The reaction of [Os6(CO)18] with carbon monoxide at 160 °C and 90 atm leads to the formation of [Os5(CO)19](1) in high yields. This complex crystallises in the triclinic space group P with a= 8.880(4), b= 10.244(5), c= 16.529(7)Å, α= 99.98(2), β= 93.44(2), γ= 110.37(3)°, and Z= 2. The structure was solved by a combination of direct methods and Fourier-difference techniques and refined by blocked-cascade least squares to R= 0.040 for 2 616 observed diffractometer data. The metal-atom skeleton consists of two triangles sharing a vertex. The Os atom common to both triangles is co-ordinated to three terminal carbonyl groups, and the other four metal atoms are each co-ordinated to four carbonyl groups, two in axial and two in equatorial sites. Complex (1) reacts with ligands L = P(OMe)3(a) or PEt3(b) to produce complexes with the general formula [Os5(CO)16 –nLn][n= 1 (2a,b) or 2 (3a,b)]. On heating, [Os5(CO)19] decarbonylates to give [Os5(CO)16](4) which subsequently reacts with ligands L = CO, P(OMe)3(a), or PEt3(b) to produce [Os5(CO)16L3](1), (5a), and (5b) respectively. The molecular structure of [Os5(CO)16{P(OMe)3}3](5a) has been solved using the same techniques as for (1), and refined by blocked full-matrix least squares to R= 0.071 for 2 144 observed diffractometer data. This complex crystallises in the triclinic space group P with a= 11.150(5), b= 11.792(6), c= 18.581(10)Å, α= 106.91(3), β= 92.67(3), γ= 109.45(3)°, and Z= 2. The molecular geometry resembles that of (1) except that three equatorial carbonyls on three different Os atoms have been replaced by phosphite groups. The relationship between the structures of (1) and (4) is discussed briefly in terms of transformations of the metal cluster skeleton. These two compounds represent the first case where two binary carbonyls with the same number of metal atoms have different numbers of carbonyls bonded to them.