Bonding in endohedral metallofullerenes as studied by quantum theory of atoms in molecules.

Metal-cage and intracluster bonding was studied in detail by quantum theory of atoms in molecules (QTAIM) for the four major classes of endohedral metallofullerenes (EMFs), including monometallofullerenes Ca@C(72), La@C(72), M@C(82) (M=Ca, Sc, Y, La), dimetallofullerenes Sc(2)@C(76), Y(2)@C(82), Y(2)@C(79)N, La(2)@C(78), La(2)@C(80), metal nitride clusterfullerenes Sc(3)N@C(2n) (2n=68, 70, 78, 80), Y(3)N@C(2n) (2n=78, 80, 82, 84, 86, 88), La(3)N@C(2n) (2n=88, 92, 96), metal carbide clusterfullerenes Sc(2)C(2)@C(68), Sc(2)C(2)@C(82), Sc(2)C(2)@C(84), Ti(2)C(2)@C(78), Y(2)C(2)@C(82), Sc(3)C(2)@C(80), as well as Sc(3)CH@C(80) and Sc(4)O(x)@C(80) (x=2, 3), that is, 42 EMF molecules and ions in total. Analysis of the delocalization indices and bond critical point (BCP) indicators such as G(bcp)/rho(bcp), H(bcp)/rho(bcp), and |V(bcp)|/G(bcp), revealed that all types of bonding in EMFs exhibit a high degree of covalency, and the ionic model is reasonable only for the Ca-based EMFs. Metal-metal bonds with negative values of the electron-density Laplacian were found in Y(2)@C(82), Y(2)@C(79)N, Sc(4)O(2)@C(80), and anionic forms of La(2)@C(80). A delocalized nature of the metal-cage bonding results in a topological instability of the electron density in EMFs with an unpredictable number of metal-cage bond paths and large elipticity values.