Halogen complexes four-coordinate

Cobalt in its trivalent state forms many stable complexes in solution. In these complexes, the coordination number of Co + is six. The Co2+ ion also forms complexes where the coordination number is four. Several complexes of both the trivalent and divalent ions with ammonia, amines, ethylene diamine, cyanide, halogens and sulfur ligands are known (see also Cobalt Complexes). 

The reaction of dithiooxamide and its tetramethyl and tetraethyl derivatives with zinc, cadmium and mercury halides leads to complexes of stoichiometry MLX2 (M = Zn, Cd or Hg X = C1, Br or I).900,9 1 M—S bonding is involved IR spectra show that the zinc and mercury complexes are four-coordinate, while the cadmium complexes are octahedral with halogen bridges. 

Addition of halogens to planar d complexes is straightforward and does not deserve further comment. However, it should be noted that halogenation can also be achieved by treatment with the carbon tetrahalides (26, 41, 67). The mechanism of this reaction is obscure but a radical path may be involved (104). The fate of the carbon fragment has not been determined but it may be anticipated that carbene like intermediates could be formed in this way. A related process is the halogenation of reactive four-coordinate complexes by vicinal dihalides which are converted into olefins. Only two examples have been described (28,56a). The more interesting involves the formation of the cyclobutadiene complex (XLVI) (56a). 

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