A key objective of this thesis was to develop Electrospray (Ionisation) Mass Spectrometry (ESMS) within organometallic chemistry, specifically as a technique for investigating transition metal carbonyl cluster chemistry. An initial survey of air-sensitive high-nuclearity rhodium and nickel clusters, such as [Rh9As(C0)21f and [H2Ni!2(CO)iif, demonstrated the usefulness of ESMS for characterisation. ESMS was applied to monitoring the reactions of ~-E[M2(CO)xh [E = Si, Ge, Sn; M = Co (x = 7), Fe (x = 8)] with anionic transition metal carbonyl reagents. Analysis of the reaction between ~-Ge[Coi(C0)7h and [Co(C0)4r revealed the inter-conversion processes between previously isolated clusters, and was crucial in identifying the unheralded [Co10Gei(C0)24f product. Conditions for optimal yields of this anion were established and the cluster subsequently isolated and structurally characterised (below). The core structure of [Co10Ge2(C0)24f is unprecedented and is discussed in terms of electron-counting theories. The analogous reaction with µ4-Si[Co2(C0)7h gave indication of novel cluster products. These included [Co5Si(C0)16r, [Co9Si(C0h2r and [Co7Si(COh1L as well as the previously reported [Co9Si(CO)iif. Reactions with [Fe2(CO)sf, [HFe(C0)4r, [Fe(C0)3(NO)L [Mn(CO)sr and [Mo(C0)3(ri5-CsHs)r were less straight-forward, but again led to the detection of new clusters. ESMS analysis confirmed the unusual result that [Co14N3(C0h6]3 • is the only highnuclearity thermolysis product of [Co6N(C0)15r. The specificity of the reaction can only be established by ESMS since the technique precludes the existence of any other significant ionic products. The reaction between PCls and [Co(C0)4r gave the known [CoJ>(CO)i6r and [Co9P(C0)21f- clusters while the analogous reaction with AsCh produced three novel cobalt arsenido species, [Co~s(C0)16L [Co1As2(C0)17r and [Co9As2(C0)21r. The previously reported [Co4Sb2(C0)11r and novel [CosSb2(C0)14r clusters were identified in the reaction between SbCh and [Co(C0)4]. The synthesis and characterisation of a series of Co4(1-4-ER)2(C0)11 clusters are described. The dimethylamino functionalised examples, Co4(1-4-ECi#4NMe2)2(C0)11 E = Si, Ge, were characterised by ESMS as their [M+Ht ions. The Co4(J.4-SiR)2(C0)11 [R = Ph, CJliOMe, (CH2)30Me] clusters underwent facile substitution with isonitrile and phosphite reagents but equivalent phosphine-substituted products were not observed by ESMS. Isonitrile-substituted clusters (n > 2) gave clear~ ions in their ES mass spectra arising from in situ oxidation. The novel Co4(1-4-SiC~OMe)2(C0)1(XyNC)4 cluster was structurally characterised (below). Linked clusters of the form [Co4(1-4- SiCJliR)(C0)11Si]2X [R = H, OMe; X = (CH2)s, CJii] were isolated, albeit in very low yield. An oligomeric -[XSiCo4(C0)11 Si]-0 species was tentatively identified. A new structural isomer of [CoFe3(C0)13r was characterised. The ligand arrangement for [NE4][CoFe3(C0)13] differs significantly from that of [PPN][C0Fe3(C0)13] and [NE4][C0Ru3(C0)13]. A structure determination of [PPN][Mo(C0)3(ri5-CsHs)] is also reported.