Reversible rearrangement of a tetraruthenium cluster containing quadruply bridging PPh groups to one containing triply bridging PPh ligands through uptake of carbon monoxide; crystal and molecular structures of [Ru4(μ3 -XPh)2(CO)13] (X  P or As)

[Ru4(μ4-PPh)2(μ2-CO)(CO)10], in which an essentially square planar array of ruthenium atoms is capped by two phenylphosphinidene groups, readily and reversibly adds carbon monoxide to afford the tridecacarbonyl derivative [Ru4(μ3-PPh)2 (CO)13] ; crystal structure determinations on this compound and its arsenic analogue reveal that the CO addition is accompanied by a re arrangement of the cluster framework, and that these compounds can be considered as dirivatives of [Ru3(μ3-XPh)2(CO)9] (X  P or As) in which an Ru(CO)4 unit has been inserted into one of the three RuP or RuAs bonds.