Unusual geometries and questions of oxidation state in potential Sn(III) chemistry

A mixed-valence compound (Sn2I3(NPPh3)3) with nonequivalent Sn atoms in characteristic 2+ and 4+ Sn geometries, raised the idea of an average Sn3+ structure. The extended structures of Sr4Sn2Se9 and Sr4Sn2Se10 contain an unusual Sn2Se6 subunit, which has two equal Se-Sn-Se angles close to 160 degrees. This was suggestive of a Sn3+/Sn3+ compound, similar to the putative transition state for the valence state interchange in the molecular compound. These interesting geometrical features of two quite different molecules prompt a series of computations, a detective story of geometries and oxidation states, which concludes tentatively that the Sn with the abnormal angle in the extended structure is still likely to be formally Sn4+.