This thesis describes the synthesis and structural characterisation of cyclometalated heteroleptic bis(tridentate)ruthenium(II) complexes based on [Ru(terpyridine)(1,3-dipyridylbenzene)]PF6, a cyclometalated analogue of [Ru(terpyridine)2](PF6)2. These complexes are investigated photophysically, photochemically and electrochemically, and also studied by computational methods. Electron donating, electron withdrawing and large aryl groups have been incorporated at three different positions, and pyrazol-1-yl or 1,2,3-triazol-4-yl used as the N-heterocycle instead of 2-pyridyl groups, as in the parent complex, [Ru(terpyridine)(1,3-dipyridylbenzene)]PF6. The ligands are substituted at the 4 position of the central benzene or pyridine ring, or attached to the N-heterocycle. The positions of these modifications have been specifically chosen to direct electronic effects into the Ru-C bond, the opposite Ru-N bond or directly into the N-heterocycle to analyze their effect. This has provided an understanding as to which stereoelectronict features are most important with regards to changing the electronic nature of the complex, and thus tuning the properties of these complexes. This proves particularly effective in analysing the energy of the visible absorptions resulting from metal-to-ligand charge transfer and emission energy and its related quantum yield.