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A series of cationic Rh(I) diphosphine and phosphine-alkene complexes have been isolated and fully characterised. The reactivity of these species towards hydrogenation, dehydrogenation and dehydrocoupling reactions has been investigated. The use of potentially hemilabile ligands DPEphos and XANTphos in the intramolecular dehydrogenation chemistry of tricyclopentylphosphine is reported. The comparison in reactivity of these isolated diphosphine phosphine-alkene complexes towards hydrogenation and with acetonitrile is discussed along with their ability to dehydrocouple secondary silane, Ph₂SiH₂, and amine-borane H₃B·NMe₂H. The acceptorless dehydrogenation of a tethered cyclopentane with cationic Rh(I) diphosphine complexes has also been extended to include thioethers. Isolated cationic Rh(I) phosphine-alkene complexes with labile fluorobenzene ligands are found to act as a source of the reactive 12-electron [Rh{PR₂(ƞ²-C₅H₇)}]+ (R = cyclopentyl (Cyp)/ iPr) fragment in solution and can coordinate two amine-borane ligands (either H₃B·NMe₃, H₃B·NMe₂H or H₃B·NMeH₂) in a novel and unique bis-σ-binding mode. The catalytic activity of some of these isolated complexes in the dehydrocoupling of H₃B·NMe₂H and H₃B·NMeH₂ has been determined. With a view to further understanding the mechanism of catalytic transition metal assisted amine-borane dehydrogenation and dehydrocoupling, known B-N intermediates H₃B·NMe₂BH₂·NMe₂H and [H₂B·NMeH]₃ were also coordinated to the [Rh{PCyp₂(ƞ²-C₅H₇)}]+ fragment and investigated with regard to their role in the catalytic cycle. Structure activity relationships determined from stoichiometric reactions of cationic Rh(I) diphosphine fluorobenzene complexes with amine-boranes enabled the design of a highly efficient homogeneous catalyst capable of dehydrogenating H₃B·NMe₂H to [H₂BNMe₂]₂ at 0.2 mol% loading in 30 minutes at 298 K. Rapid dehydrogenation and dehydrocoupling of H₃B·NMeH₂ to form high molecular weight poly(N-methylaminoborane) with a low PDI has also been achieved. Investigations using model aminoborane H₂B=NiPr₂ and intermediate B-N species H₃B·NMe₂BH₂·NMe₂H and [H₂B·NMeH]₃ has helped establish an overall mechanistic rationale for this process.

This Thesis describes the synthesis and characterisation of new Main Group and Rare Earth alkyl, amide, alkoxide and borohydride complexes and their use as catalysts for the ring-opening polymerisation (ROP) of ε-caprolactone and rac-lactide. Chapter 1 introduces ROP from an industrial and academic perspective, as well as polymer characterisation techniques. A literature review is given, with an emphasis placed on Main Group catalysts. Chapter 2 describes the synthesis and characterisation of new homo- and hetero-scorpionate Main Group complexes. An introduction to homo- and hetero-scorpionate ligands is given, as well as a discussion of the ε-caprolactone and rac-lactide ROP activity displayed by the new complexes. Chapter 3 describes the synthesis and characterisation of new neutral and cationic Main Group borohydride complexes supported by the tris(pyrazolyl)methane and tris(pyrazolyl)hydroborate ligands. A review of borohydride complexes is also given. The ε-caprolactone and rac-lactide ROP activity shown by the complexes presented is also discussed. Chapter 4 describes the synthesis and characterisation of new mono- and di-cationic yttrium complexes supported by the tris(pyrazolyl)methane and triazacyclononane ligands. An introduction to the synthesis of neutral and cationic Rare Earth complexes is given. An overview of immortal ROP is also provided. The activity of the new complexes towards the immortal ROP of rac-lactide is also discussed. Chapter 5 contains experimental details and characterising data for the new complexes reported in this thesis. CD Appendix contains .cif files for all of the new crystallographically characterised complexes.

A range of spectroscopic and computational techniques have been applied to the study of three metal cluster systems: vanadium monoxide, the Au2 molecule, and RhnN2O+ clusters. A new instrument has been built for spectroscopy experiments on metal clusters, consisting of a laser ablation cluster source and a linear time-of-flight mass spectrometer. The instrument was characterised using nitric oxide spectroscopy and applied to the electronic REMPI spectrum of vanadium oxide in the visible region. The rotational constants and band origins of several known states have been determined, and the observation of a new spin-forbidden transition has been used to connect the energies of the quartet and doublet manifolds of VO. A new 3 2Π state was also observed and characterised. The photoionisation and photodissociation of Au2 were then studied at 157 nm, and between 35500 and 37200 cm-1 with another new instrument recently constructed in the group. Excited and ground state Au photofragments were produced in both spectral regions, and have been detected and assigned using velocity map imaging. The 157 nm photodissociation produced gold atom products in the seven highest-energy accessible channels in a single-photon dissociation process. The complex near-UV spectrum involved two-photon excitation to two 0g+ excited states close to their dissociation thresholds, followed by predissociation to thirteen different Au product channels. The branching ratios for dissociation into each of these channels varied across the spectrum as different dissociation limits and curve crossings occurred. The mid- and far-infrared multiple-photon dissociation spectra of RhnN2O+ clusters have been recorded using the argon-tagging action spectroscopy technique and free electron laser radiation. The results have been compared to density functional theory calculations to deduce the nature of the binding and the likely low-lying electronic and geometrical structures. The N2O was found to be molecularly adsorbed on the surface of the cluster but, upon infrared heating of the complex via the N2O vibrational modes, was observed to undergo a reaction, producing N2 and cluster oxides RhnO+. The reaction is believed to be thermal and mode-independent, but the efficiency of the surface reaction does vary with cluster size, with the n=5 cluster showing no detectable reaction.

