The structures of [Ru5C (CO) 14L] , where L = P (4-C6H4OMe) 3, 2a, P (4-C6H4F) 3,2b,and PCy3, 2c, and of [Ru5C (CO) 13 (etpb) 2] (etpb = P (OCH2) 3CEt) ) , 3, have been determined. The Ru — Ru andRu — Pbond distances of the [Ru5C (CO) 15−nLn] clusters, 2a, 2b, 2c, 3, [Ru5C (CO) 14 (PPh3) ] , and [Ru5C (CO) 13 (PPh3) 2] , were analyzed and the resultsarecompared with those reported elsewhere for the unsubstituted parent cluster, and for theclusters [Ru3 (CO) 12−nLn] , n = 1or 2. TheRu5 clusters all maintain the square pyramidal arrangement of the parentcluster andthe general arrangement of the ligands. The average Ru — Ru distances are remarkablyconstantbut individual Ru — Ru distances vary widely. A clear feature is that the lengths of theRu — Rubonds in the basal plane that are cis to the axial Ru — P bonds are longer thanany of theother Ru — Ru distances in all the [Ru5C (CO) 15−nLn] clusters except 3. The distance lengthens slightly with increasing coneangle of thesubstituent P-donor ligand but this effect may be at least partly due to crystal packingeffects. TheRu — P distances in the Ru5 and Ru3 clusters increasesignificantly withincreasing ligand cone angle and net electron donicity. The two effects cannoteasily be separateddue to the general correlation of the donicity and size of the P-donor ligandsand both effects mayapply. A distinctive feature of these changes in bond lengths is that they donot depend onwhether n = 1 or 2, showing that the substituent effects are localized.The carbido atomout-of-plane distance increases in an approximately linear fashion with thebasicity of the P-donorligand.