The versatility of N-heterocyclic carbenes (NHCs) is demonstrated by numerous practical applications in homogeneous transition metal catalysis, organocatalysis and materials science. There remains a paucity of electropositive metal NHC complexes and so this chemistry is poorly developed with respect to that of the late transition metal and main group elements. This thesis describes the synthesis of new alkoxy-tethered NHC proligands, their use in the synthesis of reactive metal amide and metal alkyl complexes, and finally small molecule activation using these complexes. Chapter One introduces NHCs and discusses their use as supporting ligands for early transition metal and f-block complexes. Small molecule activation using organometallic complexes is examined alongside the use of electropositive metal NHC complexes in catalysis. Chapter Two contains the synthesis and characterisation of new alkoxy-tethered NHC proligands and a variety of electropositive MII (M = Mg and Zn), MIII (M = Y, Sc, Ce and U) and MIV (M = Ce and U) amide complexes. X-ray diffraction studies and a DFT study are used to probe the extent of covalency in the bonding of the MIV complexes. Chapter Three investigates the reactivity of the amide complexes prepared in Chapter Two. The MII complexes are shown to be initiators for the polymerisation of raclactide into biodegradable polymers. The MIII complexes are used to demonstrate additionelimination reactivity of polar substrates across the M-Ccarbene bond which allows the formation of new N-E (E = Si, Sn, P or B) bonds. Treatment of the UIII silylamide complex U(N{SiMe3}2)3 with CO results in the reductive coupling and homologation of CO to form an ynediolate core -OC≡CO- and the first example of subsequent reactivity of the ynediolate group. The MIV complexes are used to examine the potential for forming MIV cationic species and alkyl complexes. Chapter Four examines the synthesis of MIII (M = Ce and Sc) aminobenzyl complexes and MIII (M = Y, Sc and U) neosilyl and neopentyl alkyl complexes. The addition-elimination reactivity discussed in Chapter Three is extended to include C-E bond formation (E = Si, Sn, P, B, I or C). Chapter Five provides overall conclusions to the work presented within this thesis. Chapter Six gives experimental and characterising data for all complexes and reactions in this work.

This thesis describes the synthesis, characterisation and reactivity of new manganese and rhenium(I) NHC complexes, which have been investigated both thermally and photochemically and the results contrasted with existing phosphine analogues in the literature. Cp’Mn(CO)2(NHC) (NHC = IEt2Me2 1, IMes 2, IiPr2Me2 3 and IPr 4) were synthesised and investigated by TRIR spectroscopy. Loss of CO was observed after 355 nm irradiation to form agostically stabilised intermediates, which reformed the parent species by recombination with CO on the nanosecond timescale. Loss of NHC was not observed, in contrast to Cp’Mn(CO)2(PPh3) which lost both CO and PPh3 upon photolysis. [Re(NHC)(Bpy)(CO)3]BAr4F (NHC = IEt2Me2 5, IMes 6) were synthesised and investigated by TRIR spectroscopy and UV/Vis absorption and emission spectrometry. Inclusion of an NHC altered the excited state manifold of the complexes, favouring population of the 3MLCT over the 3IL excited state. The lowest energy excited state for both 5 and 6 proved to be a 3MLCT excited state at 298 and 77 K. In contrast, [Re(PPh3)(Bpy)(CO)3]BAr4F exhibited 3MLCT at 298 K, but 3IL at 77 K. A series of complexes, M(NHC)(CO)4X and M(NHC)2(CO)3X (M = Re, X = Cl; M = Mn, X = Br) formed upon reaction of the corresponding M(CO)5X species and free NHC. The substitution pattern was dictated by the steric bulk of the NHC. Generation of the corresponding cations by halide abstraction was investigated. M(NHC)2(CO)3X was found to form agostic stabilised species upon halide abstraction by NaBAr4F in CH2Cl2. Under the same conditions, Re(IPr)(CO)4Cl was found to form the dichloromethane complex, [Re(IPr)(CO)4(η1-CH2Cl2)]BAr4F. In C6H5F solution under an atmosphere of dihydrogen, the CH2Cl2 ligand could be displaced by H2 to form the dihydrogen species, [Re(IPr)(CO)4(H2)]BAr4F.

