The molecular structures of Ru 3(CO) 9(L) 3 (L = PMe 2(CH 2Ph), PMe 2Ph, AsMe 2Ph, PPh(OMe) 2, P(OEt) 3 and P(OCH 2CF 3) 3) and Os 3(CO) 9(PPh 3) 3 have been determined by single-crystal X-ray diffraction methods. The tertiary phosphine, arsine or phosphite ligands occupy equatorial sites, one per metal atom, so arranged that each is as far from the other two as possible. The unit cell of the PPh(OMe) 2 complex is unusual in containing four conformationally distinct molecules, two of which contain disordered Ru 3 cores. Metalmetal and metalligand separations are similar to those found in Me 3(CO) 12 and mono- or disubstituted complexes, respectively. Considerable distortion towards D 3 symmetry is found, with MMCO angles approaching those calculated for semi-bridging CO groups in the case of the P(OEt) 3 complex. Crystal data: Ru 3(CO) 9(PMe 2Ph) 3, orthorhombic, P2 12 12 1, a 21.343(2), b 14.752(4), c 12.170(2) Å, U 3832(1) Å 3, Z = 4, N 0 (number of ‘observed’ data with I > 3σ( I)) = 3601, R = 0.040, R′ = 0.048; Ru 3(CO) 9-(AsMe 2Ph) 3, triclinic, P1, a 19.113(4), b 15.375(4), c 13.756(3) Å, α 89.36(2), β 85.97(2), γ 77.61(2) °, U 3939(2) Å 3, Z = 4, N 0 = 8823, R = 0.046, R′ = 0.047; Ru 3(CO) 9{PMe 2(CH 2Ph)} 3, trigonal, P 3 , a 12.766(10), c 15.583(9) Å, U 2202(2) Å 3, Z = 2, N 0 = 1966; R = 0.093, R′ = 0.117; Ru 3(CO) 9{PPh(OMe) 2} 3, triclinic, P 1 , a 23.23(1), b 20.87(1), c 20.73(1) Å, α 98.21(4), β 111.07(3), γ 111.95(4) °, U 8237(7) Å 3, Z = 8, N 0 = 16532, R = 0.041, R′ = 0.039; Ru 3(CO) 9{P(OEt) 3} 3, triclinic, P 1 , a 18.170(6), b 13.016(6), c 10.035(3) Å, α 68.83(3), β 80.42(3), γ 78.10(4) °, U 2154(1) Å 3, Z = 2, N 0 = 4318, R = 0.051, R′ = 0.061; Ru 3(CO) 9{P(OCH 2CF 3) 3} 3, triclinic, P 1 , a 21.036(2), b 13.146(1), c 9.376(2) Å, α 82.68(1), β 88.40(1), γ 85.59(1) °, U 2564(1) Å 3, Z = 2, N 0 = 3966, R = 0.066, R′ = 0.068; Os 3(CO) 9(PPh 3) 3, triclinic, P 1 , a 18.207(5), b 17.911(6), c 12.928(3) Å, α 94.13(2), β 98.98(2), γ 110.56(2) °, U 3862(2) Å 3, Z = 2, N 0 = 3216, R = 0.092, R′ = 0.084.