Halogen ligands

Mercury(I) forms few complexes, one example is the linear [H2O-Hg Hg- H20] found in the mercury(I) nitrate dihydrate (above, p. 437), In contrast, mercury(II) forms a wide variety of complexes, with some peculiarities (a) octahedral complexes are rare, (b) complexes with nitrogen as the donor atom are common, (c) complexes are more readily formed with iodine than with other halogen ligands. 

This reaction is a principal method of forming IIIB-transition-metal cr bonds. The formation of thermodynamically favored alkali-metal halides or related salts and acids HX enhances the easy formation of those bonds. A second possible interaction between anionic metal bases and group-IIIB halides is a simple acid-base relationship without elimination of halide anions. However examples of this are rare, and they have been described often for group-IIIB compounds without halogen ligands ( 6.5.3.2). 

Figure 1. Dependence of polybutadiene cistacticity (%) on the nature of halogen ligand for different catalyst systems. The data of the first four lines are taken from Ref. 18. cis content by IR from Ref. 34.

Figure 11. Mbssbauer spectra of (CQH5) Sn and (CQH5)3SnCl. Substitution of halogen ligand for CgH., group produces a quadrupole splitting of 2.45 ...

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