Relativistic-Consistent Electron Densities of the Coinage Metal Clusters M2, M4 M42-, and M4Na2 (M = Cu, Ag, Au): A QTAIM Study.

We employ second-order Møller-Plesset perturbation theory level in combination with recently developed pseudopotential-based correlation consistent basis sets to obtain accurate relativistic-consistent electron densities for small coinage metal clusters. Using calculated electron densities, we employ Bader's quantum theory of atoms in molecules (QTALM) to gain insights into the nature of metal-metal bonding in the clusters M2, M4, M4,2-, and M4Na2 (M = Cu, Ag, Au). For the simplest case of the metal dimer, M2, we correlate the strength of the metal-metal bond with the value of the electron density at the bond critical point, the total energy density at the bond critical point, the sharing (derealization) index, and the values of die two principle negative curvatures. We then consider changes to the metal-metal bonding and charge density distribution upon the addition of two metal atoms to form the metal tetramer, M4 and then followed by the addition of an electron pair to form M42- and finally followed by die addition of two alkali metal (sodium) ions to form M4Na2. Using topological properties of die electron density, we present evidence for the existence of a-aromaticity in Au42- . We also report die existence of two non-nuclear attractors in the molecular in the molecular graph of Cu42- and large negative charge accumulation in the nonbonded Cu basins of this cluster. [ABSTRACT FROM AUTHOR]