Introduction The chemistry of transitional metal complexes bearing σ-bonded H−X (X = H, Si, C) species/fragments, the so called σ-complexes, are key intermediates in catalytic processes such as hydrogenation, hydrosilylation, alkane functionalization etc. Particularly, the σ-H2 complexes form the best-known group of σ-complexes in which H2 is bound to the metal center in η2-fashion. Several well characterized examples of η2-silane and η2-borane complexes have also been reported. Moreover, in recent years, the carbon analogues of these complexes in which alkanes are coordinated through η2-C-H bonds to the metal center have been attracting the attention of organometallic chemists. An approach towards direct functionalization of σ-bonds in simple alkanes is the heterolytic activation of the C−H bond using highly electrophilic complexes. After all, for fine catalyst design and the selective functionalization of H−H, silanes or simple alkanes, it is necessary to understand the bonding nature of these σ-complexes in depth. Objectives The objectives of this work are as follows a) An attempt to stabilize and gain insights into the bonding nature and reactivity behavior of various sigma ligands on ruthenium center [Ru(η2-HX)(Tpms)(PPh3)2][OTf], (X = H, SiR (R = Me3 or Me2Ph) and CH3). b) Synthesis, characterization and reactivity studies of electrophilic ruthenium(II) complexes bearing (C6F5)2PCH2CH2P(C6F5)2 (dfppe) ligand towards heterolysis of H2. c) An approach towards preparation of insoluble molecular clusters from [Ru(P(OH)3)(dppe)2][OTf]2 complex and Zn, Cd and Cu acetates to realize σ-bond activation under heterogeneous conditions. Significant results In our attempts to gain insights into the bonding nature and reactivity behavior of σ-H2, silane and methane complexes, we followed two strategies to generate these complexes in solution. First, we synthesized and well characterized two new Ru(II)-complexes [RuH(Tpms)(PPh3)2] and [Ru(OTf)(Tpms)(PPh3)2], (OTf = trifluoromethane sulfonate) where Ru-H and Ru-OTf are the key reactive centers, followed by their subsequent reactions with electrophilic reagents such as HOTf, Me3SiOTf and CH3OTf and with H2, PhMe2SiH and CH4 at low temperature, respectively. These reactions finally resulted in the characterization of σ-H2 and σ-silane complexes, however, no σ-methane complex was observed even at low temperature (Scheme 1). Scheme 1 In order to realize highly eletrophilic metal complexes, a chelating fluorinated phosphine ligand 1,2-bis-(pentafluorophenylphosphino)ethane, (C6F5)2PCH2CH2P(C6F5)2 (dfppe) was employed and the synthesis and structural characterization of a series of new, Ru(II) hydride complexes [RuH(P(OMe)3)(bpy)(dfppe)][OTf], cis-[RuH2(dfppe)(PPh3)2] and [RuH(CO)Cl(PPh3)(dfppe)] were accomplished. Protonation reaction of the hydride complexes [RuH(P(OMe)3)(bpy)(dfppe)][OTf] (Scheme 2) and [RuH(CO)Cl(PPh3)(dfppe)] (Scheme 3) with HOTf at low temperature gave free H2 and five-coordinate species [Ru(P(OMe)3)(bpy)(dfppe)][OTf]2 and [Ru(CO)Cl(PPh3)(dfppe)][OTf], respectively. Surprisingly, in all these reactions, dihydrogen complexes are formed which were unobservable in which the H2 ligand was found to be highly labile. Reaction of is-[Ru(bpy)(dfppe)(OH2)(P(OMe)3)][OTf]2 with H2 however, resulted in the heterolytic activation of the H–H bond and concomitant protonation of H2O to give the corresponding hydride complex cis-[Ru(H)(bpy)(dfppe)(P(OMe)3)][OTf] and H3O+ (Scheme 2) . Scheme 2 Scheme 3 In an attempt to prepare insoluble molecular clusters in order to realize σ-bond activation under heterogeneous conditions, we studied the reactivity of highly electrophilic [Ru(P(OH)3)(dppe)2]2+ (dppe = (C6H5)2PCH2CH2P(C6H5)2) complex with various metal acetates. Usage of Zn(OAc)2.2H2O afforded a novel [Ru2(dppe)4P2(OH)2O4Zn2(OAc)(DMP)(OTf)][OTf]2 (Ru-Zn ) soluble bimetallic complex (Scheme 4) which was characterized in detail by NMR and single crystal X-ray crystallography. To achieve the expected insoluble molecular cluster further studies are required to tune the electronics and the sterics around the phosphorous acid moiety. Scheme 4

