| Summary: | This thesis is concerned with the electronic absorption spectroscopy and photochemical relaxation mechanisms of binary metal carbonyl complexes. These paradigm complexes exhibit a wide range of photoinduced vibronic coupling related phenomena that are only recently beginning to be understood with the development of modern experimental and computational techniques. These experiments have shown that after irradiation using ultrafast (femtosecond) laser pulses an unsaturated photoproduct is produced, and possibly relaxes through a conical intersection at a Jahn-Teller active geometry, on the same ultrafast timescale. However while experiment can imply the presence of conical intersection, only theoretical methods can confirm this and accurately probe the appropriate part of the potential energy surfaces relevant to this mechanism. The accurate assignment of the electronic excited states of these carbonyls is also a matter of debate with different theoretical and experimental techniques applied to these systems over the years. The large density of excited states of different character within a small energy range and the high computational expense of studying transition metal complexes with highly correlated methods presents a considerable challenge to the theoretical chemist. The research presented in this thesis falls into two main parts, firstly the electronic excited states of the binary transition metal carbonyl complexes Fe(CO)5, Cr(CO)6 and Ni(CO)4 were studied with highly correlated coupled cluster methods as well as their one-photon and two-photon absorption spectra. These results were compared with previous experimental and theoretical results. The electronic excited states and one-photon absorption spectra were also studied for the group 7 mixed-metal bimetallic carbonyls (MnTc(CO)10, MnRe(CO)10 and TcRe(CO)10) for the first time with time-dependent density functional theory (TD-DFT), the ability of TD-DFT methods to describe charge-transfer states was also investigated here. The second part of this thesis focussed on the relaxation pathways of the 2Mn(CO)5 and 1Fe(CO)4 initial photoproducts of the photodissociation of Mn2(CO)10 and Fe(CO)5 respectively using CASSCF. Both were found to relax to their lowest energy state through a Jahn-Teller induced conical intersection at a Jahn-Teller active geometry in agreement with experimental observation.
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