In Chapter One the concept of the surface cluster analogy is discussed and examples are given of its application in resolving the nature of chemisorbed molecules and molecular fragments on metal surfaces. This is then followed by a review of previous studies into the reactivity of metal carbonyl clusters with nitrogen heterocyclic ligands. This highlights the substitution of carbonyl ligands for nitrogen heterocycles and the process of orthometallation, and emphasises how earlier work has typically concentrated on the trinuclear clusters, with examples involving higher nuclearities being rare. Consequently the need to study such cluster's reactivity further is made apparent. The second chapter details the synthesis of a series of disubstituted tetranuclear clusters of the general formula M₄(μ-H)₄(CO)₁₀(L-L) {M= Ru, Os; L-L= bidentate heterocyclic ligand or (pyridine)₂} which have been prepared from the reaction between the parent cluster M₄(μ-H)₄(CO)₁₂ and principally 2.2 equivalents of the decarbonylating reagent Me₃NO. Our studies have shown that disubstitution of the cluster is favoured whether 1.1, 2.2, or 4.2 equivalents of Me₃NO are used, and thus implies the inherent stability of the M₄(μ-H)₄(CO)₁₀(L-L) species. In particular, it is found that in its reaction with pyridine and 1.1 equivalents of Me₃NO the M₄(μ-H)₄(CO)₁₀(py)₂ cluster is formed as the major product, being present in a significantly higher yield than the expected monosubstituted derivative. This reactivity is accounted for mechanistically by comparison with similarly observed substitution patterns for the Ir₄(CO)₁₂ cluster. Full X-ray and spectroscopic analyses are discussed in detail. In Chapter Three the previous work of Dutton ¹ into thermal activation of the Ru₅ cluster, primarily with pyridine, is discussed and contrasted to the chemical activation process using Me₃NO utilised in this study. The Ru₅C(CO)₁₅ cluster has been found to react in the presence of 2.2 equivalents of Me₃NO with both 2,2'-bipyridyl and 1,10-phenanthroline to yield the Ru₅C(CO)₁₄(L-L) cluster as the major product. Hence loss of only a single carbonyl ligand occurs to yield a 76 electron bridged butterfly cluster which has been characterised both spectroscopically and by single crystal X-ray diffraction. When a second carbonyl ligand is removed from the cluster the orthometallated species Ru₅(μ-H)C(CO)₁₃(L-L) is produced, but in relatively lower yield. Again this species has been characterised by spectroscopy, and in addition for the 2,2'-bipyridyl derivative, by X-ray crystallography. In addition, it has been discovered that the Ru₆C(CO)₁₇ cluster undergoes cluster degradation in the presence of 2,2'-bipyridyl or 1,10-phenathroline and 2.2 equivalents of Me₃NO with the loss of an Ru(CO)₃ fragment in the formation of the Ru₅(C(CO)₁₄(L-L) cluster, which had been previously prepared directly from the Ru₅C(CO)₁₅ cluster. Also produced in comparable yield was the expected Ru₆ species Ru₆C(CO)₁₅(L-L) which is relatively unstable and decomposes over time.