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This thesis describes the development of nickel(I) complexes incorporating ring expanded N-heterocyclic carbene (RE NHC) ligands and examines their electronic characterisation, activation of O2, reactivity and catalytic applications. A series of three coordinate, paramagnetic Ni(I) complexes of the form Ni(RE NHC)(PPh3)Br (1 – 10) were prepared by comproportionation of Ni(COD)2 and Ni(PPh3)2Br2 in the presence of RE NHCs. The RE NHCs employed varied in the degree of ring expansion (6-, 7- and 8-membered), extent of N-substituent steric bulk (Mes, oTol, oAnis) and the donor/acceptor properties of the carbene (diamino vs. diamido). EPR and DFT electronic characterisation of 1 – 10 confirmed that the unpaired electron was located on the nickel ion in a mixed orbital of predominantly 3dz2 character. Yellow solutions of 1 or 6 (RE NHC = 6Mes and 7Mes respectively) immediately became purple in the presence of O2 due to O2 activation and incorporation of the oxygen atoms as bridging ligands resulting from C-H activation/oxygenation of an RE NHC N-substituent. This generated the dimeric Ni(II) complexes Ni(6/7Mes)Br(µ-OH)(µ-O-6/7Mes)’NiBr (6Mes = 13; 7Mes = 14). Mass spectrometry demonstrated that the doubly activated complexes [NiBr(µ-O-6/7Mes)’]2 (15 and 16 respectively) were also formed in the reactions. UV-vis spectroscopy revealed the reactions took place rapidly, even at 190 K. Contrasting reactivity was observed when 2 or 7, bearing the less sterically encumbered N-oTol substituents 6oTol and 7oTol respectively, were exposed to O2, which led to the ligand redistribution products NiII(6/7oTol)(PPh3)Br2 (17 and 18 respectively). The less electron rich diamido analogue containing 6MesDAC (5), underwent dissociation and oxidation of the RE NHC and PPh3 ligands. Attempts to abstract the bromide from 1 generated novel two and three coordinate Ni(I) products. Reaction with additional 6Mes produced the two coordinate cation [Ni(6Mes)2]+ (19), which could be reduced with KC8 to Ni(6Mes)2 (20). SQUID analysis of 19 revealed it to be the first example of a nickel containing mononuclear single molecular magnet (SMM). Addition of [Et3Si]+ to 1 followed by work up in toluene led to the isolation of the Ni(I)-(η2-toluene) complex [Ni(6Mes)(η2-C6H5CH3)]+ (21). Mesitylene generated the analogous [Ni(6Mes)(η2-C6H3(CH3)3)]+ (23), but neither 1,4-xylene nor naphthalene gave isolable products. In all cases, cocrystallisation of [6MesH]+…arene was observed in variable amounts, which compromised reaction studies of the Ni-arene complexes. Removal of bromide from 1 with TlPF6 in THF generated the solvent coordinated cationic species [Ni(6Mes)(PPh3)(THF)]+ (24). Attempts to remove the ligated THF molecule were unsuccessful, however, it could be directly substituted by CO to form [Ni(6Mes)(PPh3)(CO)]+ (26). Similarly to 1, complex 24 activated O2, generating a dimer analogous to the singly activated complex 13 (Ni(6Mes)(PPh3)(µ-OH)(µ-O-6Mes)’NiBr (25)). Reactivity of 1 with NaBH4 produced [Ni(6Mes)(κ2-BH4)]2 (28), a Ni(I) dimer bridged by two BH4 ligands. The catalytic efficiency of neutral 1 in Kumada cross-coupling of aryl halides and PhMgCl or MesMgBr was probed. Of note was the high activity towards both aryl chlorides and aryl fluorides. Comparisons with cationic 24, larger 7- (7) and 8-membered ring (8 and 9) variants and the Ni(II) complexes Ni(6Mes/6oTol/7oTol)(PPh3)Br2 (29, 17 and 18 respectively) revealed that 1 exhibited the highest reactivity of all the precursors.

Earth-abundant first-row transition metals have seen a renaissance in chemistry in recent years due to their relatively low toxicity and cost in comparison to precious metals. Furthermore open-shell transition metal complexes exhibit useful one-electron redox processes which contrasts to their heavier d block anologues. This thesis aims to synthesize and analyse the structure and reactivity of low-coordiante first-row transition metal complexes of from groups 7-9 with an aim to utilize these species in catalysis. The divalent compound [Co{N(SiMe3)2}2] reacts with the primary phosphines PhPH2 in the presence of an NHC ligand (IMe4) to yield the phosphinidene bridged dimer [(IMe4)2Co(µ-PMes)]2. The complex has interesting magnetic properties due to strong antiferromagnetic coupling between the two cobalt(II) centres. Increasing the steric bulk of the NHC yielded carbene-phosphinidene adducts (NHC·PAr). This transformation was shown to be catalytic. The structure and reactivity of complexes of the type [(NHC)xMn{(N(SiMe3)2}2] were investigated. The complexes exhibit similar structural properties to their iron and cobalt analogues; however their reactivity has been shown to differ. The addition of primary phosphines to complexes of the type [(NHC)xMn{N(SiMe3)2}2] yielded a range of manganese phosphide complexes. [Mn{N(SiMe3)2}2] also reacts with imidazolium salts at elevated temperatures to yield heteroleptic manganese NHC complexes. The reaction of [Mn{N(SiMe3)2}2] with IPr·HCl afforded the abnormal carbene complex [(aIPr)Mn{N(SiMe3)2}µ-Cl]2. A new monoanionic bidentate ligand is reported which has shown to be a useful ligand system to stabilize three-coordiante iron(II) complex. The reaction of [(L)Fe(Br)] with mesitylmagnesium Grignard or n-butyllithium yield the iron hydrocarbyls [(L)Fe(Mes)] and [(L)Fe(nBu)] with the latter being stable to β-hydrogen elimination. Finally [(L)Fe(nBu)] has been utilized as a pre-catalyst in the hydrophosphination of internal alkynes, showing selectivity for the E-isomeric vinylphosphine.

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