Metal was first discovered in 1979 (? ), he used metal sodium to react with bicyclic polyoxydiamine to obtain Na2C 18H36N2O6, where Na is half Na+ and half Na-. This is the first time to synthesize negative-valent metal ions, which belong to metal anions-crown ether metal cation salts.

In addition, there is another kind of metal with negative valence, which is a polycarbonyl compound, such as NaMn(CO)5, manganese is negative monovalent, H2Fe(CO)4, and Fe is negative divalent, which is a dibasic acid.

Others are more advanced, such as free negative valence metal cluster ion [Pb9]4-.

I only know the latter two, but I don't know how they came from.

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