The interactions between main group elements and d- and f-block metals are important in many chemical applications. Calculated bond dissociation energies (BDE) and heats of formation of the Group 3 metal halide dimers (MX, where M = Sc, Y, La and X = F, Cl, Br, I) for the X1Σ+ and a3δ state were used to explain the chemiluminescent reactions of the Group 3 metals Sc and Y with F2, Cl2, Br2, ClF, ICl (Sc), IBr (Y) and SF6 and La with F2, SF6, Cl2, and ClF and show that the observed spectra are due to metal monohalide emission. The BDE calculations were performed using the Feller-Peterson-Dixon (FPD) approach including molecular spin-orbit (SO) corrections. The initial steps in the selective catalytic reduction (SCR) of NO by TiO2 supported vanadium oxides and surface adsorbed NH3 were predicted at the density functional theory (DFT) level with the B3LYP functional benchmarked at the coupled cluster CCSD(T) level. Different proton transfer pathways which depend on the initial neutral or protonated sites coupled with addition of NO lead to formation of a NH2NO surface species and reduction of a vanadium and spin transfer to the metal oxide surface. NH2NO subsequently desorbs and decomposes in the gas phase following a series of intramolecular rearrangements with barriers comparable to its generation on the surface. Spectroscopic observation of surface adsorbed NH3 and NH4+ in the SCR reaction is supported by vibrational frequency calculations. However vanadium bound NH2 is not predicted to be present in significant amounts, consistent with experiment. NO2 may also be present in combustion gas streams with the NO so the interactions of NO2 with cluster models of vanadium oxides and supported vanadium oxides were studied at the CCSD(T)//B3LYP level. A qualitative covalent vs, ionic bonding model is further developed, and the key factors in the favorability of nitrate formation on vanandium oxides are proposed. Calculations predict that saturating the vanadia with V-O bonds strengthens the V=O bond, raises the excitation energy, and precludes NO2 chemisorption so that only weak physisorption will occur. A series of organic ligands is investigated for metal selectivity in the separation of actinides (An) from lanthanides (Ln) at the DFT level couples with self-consistent reaction field calculations using the COSMO parameters to predict free energies in aqueous and organic solvents. A novel ligand to metal charge transfer in Eu complexes is predicted.