Transition-metal derivatives of the functionalized cyclopentadienyl ligand. XVIII. Syntheses and structures of a series of tetranuclear cyclic complexes [{(CO)3M(μ,η5-C5H4PPh2)}2M'M''] (M=Cr, Mo or W, M'=M''=Ag; M=Cr or Mo, M'=M''=Au; M=Mo, M'=Ag, M''=Au). Isotopomerism and NMR evidence of a double epimerization process