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Because of the great biological and chemical importance, porphyrins and their analogues have received extraordinary attention in the literature. As such, it becomes particularly important to develop a theoretical understanding of the complex mechanisms of these biological systems. The commonly used Density Functional Theory (DFT) method has been shown to be afflicted with uncertainties/errors for describing this type of interaction. Multi-configurational ab initio methods: CASSCF/CASPT2 and RASSCF/RASPT2, were used in this work to obtain a more accurate description of the bonding and electronic structures. Transition metal (TM) complexes are formed by coordinating ligands to the central transition metal atom. Porphyrins, naturally occurring macrocyclic compounds, play a very important role in the metabolism of living organisms. The coordination chemistry of two porphyrin analogues, namely, corroles and corrolazines, have shown unique and intriguing behaviors that are clearly distinguishable from those of porphyrin. In order to understand the biological activity of natural compounds, the interactions between first row TM and the macrocyclic ligands have been extensively studied in the literature. (Chapter 1) In quantum chemistry, the Schrödinger equation for molecular chemical systems cannot be solved exactly. Several methods have been designed to approximate the exact solutions of this equation. The Hartree-Fock (HF) method approximates the exact, many-body wave function by a single Slater determinant, but it neglects electron correlation. In CASPT2 and RASPT2, the non-dynamical correlation, important to provide an accurate description of degenerate or nearly degenerate states, is included in a multi-configurational SCF wave function, whereas the dynamical electron correlation, associated with the movement of electrons, is obtained by means of second order perturbation theory. DFT can be considered as another way to improve on HF theory, where electron correlation is modeled by a functional of the electron density. The main disadvantage of DFT is that there is no systematic approach for improving the results towards the exact solution. In case of a spin unrestricted DFT solution, there will be some error, because the wave function is no longer an eigenfunction of the total spin. This error is called spin contamination. Furthermore, calculations of molecular properties are susceptible to basis set superposition errors (BSSE). The Counterpoise method (CP) is an approximate method for estimating the size of the BSSE. (Chapter 2) In Chapter 3, we have focused on the study of the spin-state energetics for three small FeII and FeIII heme models as well as two large models: FeP(SH), a model of the active site of cytochrome P450 in its resting state and FeP(Im), a model of the active site of myoglobin. By comparing with the recently available coupled cluster results, the CASPT2 calculations were found to be very accurate for the studied FeIII complexes, but there is a strong indication of a systematic error (around 5 kcal/mol) in favor of the high-spin state for the studied FeII complexes. With DFT, the overall performance of the recently developed M06 and M06-L functionals is not any better than that of the traditional functionals, like B3LYP or OLYP. Moreover, none of the tested density functionals consistently provides a better accuracy than CASPT2.In the next part (Chapter 4), we have presented a comparative study of the electronic and geometric structure of copper corroles. The effect of saddling on the electronic structure was investigated by comparing the results obtained for planar (C2v) and saddled (C2) structures. With DFT, the origin of the saddling distortion is found to be dependent on the applied functional: covalent Cu 3d-corrole σ interactions with pure functionals (BP86, OLYP), antiferromagnetic exchange coupling between an electron in the corrolate (C2) b type π orbital, and an unpaired CuII 3d electron with hybrid functionals (B3LYP, PBE0). The CASPT2 results essentially confirm the suggestion from the hybrid functionals that copper corroles are noninnocent, although the contribution of diradical character to the copper-corrole bond is found to be limited to 50% or less. The lowest triplet state is calculated at 0-10 kcal/mol, conform with the experimental observation that this state should be thermally accessible. Because of the size limitations of the CAS space, RAS may provide a considerable and valuable improvement. In Chapter 5 a systematic study was carried out on a series of model transition-metal complexes to give an overview of the possibilities and limitations of the RASSCF/RASPT2 method. Highly accurate RASPT2 results were obtained for the heterolytic binding energy of ferrocene and for the electronic spectrum of Cr(CO)6. For ferrous porphyrin the intermediate spin 3A2g ground state is correctly predicted by including the (3s,3p)-3d inter-shell correlation effects. The high magnetic moment associated with this triplet state is correctly reproduced after introducing spin orbit coupling with the close-lying 5A1g and 3Eg states. For cobalt corrole the largest active space comprises the entire set of corrolate π orbitals. However, an active space with 33 orbitals, combining an optimal RAS2 with RASPT2(SDT), is computationally unfeasible. This increases the uncertainty of the RASPT2 results by 0.1-0.2 eV. Nevertheless, the RASPT2 calculations give a considerable improvement for the splitting between the a2 and b2 radicals on the corrolate ligand. With the success of the above described method RASSCF/RASPT2, more calculations were performed on manganese(V)-oxo corrole/corrolazine (Chapter 6). The calculations confirm the expected singlet ddelta2 ground states for both complexes. The lowest excited states are a pair of Mn(V) triplet states with ddelta1dπ2 configurations 0.5-0.75 eV above the ground state. The Mn(IV)O-(Cor/Cz)●2- radical states are much higher in energy, ≥1.0 eV relative to the ground state. The macrocyclic ligands in the ground states of the complexes are thus unambiguously “innocent”. In Chapter 7, time dependent IR measurements indicate that two Mn(TPP)(NO) isomers with either a linear or a bent Mn-N-O geometry may be obtained by the reaction of NO with amorphous Mn(TPP) layers. Although the results obtained from DFT were strongly functional dependent, the linear (S = 0) and bent (S = 1) structures were found to be close-lying at all levels of theory. The pure GGA functionals BP86 and TPSS tend to overstabilize the S = 0 with respect to the S = 1 state. B97D performs better, predicting the S = 0 and S = 1 states very close in energy. On the other hand, the hybrid functional B3LYP seems to significantly overstabilize the triplet state. CASPT2 also predicts the S = 1 state slightly lower in energy (-1.5 kcal/mol).