Reversible Formation of (μ‐H)Ru4(CO)10(μ‐PPh2)(μ4‐η1(P),. eta.1(P),η1(P),η1,η2‐(C6H4)PPh) from the Electron‐Rich Cluster Ru4(CO)13(μ‐PPh2)2: The First Example of a Molecule with a Five‐Coordinate Bis(aryl)phosphido Bridge