HETEROCYCLIC-SYSTEMS CONTAINING TIN(IV)-XIII .1. POSSIBLE CEASING OR INVERSION OF THE STRUCTURAL TRANS INFLUENCE DURING THE COURSE OF A BIMOLECULAR NUCLEOPHILIC-ATTACK OF A DONOR GROUP IN A SERIES OF SN-IV RINGS

In the series of the heterocycles X[(CH2)(3)](2)SnR2 (16 newly synthes ized compounds and two taken from the literature) and in the germanium analog (BuN)-N-i[(CH2)(3)](2)GeCl2, the donor group X ( = NMe, NBz, N Bui, NPri, O, S) intramolecularly attacks the Lewis acidic atoms Sn or Ge. Eight structure determinations (and, in addition, 2 taken from th e literature) are compared. The nucleophilic attack at Sn is Stronger for-the ligands R-2 = Cl-2 than for the more electronegative R-2 = (OS iPh3)(2). Overall the familiar trans influence holds: simultaneous app roach of X and weakening of the ligand R(trans). In three cases of sub tly graded donor strength a slight inversion or-ceasing is observed: s horter distances X ... Sn and simultaneously shorter distances Sn-R(tr ans). Also in a series of 8 compounds with O ... Si(Hal) interaction, taken from the literature, the dominant trans influence ceases to zero in the central part of the oxygen approach. Actually, the discussed r anges of inversion or ceasing of the trans influence are small and in terms of e.s.d.s at the borderline of statistical significance. Despit e this objection, the existence of four alike cases is noticeable. As an explanation for this secondary effect, a bonding scheme is discusse d in which the electronegativity of the ligands inhibits the nucleophi lic attack and in which its drive originates from the pi-basicity of t he ligands into a sigma-LUMO at Sn or Ge-1. This bonding scheme is su pported by semi-empirical MO calculations on the extended Huckel level for the Ge compound and its tetrahedral analogue Cl2GeBu2n. In additi on, a qualitative-discussion df Sn-119 NMR chemical shifts is given, w hich uses the same line of arguments as applied above on the basis of interactions of frontier orbitals. (C) 1997 Elsevier Science Ltd.