High-valence cluster metal preferences

The nitride cluster in TNT EMFs can also be viewed as a whole, which always gives six electrons to the carbon cage [121], This is interpreted by the high electron affinity of the //,-Cso cage six electrons are necessary to make it the most stable hexa-anion. Because N prefers to take the -3 valence, each metal in M3N has a +3 valence. Therefore, it is easy to understand why Sm, Eu, and Yb have not been trapped as TNT EMFs because they can donate only two electrons from their 6s orbitals. 

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