Protonation of metal–metal bonds: X-ray crystal structures of the complex salts [Ru2(CO)4(μ-H)(μ-Cl)(μ-PtBu2)(μ-Ph2PCH2PPh2)][BF4] and [M2(CO)4(μ-H)2(μ-PtBu2)(μ-Ph2PCH2PPh2)][BF4] (M=Fe, Ru)