of indeterminacy of the computed quantities, thus indicating that the choice of selected geometries is of the utmost importance if, in order to achieve a more quantitative description of the coupling constants in organometallic derivatives, higher levels of sophistication are introduced into the computation methods.