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Tungsten dichalcogenide monolayers have unique properties which lead them to have potential uses as a channel material in next generation semiconductor devices, an atomically thin diffusion barrier in interconnect structures, or as a low dimensional support for catalysis. In order to enable these applications, these monolayers must be interfaced with a metal, such as a source and drain contact for their use as a channel material, or an interconnect metal for use as a diffusion barrier. The morphology of the metal that is interfaced with the TMD monolayer typically determines the possible applications. Therefore, an atomistic level description of how metals interact and grow on TMDs is required. Theoretical studies of metal-monolayer interaction have primarily focused on the behaviour of either single metal atom adsorption and doping, or pristine metal-monolayer interfaces. There is therefore a clear gap in understanding the early stages of metal growth on TMD monolayers. In this work, we present a density functional theory (DFT) study on the adsorption of Co and Ru adatoms, dimers and 4 atom nanoclusters onto WS 2, WSe 2, and WTe 2 to develop a comprehensive understanding of the fundamental interactions between adsorbed metal atoms and tungsten TMD monolayers, and the role different metal-substrate and metal-metal interactions have on the preferred metal adsorption sites and nanocluster morphology. We find that Co and Ru interact strongly with all three TMDs, but can also cause significant structural alterations which will affect thin-film growth. In particular, there is a strong drive to adsorb interstitially in WSe 2 and WTe 2, due to their larger lattice parameter compared to WS 2. In WTe 2, this interstitial adsorption also causes a localised phase change from hexagonal to orthorhombic due to displacement of W atoms by the interstitial metal atom. Additionally, both Co and Ru can act as a catalyst for chalcogen vacancies by displacing the chalcogen atom and occupying the resulting vacant site. Overall, Ru has a strong drive towards vertical growth, as stability is driven largely by metal-metal interaction, while Co adsorption is most stable at an interstitial site.