N-heterocyclic carbenes (NHCs) can be used as ligands for organometallics complexes, which can then facilitate numerous catalytic applications, such as, C-H activation, small molecule activation and numerous materials applications. The use of anionically-tethered NHCs for usage with electropositive metals has been pioneered by the Arnold group within the last decade. This thesis describes the synthesis of both aryloxide- and amide-tethered NHC organometallic complexes of s-, p-, d- and f-block metals to provide a platform for small molecule activation. Once synthesised, the reactivity of some of these complexes were tested by reaction with CO2 with the aim of turning a molecule considered a harmful (environmentally), waste product into value added products, potentially providing an alternative fuel source. Chapter One introduces the use of anionically-tethered NHCs for use in a number of organometallic complexes as well as their current potential as catalysts for a number of important small molecules. This chapter focuses upon the differences between complexes tethered with anionic O, N, P, S elements, f-element NHC complexes and the use of d-block NHC complexes for catalysis. Chapter Two contains the synthesis and characterisation of a number of aryloxy-tethered NHC p-, d- and f-block organometallic complexes using the ligand H2(LArO R)2. The synthesis of SnII complexes including the synthesis of new ‘normal’ ‘abnormal’ complexes given enough steric bulk around the Sn centre due to the lone pair present in Sn complexes, preventing one of the ligands binding through the classical carbene position and therefore binding through the backbone C4 carbon. The synthesis of MII (Zn, Co and Fe) complexes to compare the solid-state structure and binding mode of the carbenes. The synthesis and characterisation of MIII (Ce and Eu) complexes to assess the solid-state structure and binding modes within f-bock complexes. Chapter Three investigates the reactivity of the MII complexes (Sn, Zn, and Fe) with CO2. Successful reactions were characterised using NMR and further treated with alkynes to target catalytic reactions. Chapter Four contains reactions to target a number of amide-tethered bis (NHC) s-, p-, d- and f-block organometallic complexes using the proligand, H4(LN Mes)Cl3. Deprotonation studies undertaken with a number of bases to give the MI (Li and K) salts and MII (Mg) salts and proved to be unsuccessful upon isolation. Reactions to synthesise the p-, d- and f-block complexes were then undertaken using in situ free carbene production as well as the attempted isolation of the free carbene, both of which also proved unsuccessful. Chapter Five provides an overall conclusion to the work presented in Chapters Two, Three and Four within this thesis. Chapter Six gives the experimental and characterising data for the complexes and reactions.

2-Phosphanyl-substituted imidazolium salts 2-PR2(4,5-Cl-Im)[OTf] (9a,b[OTf]) (4,5-Cl-Im = 4,5-dichloro-1,3-bis(2,6-di-isopropylphenyl)-imidazolium) (a: R = Cy, b: R = Ph) are prepared from the reaction of R2PCl (R = Cy, Ph) with NHC 8 (4,5-dichloro-1,3-bis(2,6-di-isopropylphenyl)-imidazolin-2-ylidene) in the presence of Me3SiOTf. 5-Phospanyl-substituted imidazolium salts 5-PR2(2,4-Cl-Im)[OTf] (10a,b[OTf]) are obtained in quantitative yield when a slight excess of the NHC 8 is used. 5-Phosphonio-substituted imidazolium salts 5-PR2Me(2,4-Cl-Im)[OTf]2 (14a,b[OTf]2) and 5-PR2F(2,4-Cl-Im)[OTf]2 (16a,b[OTf]2) result from methylation reaction or oxidation of 10a,b[OTf] with XeF2 and subsequent fluoride abstraction. According to our quantum chemical studies the Cl1 atom at the 2-position at the imidazolium ring of dication 14b2+ carries a slightly positive charge and is therefore accessible for nucleophilic attack. Accordingly, the reaction of 14a,b[OTf]2 and 16a,b[OTf]2 with R3P (R = Cy, Ph) affords cationic 5-phosphonio-substituted NHCs 5-PR2Me(4-Cl-NHC)[OTf] (17a,b[OTf]) and 5-PR2F(4-Cl-NHC)[OTf] (18a,b[OTf]) via a SN2(Cl)-type reaction. A series of transition metal complexes such as [AuCl(5-PPh2Me(4-Cl-NHC))][OTf] (19[OTf]), [CuBr(5-PPh2Me(4-Cl-NHC))][OTf] (20[OTf]), [AuCl(5-PPh2F(4-Cl-NHC))[OTf] (21[OTf]) and [RhCl(cod)(5-PPh2Me(4-Cl-NHC))][OTf] (23[OTf]) are prepared to prove the coordination abilities of carbenes 17b[OTf] and 18b[OTf]. The isolation of a rare example of a tricationic bis-carbene silver complex [Ag(5-PPh2Me(4-Cl-NHC))2][OTf]3 (22[OTf]3) is achieved by reacting 14b[OTf] with Cy3P in the presence of AgOTf. NHC 17b[OTf] represents a very effective dehydrocoupling reagent for secondary (R2PH, R = Ph, Cy, iBu) and primary (RPH2, R = Ph, Cy) phosphanes to give diphosphanes of type R4P2 (R = Ph, Cy, iBu) and oligophosphanes R4P4, R5P5 (R = Ph, Cy), respectively. Methylation of 17b+ and subsequent deprotonation reaction with LDA affords the cationic NHO (N-heterocyclic olefin) 35+ of which the gold complex 36+ is readily accessible via the reaction with AuCl(tht).

