Halogen bridges

Both 2-formylpyridine and 6-methyl-2-formylpyridine N-methylthiosemi-carbazones, 30 and 31, respectively, form high spin NiLAj (A = Cl, Br, NCS) and [Ni(L)2]A2 (A = CIO4, NO3) [170]. The former complexes are 5- or 6-coordinate with bridging halogen or psuedohalogen ligands while the latter are octahedral with NNS coordination. 

On the basis of infrared and Raman data 102), the structure is believed to have bridging halogen atoms between the metal atoms, the coordination geometry around each... 

The following dimeric species are associated via two bridging halogen atoms. What are then-... 

In the gas phase, the dihalides have an angular structure, but in the solid state, most have complex structures held together by bridging halogen atoms. The gaseous EX4 molecules are tetrahedral, nonpolar, and more soluble in organic solvents than the EX2 compounds. 

In the solid state, the Grignard complexes 170-177 (Table 3) aU have a dimeric structural motif, via two bridging halogen atoms between the two magnesium atoms, as shown schematically in Figure 78. 

The diphenyliodine(III) halides have dimeric structures in the solid state,63 directly comparable with that of IC13.29 Thus, these compounds have a planar coordination of the iodine atom by two carbon and two bridging halogen atoms. This corresponds to the 12-electron, octahedral arrangement. The I—Cl bonds in the chloride, of 3.085 A, are longer than those of 2.70 A in the trichloride and presumably reflect the significance63 of the ionic form [Ph2I]+[Cl] 2. 

The best-known examples of noncarbonyl clusters containing three metal atoms are the rhenium trihalides [(ReO,)iJ and their derivatives. The basic structural unit is shown in Fig. 16.62a. Each rhenium atom is bunded to the other two rhenium atoms directly by metal-metal bonds and indirectly by a bridging halogen ligand. In addition. 

The results are relevant to the mechanism of the isomerization and racemization of triorganotin halides, which may include the selective cleavage of the bridging halogen bonds in unsymmetrical intermediates X—SnR3—Y—SnR3—D (D = donor) like 53. 

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