The thermolysis of Ru 3 (CO) 1 1 [Ph 2 PC 6 H 5 Cr(CO) 3 ] affords two isomers of the benzyne derivative Ru 3 (CO) 7 (C 6 H 4 )[PPhC 6 H 5 Cr(CO) 3 ] 2 ((5) and (6)). The crystal structure of 5, previously determined, shows the phenyl groups are trans to the Ru 3 -triangle; in 6 they are probablycis and on the same side of the Ru 3 -triangle as the benzyne moiety. Os 3 (CO) 1 1 [Ph 2 PC 6 H 5 Cr(CO) 3 ] decomposes on heating to afford a benzyne derivative of a less common class Os 3 (CO) 9 (C 6 H 4 )[PC 6 H 5 Cr(CO) 3 ] (7), and an electron deficient molecule Os 3 (CO) 8 (PhPC 6 H 4 )[Ph 2 PC 6 H 5 Cr(CO) 3 ] (8), in which the PhPC 6 H 4 moiety acts as a four-electron donor. The isopropylphosphine derivatives M 3 (CO) 1 1 [Pr i 2 PC 6 H 5 Cr(CO) 3 ] (M = Ru, Os), also undergo thermolysis to afford HRu 3 (CO) 8 [C 6 H 4 Cr(CO) 3 ](PPr i 2 ) (10), Ru(CO) 3 [Pr i 2 PC 6 H 5 Cr(CO) 3 ] 2 (11), H 2 Os 3 (CO) 8 [Pr i PC 6 H 4 Cr(CO) 3 ] (12) and H 2 Os 3 (CO) 8 [Pr i PC 6 H 4 Cr(CO) 3 ] [Pr i 2 PC 6 H 5 Cr(CO) 3 ] (13). Complex 10 is probably a benzynechromium tricarbonyl derivative with a Cr-Ru bond. The phosphines are trans in complex11 , a derivative of Ru(CO) 5 . The structures of complexes 12 and 13 are closely related and the orthometalated [PC 6 H 4 Cr(CO) 3 ] moiety is acting as a four-electron donor to the cluster. Crystal data: for 8 (C 4 1 H 2 4 CrO 1 1 Os 3 P 2 CH 2 Cl 2 ): monoclinic, a = 13.039(3),b = 18.001(2),c = 20.125(2) 9, β = 108.42(1)°, Z = 4, space group P2 1 /n; for 11 (C 3 3 H 3 8 Cr 2 O 9 P 2 Ru): monoclinic, a = 15.390(1), b = 10.241(2),c = 23.490(1) 9, β = 96.686(6)°, Z = 4, space group P2 1 /a; for 12 (C 2 1 H 1 3 CrO 1 2 Os 3 P): orthorhombic, a = 14.695(2), b = 16.696(2),c = 11.186(1) 9, Z = 4, space group P2 1 2 1 2 1 ; for 13 (C 3 5 H 3 2 Cr 2 O 1 4 Os 3 P 2 2CH 2 Cl 2 ): monoclinic, a = 11.536(2), b = 18.354(4),c = 23.176(3) 9, β = 98.43(2)°, Z = 4, space group P2 1 /n. The structures were solved by Patterson methods and were refined by full-matrix least-squares procedures to R = 0.057, 0.031, 0.030 and 0.032 (R w = 0.065, 0.030, 0.028 and 0.030) for 6320, 6103, 3529 and 6042 reflections with I ≥ 3σ(I), respectively.