This dissertation, "Ab Initio Relativistic-consistent Calculations and Charge Density and Experimental Mass-spectroscopic Analysis of Mono and Poly-nuclearclusters of Group 11 and 12 Transition Metals and Metal Chlorides: YSeyedabdolreza Sadjadi." by Seyedabdolreza, Sadjadi, was obtained from The University of Hong Kong (Pokfulam, Hong Kong) and is being sold pursuant to Creative Commons: Attribution 3.0 Hong Kong License. The content of this dissertation has not been altered in any way. We have altered the formatting in order to facilitate the ease of printing and reading of the dissertation. All rights not granted by the above license are retained by the author. Abstract: The electron density function of molecular systems supplies a package of information. Quantum mechanical methods of producing and analyzing this function have been significantly improved during the past few years. The advent of accurate pseudopotentials and corresponding basis sets for Kohn-Sham density functional and for post-Hartree-Fock electron-correlated approaches have enabled the inclusion of scalar relativistic and spin-orbit coupling effects as well as electron correlation effects into the electron density function. The unpacking of the information embedded in such a function via the quantum theory of atoms in molecules (QTAIM) became possible by utilizing the very new subshell fitting method of reconstructing the density distribution of core electrons that had been replaced by the pseudopotentials. These theoretical advances were applied in this thesis to characterize and explore the topological features of metal-metal bonding as one of the fundamental types of bonds formed between two elements. Group 11 and 12 transition metals which include gold and mercury as the most relativistic elements were the main focus of this work. Mono and poly-nuclear compounds (with up to 4 metal atoms) in both pure metal clusters and chloro-complexes were studied by ab initio MOller-Plesset perturbation calculations followed by QTAIM analysis on the relaxed density. Some of these chloro-complexes of copper, gold, zinc and cadmium metals were identified in the gas phase by mass spectrometric experiments. The general formulas of the set of molecules studied in group 11 were: M2, MCl, MCl+, MCl2, MCl2+, M2Cl+, M2Cl2 DEGREES(s+), M2Cl3+, M3Cl2+, M3Cl3+, M3Cl5+, M4Cl5+ and M4Cl7+ and in group 12 were: M2, MCl, MCl+, MCl2, M2Cl3+, M3Cl5+, M4Cl7+ and M2 DEGREES(s+). The topological features of metal-metal bonding were calculated along with atomic properties for each individual local minimum isomer found. The comparison ofthe metal-metal bonding within the complexes and with the dimers revealed newfeatures of metal-metal bonding in 3d, 4d and 5d transition metal elements ofgroups 11 and 12. With the aid of strong correlation between bond dissociationenergy and electron density at the location of the bond critical points found in thecase of dimers, the strength of the metal-metal bonding in the complexes wasestimated. The electron density's basin properties calculated accurately for all theclusters and their isomers in this thesis provided more insight also into the natureof M-Cl bondings in the group 11 and 12 chloride clusters. Ultimately the bondinginformation was used to predict the viability of these clusters in the gas phase. DOI: 10.5353/th_b5060577 Subjects: Metal-metal bondsTransition metalsChlorides