This thesis discusses the development of permethylindenyl constrained geometry complexes (CGCs) as highly efficient slurry phase polymerisation catalysts capable of incorporating olefins at high and controllable levels and a niobium imido complex as an ethylene oligomerisation catalyst. Finally, a CGC and a niobium imido complex were combined on a single inorganic solid support with the aim of forming short chain branched linear low density polyethylene (LLDPE) from a single ethylene feedsource. Ethylene (co)polymerisation, oligomerisation and tandem catalysis are introduced, with discussion of previously reported examples of catalysis of these processes. The synthesis, characterisation and immobilisation on inorganic solid supports of a range of permethylindenyl based CGCs (^Me,R1SB(^tBuN,^3-RI*)MX₂ where (^Me,R1SB(^tBuN,^3-RI*) = {(3 R η⁵-C₉Me₅)(Me,R¹)Si(^tBuN)}^2–), ^Me₂SB(^RN,I*)TiCl₂ and ^Me₂SB(^tBuN,I*)Ti(Cl)CH₂SiMe₃) is discussed. Slurry phase ethylene polymerisation studies were undertaken to investigate the effect of the amido fragment, initiator group, ansa bridge, metal centre and permethylindenyl moiety. Factors including temperature, time, scale, catalyst loading, scavenger, pressure and inorganic solid support were all investigated. High throughput polymerisations were also undertaken for the most active catalysts, and hydrogen response also studied. Selected complexes were tested as ethylene/1 hexene, ethylene/1 octene and ethylene/styrene copolymerisation catalysts, with ethylene/1 hexene copolymerisations carried out under high throughput conditions and ^Me₂SB(^tBuN,I*)TiR₂ (R = Cl and CH₂Ph) tested on a large scale at SCG Chemicals Co., Ltd. (Rayong, Thailand). Solution copolymerisations were also carried out with ^Me₂SB(^tBuN,I*)TiCl₂,^Me₂SB(^tBuN,^3-EtI*)TiCl₂ and ^Me₂SB(^tBuN,Cp*)TiCl₂. Nb(NDipp)Cl₃.THF.pyr was synthesised and used as a slurry and solution phase oligomerisation catalyst, with work undertaken to optimise conditions for ethylene trimerisation. Finally, both Nb(NDipp)Cl₃.THF.pyr and ^Me₂SB(^tBuN,I*)TiCl₂ were simultaneously supported on solid polymethylaluminoxane (sMAO) for use in the tandem catalysis of ethylene to form short chain branched LLDPE from a single ethylene feedsource.