The preparation and reactivity of ruthenium(II) complexes that incorporate the chelating amidophosphine ligands [NPN] (where [NPN] = PhP(CH2SiMe2NPh)2) or [P2N2 ] (where [P2N2 ] = PhP(CH2SiMe2NSiMe2CH2)2PPh) are presented. The reaction of the dilithium salt [P2N2]Li2(S) (S = 1,4-dioxane) with the ruthenium starting material [RuCl2(cod)]x (cod = 1,5-cyclooctadiene, r]2:ri2-C8Hi2) generates the diamido complex (P2N2]Ru(r|2:r|2-C8Hi2) and the reaction with RuCl2(PPh3)3 gives the ort/zo-metalated species [P2NNH]Ru(C6H4PPh2). The [P2N2] cyclooctadiene complex reacts with hydrogen gas (one atmosphere) to give the fluxional dihydrogen-hydride species [P2NNH]Ru(T]2-H2)(H). The minimum longitudinal relaxation time (7i(min)) for the hydride ligands in this complex is 62 ms (at 240 K) and the 1JHD coupling constant in the HD isotopomer is 15 Hz. These data both lead to an estimated H-H distance of 1.2 A corresponding to an elongated H2 moiety. The or^o-metalated species reacts with H2 forming the monohydride complex [P2NNH]RuH(PPh3). Exposure of this hydride species to an atmosphere o f deuterium gas results in the incorporation o f deuterium into both the N-H and Ru-H sites. Whereas the dihydrogen complex catalyzes the hydrogenation of olefins under mild conditions the monohydride complex is inactive. Reaction of pSTPN]Li2(S)2 (S = tetrahydrofuran) with [RuCl2(cod)]x generates the 3 2 ruthenium cyclooctadienyl complex [NPNH]Ru(r| :r\ -CgHn) that forms via deprotonation of the cyclooctadiene ligand by one of the amido donors of the tridentate ligand. This product exists in equilibrium as a mixture of two diastereomers; inter-conversion of the two isomers occurs via intramolecular proton transfer between the amido side-arms of the [NPNH] ligand. The solid-state molecular structure of one of the isomers was determined by X-ray crystallography and it shows that the complex adopts a distorted trigonal bipyramidal (a Y-shape in the trigonal plane) coordination geometry. This structure allows for maximal 7t-overlap of the amido lone pair with the metal LUMO. Exposure of [NPNH]Ru(r|3:r|2-C8Hii) to H2 gas yields the three ruthenium hydride complexes [NPNH(r|6-C6H5)]RuH, [NPNH2]Ru(H)2(C7D8) and [NPNH2(r|6- C6Hs)]Ru(H)2. Each of these complexes contains an r|6-bound arene group; in two of the complexes this moiety is the amino phenyl group of the chelating ligand set and in the other it is a coordinated solvent molecule. Each of these three hydride products is inactive toward olefin and imine hydrogenation reactions, however, the cyclooctadienyl complex [NPNH]Ru(r|3:r|2-C8Hii) does reduce these substrates. The catalytic hydrogenation studies that have been performed with all of these complexes are also discussed. The attempted preparation of [NPN] and [P2N2] ruthenium alkylidene and vinylidene complexes is reported. The terminal alkynyl complex [NPNH](PlPr3)Ru(CCPh) forms from the reaction of the dilithium salt of the [NPN] ligand with Cl2(PiPr3)2Ru(=CCHPh) via deprotonation of the vinylidene ligand. The addition of hydrogen gas to this complex produces the thermally unstable bis-dihydrogen dihydride complex pSTPNH2]Ru(ri2-H2)2(H)2. The Ti(min) value for the metal-bound hydrogen atoms within this complex is 51 ms observed near 220 K. The dilithium salt of the [P2N2 ] ligand reacts with Cl2(P'Pr3)2Ru{=CC(SiMe3)(Ph)} to give the five-coordinate vinylidene complex [P2N2]Ru{=CC(SiMe3)(Ph)}. Olefin metathesis reactions including the ring opening metathesis polymerization of norbornene and cross metathesis with styrene are unsuccessful with this complex, however, it does react with H2 (4 arm) to generate the previously described dihydrogen-hydride complex [P2NTSrF£]Ru(ri2-H2)(H).