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Why is mercury (I) ion diatomic?

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user529401:
Hello, Why is mercury (I) ion diatomic?

Mitch:
Why Mercury(I) is diatomic is an excellent question that I'm not sure is well understood. It goes against the trend in that group, Zinc and Cadmium don't behave like this. The obvious answer to give you is that the 4f electrons shield the 4s electrons poorly, so it can ionize to Hg2+ readily.

A more qualitative answer is that this ion is not found in nature as Hg2+, but as salts like Hg2(H2O2)22+

Mitch:
Forgot to put the references. I took some of the above information from Advanced Inorganic Chemistry fith edition by Cotton and Wilkinson page 601.

And also from W. R. Mason, Inorg. Chem., 1983, 22, 147

AWK:
Scientists answer for troublesome question is always "good (or excelent of fascinating) question. Mitch, this is not a cutting remark to your answer, this is real truth.

This is structure of Hg2(H2O)22+ . 2NO3- (water hydrogen atoms are not included in ICSD - very old work). Note, there are covalent and ionic bonds between Hg and O atoms (lengths are in Angstrems, drawn in Mercury 1.2)
Oxygen -red
Nitrogen - blue
Mercury - violet

Mitch:
Do you have too many atoms? I count 12? Can you tell us what the colors are?

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