This paper is dedicated to Professor Kurt Dehnicke on the occasion of his 60th birthday MARGARET ANN JAMES, OSVALD KNOP, and T. STANLEY CAMERON. Can. J. Chem. 70, 1795Chem. 70, (1992. anion. The actual composition of the centrosymmetric p2-dimer in C is Te2Cls(Cl, OH)!-, the replacement of C1 by OH being at the axial ligand positions of the TeC1,-halves of the dimer. In the centrosymmetric p 2 -~e 2 0 2~1 2 -dimer in D the Se-0 bonds are perpendicular to the Se(Cl),Se 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 p2-&~al,,2"-(Z = Se, Te) and p,-&~al:-(Z = As, Sb, Bi) as well as in the p 2 -~e 2~2~a 1 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),Z 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 &~al,,2"-dimer on the one hand and of a discrete ZHa1,"-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 k-dimer, with results that are important when correcting bond lengths for the effects of thermal motion. A variety of general observations on p,-dimers in crystals is proffered for good measure. On exploite une quantification empirique du TE pour les dimkres du Se et du Te pour authentifier la composition de dimkres dans des situations oh la nature du ligand n'est pas certaine (dans le cas examine ici, il s'agit du produit d'hydrolyse partielle). On a examine I'asymCtrie du double pont Z(Hal)2Z en rapport avec la stCrCochimie globale du dimkre. On a aussi examine les circonstances qui peuvent conduire, dans un cristal, & la presence d'une part d'un dimkre ~,~a l , , 2 " -et d'un monomkre ZHa1,"-individuel, dans des conditions chimiques qui sont par ailleurs chimiquement semblables. On a utilisC l'analyse TLS du mouvement thermique pour juger de la force de l'association de deux parties de monomeres dans un dimkre-p,; les resultats sont importants pour coniger les longueurs de liaison pour les effets des mouvements thermiques. On prksente aussi un nombre d'observations gCnCrales sur dimeres-p2 dans les cristaux.[Traduit par la redaction]In the course of investigating the actual geometry of tallographic studies showed three of them (see title) to conoctahedral 12-and 14-valence-electron ML,"-anions in tain uncommon Se(1V)-and Te(1V)-derived anions and thus crystals (cf. ref. 1) we obtained, serendipitously, several tetrato merit closer investigation. The crystal structures of these n-propy...