The accurate selection of a ligand for a metal-catalyzed homogeneous reaction can be as decisive as the selection of the metal. Incorporating a carborane cage as a backbone to produce new phosphines often leads to remarkable properties, which are not accessible employing conventional organic entities. The synergy effects between the o-carborane cluster and the P atom in these closo- and nido-carboranylphosphines is observed from two points of view: the cluster and the P atom. The P atoms in the closo-carboranylmonophosphines grant to the cluster a unique plasticity on the CclusterCcluster distance. The cooperation between the carboranyl moiety and the P atom may result in mono-, di- and tridendicity according to the necessities of the metal, not an ever-available property for organic phosphines.