Correlated ab initio approaches to metal-metal bonding in the tricobalt extended metal atom chains
Keywords: MCSCF
X-ray Structure
Coodination Complexes
EMAC
Transition Metal Complexes
DFT
Polymorphism
Extended Metal Atom Chains
Transition Metal
Spin Crossover
Organometallic
CASSCF
CASPT2

The extended metal atom chains are a family of complexes with a linear, metallic chain surrounded by four equatorial polydentate ligands and capped by two axial ligands. The tricobalt extended metal atom chains studied in this work have a trimetallic core with halide axial ligands: the fluoride, chloride, bromide and iodide complexes. In this work we adopt correlated ab initio approaches (DFT and MCSCF) to the study of the metal-metal bonding between the cobalt atoms in the chain.
The literature surrounding the tricobalt series is extensive, reflecting the vast array of experimentally observed phenomena. X-ray crystallography has been used to determine crystal structures at a range of temperatures, and has shown the structure of the tricobalt chain can vary continuously between two extremes: symmetric with equal Co- Co bond lengths and unsymmetric with one Co-Co bond markedly lengthened. Magnetic measurements show an increase in temperature initiates a spin crossover in all known instances of the complex, in both the solution- and crystalline-phase. The data reported in the literature is consistent with a low-spin doublet ground state at low temperatures, however resolution of the high-spin state multiplicity is complicated by a lack of saturation in the effective magnetic moments at the highest available temperatures. In Chapter 1 we present a detailed review of the entire literature regarding the tricobalt extended metal chains such that the reader can put into context the entire range of remarkable behaviours. In Chapter 2 we discuss the theoretical techniques underpinning this thesis, while Chapter 3 describes the specific computational methodologies used.
In Chapter 4 we undertake a comprehensive and systematic study of the symmetric form of the bromide complex. We detail electronic configurations of potential low-lying excited states that arise from the low-energy excitations within the ground state. The properties of the ground and excited states are used to explain the experimentally observed spin crossover and structural temperature dependence. We identify a symmetric sextet state, the thermal population of which is found to be consistent with the behaviour of those crystals that remain symmetric in their observed temperature ranges. In Chapter 5 we adopt the same systematic approach to study the unsymmetric form, again focusing on the bromide complex, and identify a quartet state that fits the behaviours seen in those crystals that show a transition between the symmetric and unsymmetric limits in their observed temperature ranges. In Chapter 6 we describe the electronic mechanisms by which the tricobalt bromide system makes this transition, examining low energy distortions in the symmetric low-spin state structures and the effect of the high-spin state population on the X-ray crystal structure. In Chapter 7 we extend the analysis developed in the first three chapters to study the complexes with the other axial halide ligands. Our results agree with the experimental findings of a decreasing spin crossover temperature going down the group VII halides F > Cl > Br > I, and we present a re-evaluation of the link between the experimental structures and the spin crossover process.