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This thesis examines the interaction of hydrides of the group 13 metals aluminium and gallium with transition metal centres. Furthermore, a gallium-based system is developed which activates a wide range of E-H bonds, with the product of H₂ activation found to act as a catalyst for the reduction of CO₂ to a methanol derivative. Chapter 3 details the synthesis of a number of alane and gallane adducts of expanded-ring N-heterocyclic carbene (NHC) ligands, which are more strongly σ-donating and sterically shielding analogues of classical NHCs. These NHC adducts are found to be apposite for the formation of σ-alane and σ-gallane complexes at group 6 metal carbonyl fragments, which has allowed the characterisation of the first κ² σ-gallane complexes. The attempted formation of a terminally coordinated κ³ σ-alane complex leads instead to the isolation of a novel dinuclear cluster featuring both μ:κ¹,κ¹ and μ:κ²,κ² coordination to Mo(CO)₃ units. The work presented in Chapter 4 probes the interaction of the β-diketiminate stabilised gallane Dipp₂NacNacGaH₂ with transition metal carbonyls. Far from simply mimicking the chemistry of the alane congener Dipp₂NacNacAlH₂, which forms simple κ¹ and κ² σ-alane complexes, the gallane shows a marked propensity towards dehydrogenation and formation of direct M-Ga(I) bonds. This represents a rare mode of reactivity among group 13 hydrides, being unprecedented beyond boron chemistry, and provides a new route to M-Ga bond formation. Experimental and computational investigations of the mechanism suggest that initial Ga-H oxidative addition is facile, and is generally followed by rate-limiting loss of H₂. The reaction of Dipp2NacNacAlH2 with Co₂(CO)₂ is shown to yield an unusual alane complex which displays an unprecedented degree of Al-H activation in a σ-alane complex. Chapter 5 represents an extension of the work described in Chapter 5, investigating the interaction of Dipp₂NacNacMH₂ (M = Al, Ga) with cationic group 9 transition metal fragments supported by ancillary phosphine ligands. While attempts to isolate unsupported, cationic σ-alane complexes prove unsuccessful, Dipp₂NacNacGaH₂ readily binds to cationic rhodium and iridium centres, forming the first cationic σ-gallane complexes as well as cationic gallylene complexes resulting from complete Ga-H oxidative addition. The extent of Ga-H bond activation is shown to be markedly dependent on the nature of the phosphine co-ligands. In particular, a series of rhodium complexes is reported which represents snapshots of the oxidative addition process, from a Rh(I) σ-gallane complex to a Rh(III) gallylene dihydride, with two further complexes which are on the cusp of these two oxidation states. Described in Chapter 6 are the synthesis and reactivity studies of an ambiphilic system, Dipp₂NacNac′Ga(^tBu), featuring a three-coordinate gallium centre supported by a deprotonated NacNac ligand. The combination of this electrophilic gallium centre with the highly nucleophilic exocyclic alkene functionality facilitates the cooperative activation of protic, hydridic and apolar E-H bonds. Accordingly, molecules including H₂, NH₃, H₂S and SiH4 may be cleaved under mild conditions. Moreover, the hydride product of H₂ activation is shown to be a competent catalyst in conjunction with HBpin for the reduction of CO₂ to the methanol derivative MeOBpin.