Pseudohalogen

Pseudohalogens are binary inorganic compounds of the general form XY, where X is a cyanide, cyanate, thiocyanate etc. group and Y is any of X, or a true halogen. Not all combinations are known to be stable. Examples include cyanogen, (CN)₂, and iodine cyanide, ICN. These anions behave as halogens and the presence of the internal double bonds or triple bonds do not appear to affect their chemical behavior.

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Nanoclusters of aluminium (often referred to as superatoms) are sometimes considered to be pseudohalogens since they, too, behave chemically as halide ions, forming Al₁₃I₂⁻ (analogous to I₃⁻) and similar compounds. This is due to the effects of metallic bonding on small scales.

Another complex pseudohalogen is dicobalt octacarbonyl, Co₂(CO)₈. This substance can be considered as a dimer of the hypothetical cobalt tetracarbonyl, Co(CO)₄. It can easily be reduced to the "pseudo-halide", Co(CO)₄^-. The acid HCo(CO)₄ is in fact quite a strong acid, though its low solubility renders it not as strong as the true hydrohalic acids.

References

• New Scientist, issue 2495 (16th April 2005), pp30-33, "A new kind of alchemy", Philip Ball.