Electronic Structures of EMFs Intramolecular Charge Transfer

2 Electronic Structures of EMFs Intramolecular Charge Transfer 

For mono-metallofullerenes with rare earth metals, three electrons are donated from the encaged metal to the fullerene cage, such as Y and most lanthanides (La, Ce, Pr, Nd, Gd, Tb, Dy, Ho, Er, andLu) [2, 108, 109]. However, Sc, Sm [110], Eu [111], Tm [112], and Yb [113] are found to donate only two electrons and prefer to take the valence of +2. Consequently, the EMEs containing rare earth metals are classifiable into two categories according to the electronic state of the encaged metal(s) trivalent EMFs and divalent EMFs. Eor example, La Cs2 should be expressed as La + Cg2. whereas Yb Cs2 has the formula of Yb + CgJ. These two types of EME differ markedly from each other in their structures and properties, (i) When three electrons are transferred to the carbon cage, the resultant -type EMFs 

Examples of isolated di-metallofullerenes are fewer than the corresponding mono-EMFs. The most accessible di-metallofullerenes are M2 Cso (M = La, Ce, Pr, and so on) in which six electrons are transferred from the two metal atoms (each denotes three) to the carbon cage [116]. Recent examples are M2 C2n (M = La, Ce 2n = 72, 78) [90, 117, 118], in which the metals also take the -1-3 valency. To the best of our knowledge, no examples of di-metallofullerenes containing divalent metals have been recognized unambiguously. 

It is significant that the electronic state of Sc is variable, and sometimes controversial. Both experimental and theoretical results recommend the divalent state for Sc in mono-metallofullerenes and di-metallofullerenes [ 119], but it was determined that the three Sc atoms in Sc3N Cso take the - -3 valency. Variable electronic states of Tm in different EMFs were also found. For example, Tm was found to be -1-2 in Tm Cs2, but -1-3 in Tm3N Cso [120]. It is worth noting that a purely ionic picture is not valid to describe the electronic structure of the encaged metals because the charges are not observable quantities [2]. 

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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