2-Phosphanyl-substituted imidazolium salts 2-PR2(4,5-Cl-Im)[OTf] (9a,b[OTf]) (4,5-Cl-Im = 4,5-dichloro-1,3-bis(2,6-di-isopropylphenyl)-imidazolium) (a: R = Cy, b: R = Ph) are prepared from the reaction of R2PCl (R = Cy, Ph) with NHC 8 (4,5-dichloro-1,3-bis(2,6-di-isopropylphenyl)-imidazolin-2-ylidene) in the presence of Me3SiOTf. 5-Phospanyl-substituted imidazolium salts 5-PR2(2,4-Cl-Im)[OTf] (10a,b[OTf]) are obtained in quantitative yield when a slight excess of the NHC 8 is used. 5-Phosphonio-substituted imidazolium salts 5-PR2Me(2,4-Cl-Im)[OTf]2 (14a,b[OTf]2) and 5-PR2F(2,4-Cl-Im)[OTf]2 (16a,b[OTf]2) result from methylation reaction or oxidation of 10a,b[OTf] with XeF2 and subsequent fluoride abstraction. According to our quantum chemical studies the Cl1 atom at the 2-position at the imidazolium ring of dication 14b2+ carries a slightly positive charge and is therefore accessible for nucleophilic attack. Accordingly, the reaction of 14a,b[OTf]2 and 16a,b[OTf]2 with R3P (R = Cy, Ph) affords cationic 5-phosphonio-substituted NHCs 5-PR2Me(4-Cl-NHC)[OTf] (17a,b[OTf]) and 5-PR2F(4-Cl-NHC)[OTf] (18a,b[OTf]) via a SN2(Cl)-type reaction. A series of transition metal complexes such as [AuCl(5-PPh2Me(4-Cl-NHC))][OTf] (19[OTf]), [CuBr(5-PPh2Me(4-Cl-NHC))][OTf] (20[OTf]), [AuCl(5-PPh2F(4-Cl-NHC))[OTf] (21[OTf]) and [RhCl(cod)(5-PPh2Me(4-Cl-NHC))][OTf] (23[OTf]) are prepared to prove the coordination abilities of carbenes 17b[OTf] and 18b[OTf]. The isolation of a rare example of a tricationic bis-carbene silver complex [Ag(5-PPh2Me(4-Cl-NHC))2][OTf]3 (22[OTf]3) is achieved by reacting 14b[OTf] with Cy3P in the presence of AgOTf. NHC 17b[OTf] represents a very effective dehydrocoupling reagent for secondary (R2PH, R = Ph, Cy, iBu) and primary (RPH2, R = Ph, Cy) phosphanes to give diphosphanes of type R4P2 (R = Ph, Cy, iBu) and oligophosphanes R4P4, R5P5 (R = Ph, Cy), respectively. Methylation of 17b+ and subsequent deprotonation reaction with LDA affords the cationic NHO (N-heterocyclic olefin) 35+ of which the gold complex 36+ is readily accessible via the reaction with AuCl(tht).