The work described in this Thesis is a study of the synthesis and reactivity of some giant osmium carbonyl clusters and an investigation into their magnetic and electronic properties. Chapter One contains an overview of the concept of the chemical bond from Dalton to the present day ideas of Wade and Mingos, and their application to transition meial clusters. Chapter Two is an investigation into the fluxional and dynamic behaviour of mixed-metal clusters in solution using multinuclear N.M.R. (lH,iaC, and 3lP{1H}). The clusters [Os10C(CO)24MPR3]-(M = Cu, Ag or Au) and [Os1oC(CO)24(MPR3)(M>PR3)] (M = M'= Ag or Au, and M 5* M' Ag and Au) have been examined using these techniques. Chapter Three reports the synthesis of the dianion [Os11C(CO)27]2~, a cluster containing a trigonal prismatic geometry and its reaction with elec-trophiles such as H+, I2, [Cu(NCMe)4]PF6, AuPR3Cl and AgBF4. THe X-ray structure of [HOsnC(CO)27]~ is described, and the conversion of [OSllC(CO)27]2- to [Os10C(CO)24]2-. Chapter Four is concerned with the synthesis of giant osmium clusters using [Os10C(CO)24]2"~ as a precursor and reacting it with Hg(CF3COO)2 to form [Os20Hg(C)2(CO)42]2-, [Os30Hg2(C)3(CO)72]2-and [Os40Hg3(C)4(CO)oe] • The corresponding gold derivatives, [Os10C(CO)24AuBr]- and [Os20Au(C)2(CO)40]2~ have also been prepared and fully characterised by X-ray crystallography. Chapter Five describes the use of l87Os-1H satellites and Inversion-Recovery 1H n.m.r. as aids in the study of the dynamic behaviour of hydrido osmium carbonyl clusters in solution. In particular the work has focussed on differentiating between hydride ligands in tetrahedral or octahedral interstitial sites. The improved synthesis of [H4Osio(CO)24]3~ is reported and its reactions with electrophiles are discussed. The dianion reacts with H+ to form [H5Os10(CO)24]~ which has been characterised by an X-ray structural determination. Chapter Six presents UV/visible, magnetic and EPR measurements on a range of osmium carbonyl clusters prepared in Chapters Two to Five. A molecular orbital framework for understanding these results is presented. Finally Chapter Seven contains the experimental section and spectroscopic data.

A combined experimental and computational study on the reversible ortho-metalation exhibited by the triosmium cluster Os3(CO)10(dppm) (dppm = 1,1-bis(diphenylphosphino)methane is reported. The conversion of nonacarbonyl cluster HOs3(CO)9[-PhP(C6H4)CH2PPh2] to Os3(CO)10(dppm) is independent of added CO and exhibits a significant inverse equilibrium isotope effect (EIE). Reductive coupling of the C-H bond in HOs3(CO)9[-PhP(C6H4)CH2PPh2] leads to the formation of agostic C-H and two distinct aryl-π species prior to the rate-limiting formation of the unsaturated cluster Os3(CO)9(dppm). Heating the unsaturated dimer H2Re2(CO)8 with Cp*Rh(CO)2 (Cp* = 1,2,3,4,5-pentamethylcyclopentadiene) at elevated temperature affords the new trimetallic clusters H2RhRe2Cp*(CO)9 and HRh2ReCp*2(CO)6, and the spiked-triangular cluster HRhRe3Cp*(CO)14. H2Re2(CO)8 reacts with Cp*2Rh2(CO)2 under identical conditions to furnish H2RhRe2Cp*(CO)9 and HRh2ReCp*2(CO)6 as the principal products, in addition to the tetrahedral cluster H2Rh2Re2Cp*2(CO)8. H2RhRe2Cp*(CO)9 undergoes facile fragmentation in the presence of halogenated solvents and the thiols RSH (where R = H, C6H4Me-p) to afford the structurally characterized products Cp*Rh(-Cl)3Re(CO)3, S2Rh3Cp*(CO)4, Cp*Rh(-Cl)(-SC6H4Me-p)2Re(CO)3, and Cp*Rh(-SC6H4Me-p)3Re(CO)3. The new hydrazido-substituted compounds TaCl(NMe2)3[N(TMS)NMe2] (TMS = tetramethylsilyl) and Ta(NMe2)4[N(TMS)NMe2] have been synthesized and their structures established by X-ray crystallography. The latter product represents the first structurally characterized octahedral tantalum(V) complex containing a single hydrazido(I) ligand in an all-nitrogen coordinated environment about the metal center. The fluxional properties of the amido and hydrazido ligands in these new compounds have been established by VT 1H NMR spectroscopy (VT = variable temperature). Preliminary data using Ta(NMe2)4[N(TMS)NMe2] as an ALD (ALD = atomic layer deposition) precursor for the preparation of tantalum nitride and tantalum oxide thin films are presented.