The secondary bond concept

Two examples (from our own work) illustrate intramolecular secondary interactions. Thus, in triphenyltellurium(IV) tetraphenyldithioimidodiphosphinate, Ph3Te [SPh2PNPPh2S] , 27, the tellurium-sulfur interatomic distances, 3.264 and 3.451 A, are significantly longer than the sum of the covalent radii (2.443 A) but significantly shorter than the sum of the van der Waals radii (3.86 A) [56], In bis(diphenyldithiophosphinato)ditellurium(I), Te2(S2PPh2)2, 28, there are two types of tellurium-sulfur interaction - normal covalent, i.e. primary bonds (Te-S 2.471 and 2.493 A) and secondary bonds (Te- -S 2.989 and 3.066 A) [57]. 

Cyclic systems in which ring closure occurs via secondary bonds were called quasi-cyclic systems [29, 58]. 

An example of intermolecular secondary bonds for the same tellurium-sulfur pair, leading to a regular supramolecular self-assembly, 29, is provided by phenyltellurium(II) diphenyldithiophosphinate, PhTe-S(S)PPh2, in which intermolecular, secondary Te- -S bonds (3.383 A) are observed the intramolecular primary Te-S bonds are shorter (2.406 A) [59, 60] and fall within the range of single, covalent tellurium-sulfur bonds [61], 

Interatomic distances between a given pair of elements can vary substantially, from covalent bond lengths and dative (or donor-acceptor) bond lengths to secondary bond interatomic distances. Bond lengths vary to some extent with the coordination number of the central atom, and there is no clear-cut distinction between these three types of bond. Thus, for tin-oxygen bonds, there is a broad range of observed interatomic distances. Covalent Sn-0 bonds are shortest in four- 

All these interatomic distances correspond to two-electron bonds, i.e. covalent and dative. When much longer tin-oxygen interatomic distances are observed, associated with short distances, as in dimeric dioxastannolanes, 30, (dioxastanna-cyclopentanes, R = Bu , with Sn-O 1.98 and Sn- 0 2.50 A) [73] or dioxastanna-cyclohexanes, 31 (R = Bu, Sn-O 2.05 A, Sn- -O 2.57 A) [74], the longer distances should be considered as secondary bonds. 

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There are two types of objects in supramolecular chemistry supermolecules (i.e., well-defined discrete oligomolecular species that result from the inter-molecular association of a few components), and supramolecular arrays (i.e., polymolecular entities that result from the spontaneous association of a large, undefined number of components) (4, 5). Both are observed in some metal-xanthate structures to be described herein. The most frequent intermolecular forces leading to self-assembly in metal xanthates are so-called secondary bonds . The secondary bond concept has been introduced by Nathaniel W. Alcock to describe interactions between molecules that result in interatomic distances longer than covalent bonds and shorter than the sum of van der Waals radii (6). Secondary bonds [sometimes called soft-soft interactions (7)] are typical for heavier p-block elements and play an important role as bonding motifs in supramolecular organometallic chemistry (8). Other types of intermolecular forces (e.g., Ji- -ji stacking) are also observed in the crystal structures of metal xanthates. 

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