The synthesis and properties of metal phosphonates prepared using piperazine-based bisphosphonic acids have been investigated. The ligands N,N’-piperazinebis(methylenephosphonic acid) (H₄L), and the 2-methyl (H₄L-Me) and 2,5-dimethyl (H₄L 2,5-diMe) derivatives have been prepared using a modified Mannich reaction. Hydrothermal reaction of gels prepared from metal (II) acetates and the bisphosphonic acids results in the synthesis of four structures: STA-12, Ni VSB-5, Co H₂L.H₂O and Mg H₂L. STA-12, synthesised by reaction of Mn, Fe, Co or Ni acetate with H₄L or H₄L-Me, has been investigated further. STA-12 crystallises in the space group R⁻₃, and Ni STA-12 is the most crystalline version. Its structure was solved from synchrotron data (a = b = 27.8342(1) Å, c = 6.2421(3) Å, α = β = 90°, γ = 120°), and it has large 10 Å hexagonal shaped pores. Helical chains of Ni octahedra are coordinated by the ligands, resulting in phosphonate tetrahedra pointing towards the pore space. Water is present, both coordinated to the Ni²⁺ cations and physically adsorbed in the pores. Mixed metal structures based on Ni STA-12, where some Ni is replaced in the gel by another divalent metal (Mg, Mn, Fe or Co) can also be synthesised. Dehydration of STA-12 results in two types of behaviour, depending on the metal present. Rhombohedral symmetry is retained on dehydration of Mn and Fe STA-12, the a cell parameter decreasing compared to the as-prepared structures by 2.42 Å and 1.64 Å respectively. Structure solution of dehydrated Mn STA-12 indicates changes in the torsion angles of the piperazine ring bring the inorganic chains closer together. Fe and Mn STA-12 do not adsorb N₂, which is thought to be due to the formation of an amorphous surface layer. Dehydration of Ni and Co STA-12 causes crystallographic distortion. Three phases were isolated for Ni STA-12: removal of physically adsorbed water results in retention of rhombohedral symmetry, while dehydration at 323 K removes some coordinated water forming a triclinic structure. A fully dehydrated structure (dehydrated at 423 K) was solved from synchrotron data (a = 6.03475(5) Å, b = 14.9156(2) Å, c = 16.1572(7) Å, α = 112.5721(7)°, β = 95.7025(11)°, γ = 96.4950(11)°). The dehydration mechanism, followed by UV-vis and Infra-red spectroscopy, involves removal of water from the Ni²⁺ cations and full coordination of two out of three of the phosphonate tetrahedra forming three crystallographically distinct Ni and P atoms. No structural distortion takes place on dehydration of Ni and Co STA-12 prepared using the methylated bisphosphonate, and the solids give a higher N₂ uptake as a result. Dehydrated Ni and Co STA-12 were tested for adsorption performance for fuel related gases and probe molecules. Investigations were undertaken at low temperature with H₂, CO and CO₂, and ambient temperature with CO₂, CH₄, CH₃CN, CH₃OH and large hydrocarbons. Due to the presence of lower crystallinity, Co STA-12 has an inferior adsorption performance to Ni STA-12, although it has similar adsorption enthalpies for CO₂ at ambient temperature (-30 to -35 kJ mol⁻¹). Ni STA-12 adsorbs similar amounts of CO₂ and N₂ at low temperature, indicating the adsorption mechanisms are similar. Also, it adsorbs 10 × more CO₂ than CH₄ at low pressure, meaning it could be used for separation applications. Ni STA-12 adsorbs 2 mmol g⁻¹ H₂ with an enthalpy of -7.5 kJ mol⁻¹, the uptake being due to adsorption on only one-third of the Ni²⁺ cations. The uptake for CO is 6 mmol g⁻¹, with adsorption enthalpies ranging from -24 to -14 kJ mol⁻¹. This uptake is due to adsorption on all the Ni²⁺, meaning the adsorption enthalpies are high enough to allow the structure to relax. This is also observed for adsorption of CH₃CN and CH₃OH, where there is a return to rhombohedral symmetry after uptake. The adsorption sites in dehydrated Ni and Co STA-12 were investigated via Infra-red spectroscopic analysis of adsorbed probe molecules (H₂, CO, CO₂, CH₃CN and CH₃OH). The results indicate the adsorption sites at both low and ambient temperature are the metal cations and the P=O groups. The metal cation sites are also characterised as Lewis acids with reasonable strength. STA-12 was shown to have acidic activity for the liquid phase selective oxidations of 1-hexene and cyclohexene, although there is evidence active sites are coordinated by products and/or solvents during the reaction. STA-12 also demonstrates basic activity for the Knoevenagel condensation of ethyl cyanoacetate and benzaldehyde. Modification of STA-12 by adsorption of diamine molecules causes a slight increase in the basicity, and the highest conversions are where water and diamine molecules are both present.