A new family of phosphorus functionalized, pentanuclear ruthenium clusters Ru 5 (CO) 1 5 (μ 4 -PR) [R = N i Pr 2 , NCy 2 , F, OMe, OEt, O i Pr, O - ] has been synthesized and representative compounds structurally characterized. The reaction of Ru 4 (CO) 1 3 (μ 3 -PNR 2 ) (1) (R = Cy, i Pr) with Ru 3 (CO) 1 2 in refluxing heptane yields H 2 Ru 3 (CO) 9 (μ 3 -PNR 2 ) (2) and Ru 5 (CO) 1 5 (μ 4 -PNR 2 ) (3). An alternative high-yield synthesis for 3 is the reaction of 1 with Ru(CO) 5 in refluxing hexane. Treatment of 3 with HBF 4 .Et 2 O produces Ru 5 (CO) 1 5 (μ 4 -PF) (4) in high yield. In contrast, refluxing 3 with HBF 4 .H 2 O in CH 2 Cl 2 yields 4 and [R 2 NH 2 ][Ru 5 (CO) 1 5 (μ 4 -PO)] (5), a cluster containing a μ 4 -phosphorus monoxide (PO) ligand. The reaction of 4 with alcohols affords the series of alkoxy phosphinidene cluster complexes Ru 5 (CO) 1 5 (μ 4 -POR') (6) (R' = Me, Et, i PR). The structures of 2a (R = NCy 2 ), 3a (R = NCy 2 ), 4 (R = F), and the phosphorus monoxide cluster [H 2 NCy 2 ][Ru 5 (CO) 1 5 (μ 4 -PO)] (5a and 6a) (R' = Me) have been determined by X-ray diffraction studies.