Density functional theory (DFT) calculations have been performed to determine the strength and geometry of intermolecular interactions of "piano-stool" ruthenium arene complexes, which show potential as anticancer treatments. Model complexes with methane and benzene indicate that the coordinated arene has C-H center dot center dot center dot pi acceptor ability similar to that of free benzene, whereas this arene acts as a much stronger C-H donor or partner in pi-stacking than free benzene. The source of these enhanced interactions is identified as a combination of electrostatic and dispersion effects. Complexes of Ru-arene complexes with base-pair step fragments of DNA, in which the arene has the potential to act as an intercalator, have also been investigated. Binding energies are found to be sensitive to the size and nature of the arene, with larger and more flexible arenes having stronger binding. pi-stacking and C-H center dot center dot center dot pi interactions between arene and DNA bases and hydrogen bonds from coordinated N-H to DNA oxygen atoms, as well as covalent Ru-N bonding, contribute to the overall binding. The effect of complexation on DNA structure is also examined, with larger rise and more negative slide values than canonical B-DNA observed in all cases.