Compositions of the invention include metal-based coordination complexes, which are preferably tunable photodynamic compounds. The compositions and complexes are useful as therapeutic agents and as in vivo diagnostic agents for treating or preventing diseases including those that involve hyperproliferating cells in their etiology, such as cancer. Compositions and complexes of the invention are further capable of destroying microbial cells, such as bacteria, fungi, and protozoa, and destroying viruses. The compositions and complexes are also capable of modulating cell function in other ways.