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