First principles calculations based on spin-polarized density functional theory and the generalized gradient approximation have been used to study the chemisorption and diffusion of Ni atoms and clusters and a Ni-thiophene complex on defect-free MoS2(000). The calculations employ slab geometry and periodic boundary conditions. We have identified several possible adsorption configurations of Ni atoms on the surface. The most stable configuration corresponds to adsorption at 3-fold hollow sites. By increasing the Ni coverage, several types of clusters can be formed that have a lower binding energy per Ni atom than single adsorbed Ni atoms. Minimum energy pathways for the diffusion of Ni atoms between selected pairs of local minima have been determined. The results indicate the existence of relatively large barriers for hopping with values between 20 and 44 kcal/mol. Additional calculations have been performed to analyze the adsorption of the thiophene molecule on bare and Ni-covered MoS2 basal planes. Ni atoms significantly increase the adsorption energy of thiophene up to 21 kcal/mol. However, the diffusion barrier of the Ni-thiophene complex between 3-fold sites remains high with a value of 21.2 kcal/mol. Finally, the role of sulfur defects upon the chemisorption of Ni atoms has been investigated. These defects further stabilize the Ni atoms on the surface and slightly decrease the binding energy of thiophene on these metallic atoms.