Structural investigations of transition metal clusters
The work described in this dissertation is concerned with structural aspects of the chemistry of transition metal cluster compounds, in particular with carbonyl clusters of the second and third transition series. The structures of a range of these complexes have been investigated, in the solid state, by single-crystal X-ray diffraction techniques, and in the solid and in solution, to a limited extent, by EXAFS spectroscopy. In Chapter One the basics of the single-crystal X-ray diffraction technique for determining the crystal and molecular structure of compounds in the solid state is described. The various steps of sample preparation, data collection, structure solution and refinement are discussed. Chapter Two is a review of the known structural chemistry of heterometallic cluster complexes which contain gold, or gold - phosphine units coordinated to the rest of the metal framework. The majority of complexes discussed in subsequent chapters are of this type. Simple 'electron counting' schemes which have been used to rationalise the structures of this class of compound are discussed critically. Chapter Three is divided up into five sections, each of which is concerned with a different class of heterometallic cluster containing an AuPR3 unit as a ligand. In section 1, the structure of [Os3(CO)10(AuPPh3)Cl], in which the Au atom bridges an Os-Os edge of the Os3 triangle, is described. In section 2, the structures of [HOs3Co(CO)13], [Os3Co(CO)13(AuPPh3)], and [HOs3Ru(CO)13(AuPEt3)] are described, and comment is made on the structural changes which occur when a bridging hydride ligand is replaced by a bridging AuPR3 group. In section 3 the structures of [Os4(CO)13(AuPEt3)], [Os4(CO)12(AuPPh2Me)2], and [Os4(CO)12(AuAsPPh3)2] are described, and a change in geometry is noted when a CO ligand is removed from the former complex and the level of unsaturation formally increased. In section 4 the structures of [Ru5C(CO)14(AuPEt3)2] and [Os6(CO)18P(AuPPh3)] are discussed. Both these clusters contain an interstitial atom which modifies the structural chemistry. Section 5 contains a description of the structures of [Os6(CO)16 lbraceP(OMe)3 rbrace (AuPEt4)2] and [Os6C(CO)20(OMe)Au]. The Os atom framework in these clusters is contrasted with a number of hexaosmium systems which do not contain gold. The structures of two hexaosmium 'raft' clusters, [Os6O(CO)18(PPh3)] and [H2Os6(CO)18(PPh)] are described in Chapter Four. The effect of the introduction of the capping O atom and the PPh group on to the previously planar metal system is discussed, and is related to molecular orbital calculations on the systems. In Chapter Five the introduction of heavy, main group metal atoms into a transition metal cluster system is discussed, with reference to the structures of [H3BiRu3(CO)9], [Bi2Os3(CO)9], and [Bi2Ru4(CO)12]. Both steric and electronic effects are shown to be of importance. Finally, in Chapter Six, the technique of EXAFS spectroscopy is described. The limitations of the technique when applied to transition metal cluster systems are discussed with reference to data obtained from two pentaosmium cluster complexes.