This work deals with the synthesis of new titanium and zirconium oxo clusters. By using functionalized ligands, these clusters can be incorporated in class II hybrid materials. Some of the clusters described in this work were obtained by the reaction of a metal alkoxide with a functional or non-functional carboxylic acid. Crystals of the clusters were obtained directly from the reaction solution. These crystals were characterized by single crystal XRD and solution NMR. New clusters were obtained by variation of the carboxylic acid. It was found that carboxylic acids with non-coordinating functional groups can be used to obtain new clusters regardless of the steric demand of the acid. When reacting imidazolylbutyric acid with titanium isopropoxide the outcome was a coordination polymer rather than a cluster. Using methacrylic acid, a carboxylic acid which is frequently used in cluster chemistry, several different clusters were synthesized to show the correlation between precursor ratio and obtained cluster. For the use in hybrid materials it is desired to control the reactivity of the clusters, and thus the number of functional groups. Therefore some control for the arrangement of the ligands on the surface of the cluster is needed. The sterically demanding triphenylacetatic acid was reacted in a mixture with acetic acid and titanium isopropoxide. The outcome, however, showed no significant difference in the ligand sphere. Therefore calixarenes were used, since they occupy four coordination sites on the cluster surface. This led to a different arrangement of the central metal atoms but also smaller structures. In another part of this work, the reactivity of cyclic anhydrides with titanium isopropoxide was investigated. Reacting equimolar amounts of phthalic anhydride with titanium isopropoxide, a dimeric complex with two monoisopropyl phthalate ligands was formed. The same reaction in presence of acetic acid led to the formation of a cluster with a Ti6O6 core and monoisopropyl phthalate ligands. The oxo-bridges of the cluster were obtained through esterification of the acetic acid, which acted as in situ source of water. In contrast, reaction of maleic anhydride with titanium isopropoxide led to the formation of a cluster with dicarboxylate ligands. Phosphonic acids are frequently used to stabilize and functionalize titania and zirconia nanoparticles. Therefore, part of this work deals with the synthesis of phosphonate-substituted titanium and zirconium oxo clusters. It was shown that bis(trimethylsilyl) phosphonates are viable precursors for the preparation of phosphonate-substituted titanium oxo clusters, of which several new structures were synthesized and characterized. The first phosphonate substituted zirconium oxo cluster was subsequently synthesized during this work. Based on the results of previous experiments of this work, a mixture of a bis(trimethylsilyl) phosphonate and acetic acid was used and reacted with titanium isopropoxide. The same cluster core was found with various phosphonates, all containing acetate ligands. These titanium oxo clusters thus have a mixed-ligand sphere of acetate, phosphonate and alkoxo ligands. The stability of these titanium oxo clusters in solution was proven by NMR-experiments.