More than 150 different structural defects in periodic molybdenum disulfide (MoS2) monolayers were investigated systematically for the present study by density-functional calculations. All defects were chosen to be triangular, but of different size and distance from one another. Molecular-dynamics simulations proved the defect stability against temperature effects. The energetic properties of the defective MoS2 monolayers - especially the density of states - can be viewed as a combination of edge and bulk properties with the defect size being of dominating influence. A dependence on the defect-edge termination was present only in the very smallest defects. The definition of a localization parameter helps to characterize the grade of localization of the states. Finally, a strong influence of the defect distance on the electronic properties could not be observed, except for very small distances.