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Mercury forms a wide range of alloys (amalgams), some of which are known from ancient times. Square-planar Hg46− units are known to be particularly favorable building blocks in many amalgams. It is reported here that the unusual Hg46− square is aromatic (the picture shows the HOMO−1 (1a2u) molecular orbital), analogous to the recently discovered all-metal Al42− ion.
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Kuznetsov et al. (2001) studied this question.
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