HETEROCYCLIC-SYSTEMS CONTAINING TIN(IV) .11. STANNOCANES CL BR/I/ME-(ME)SN(SCH2CH2)(2)X (X=O, S, NME) - SYNTHESIS AND STRUCTURAL AND VIBRATIONAL DATA - A SEMIQUANTITATIVE INVESTIGATION OF THE ENERGY GAIN OF PENTACOORDINATE TIN IN TERMS OF FRONTIER ORBITALS/

The stannocanes mentioned in the title have been synthesized from MeSn Hal(3) and (HSCH2CH2)(2)X. Crystal structure determinations and vibrat ional transitions of the coordination polyhedra around tin are given. The 12 compounds are compared with the 9 compounds Cl-2/Br-2/I-2-Sn(SC H2CH2)(2)X (X = NMe, O, S) investigated in a previous study. Apart fro m one molecule with heptacoordinated tin, the other compounds comprise pentacoordinated tin on a ''path'' from a tetrahedron to a trigonal b ipyramid. The path is discussed in terms of the molecular orbitals of the three-center, four-electron interaction X...Sn-Cl/Br/I/Me. By the use of standard single-bond distances, vibrations, and bond energies, and with the help of Morse potentials, the structural and vibrational data are transformed into semiquantitative energy diagrams of the occu pied equatorial and axial orbitals. The energy differences between the se orbitals and those of a tetrahedral configuration are added up to g ive the total energy gain of each compound (energy coordinate) and are plotted versus an angle sum of the respective trigonal bipyramid (con figurational coordinate). The total energy gain from the structural an d vibrational data includes an energy contribution fi om the LUMO of t he three-center interaction. A model is presented in which an enhanced electronegativity of the equatorial and axial ligands inhibits the fo rmation of a pentacoordinate structure. The drive for pentacoordinatio n (sigma-Lewis acidity) results from a movable charge which populates the LUMO (pi-Lewis acidity). The movable charge originates from the lo ne pairs of the halides and chalcagenides, from the pi-electrons of ar omatic rings, or from the hyperconjugation in the case of the methyl g roup. This model contradicts the generally accepted opinion that elect ronegativity promotes pentacoordination. With regard to the presented model, a short discussion of hypervalency for the other group 14 eleme nts and a mention of the broad subject of bimolecular nucleophilic att ack are given.