CRYSTAL-STRUCTURES OF (N-PR4N)2SNCL6, (N-PR4N)[TECL4(OH)], (N-PR4N)2[TE2CL10] (NOMINAL), AND (N-PR4N)2[SE2O2CL6], WITH OBSERVATIONS ON Z2L10(2N-) AND Z2L8(2-) DIMERS IN GENERAL

The crystal structures of the following four compounds have been determined: (n-Pr4N)2SnCl6 (A; Pbca, a = 29.073 angstrom, b = 15.825 angstrom, c = 14.851 angstrom, Z = 8), (n-Pr4N)[TeCl4(OH)] (B; C2/c or Cc, a = 10.800 angstrom, b = 17.429 angstrom, c = 10.604 angstrom, beta = 93.81-degrees, Z = 4), (n-Pr4N)2[Te2Cl10] (C, nominal composition; P1BAR, a = 9.635 angstrom, b = 10.900 angstrom, c = 11.547 angstrom, alpha = 73.34-degrees, beta = 70.44-degrees, gamma = 68.15-degrees, Z = 1), and (n-Pr4N)2[Se2O2Cl6] (D; P1BAR, a = 9.252 angstrom, b = 11.691 angstrom, c = 18.304 angstrom, alpha = 110.18-degrees, beta = 92.57-degrees, gamma = 97.72-degrees, Z = 2). B contains a square-pyramidal TeCl4(OH)- anion. The actual composition of the centrosymmetric mu2-dimer in C is Te2Cl8(Cl, OH)22-, the replacement of Cl by OH being at the axial ligand positions of the TeCl5- halves of the dimer. In the centrosymmetric mu2-Se2O2Cl6(2-) dimer in D the Se-O bonds are perpendicular to the Se(Cl)2Se bridge plane and trans to the presumed lone-electron pairs on the Se atoms. It is shown that the Z-Hal bonds in the known centrosymmetric homoligand mu2-Z2Hal10(2n-) (Z = Se, Te) and mu2-Z2Hal8(2-) (Z = As, Sb, Bi) as well as in the mu2-Se2O2Hal6(2-) dimers exhibit a uniform stereochemistry, in which the d(Z-Hal) bond lengths in pairs of opposing bonds trans to each other are consistently related by the trans effect (TE). Empirical quantification of Te for the Se and Te dimers is exploited to authenticate the composition of the dimer in situations where the nature of the ligand is in doubt (in the present case on partial hydrolysis). The asymmetry of the Z(Hal)2Z double bridge is examined in relation to the overall stereochemistry of the dimer. Also examined are circumstances that may lead to the occurrence, in a crystal, of a Z2Hal10(2n-) dimer on the one hand and of a discrete ZHal5n- monomer on the other, in situations which otherwise are chemically closely similar. TLS analysis of the thermal motion is used to judge the strength of association of the two monomer halves in a mu2-dimer, with results that are important when correcting bond lengths for the effects of thermal motion. A variety of general observations on mu2-dimers in crystals is proffered for good measure.