Relationships between structure and ligand dynamics. II. Alkyne and carbonyl dynamics in Os₃(CO)₉(alkyne)(L) (L = CO, PR₃)

The synthesis of the phosphine substituted complexes Os₃(CO)₉(μ₃-η²-CH₃CH₂-CCCH₂CH₃)L (L = P(C₆H₅)₃ (III), P(CH₃)₃ (IV)) and Os₃(CO)₉(μ₃-η²-CH₃CCCH₃)L (L = P(OCH₃)₃ (V)) are reported. A detailed analysis of the VT-¹H and VT-¹³C NMR of these complexes is presented and compared with the same studies on the parent complexes Os₃(CO)₉(μ₃-η²-RCCR)(μ-CO) (R = CH₂CH₃ (I), CH₃ (II)). In the parent complexes I and II a two stage exchange process is observed: (1) a low energy process involving axial radial exchange at the carbonyl bridged osmium atoms, (2) a higher energy exchange process in which alkyne motion over the face of the metal triangle is coupled with bridge-terminal exchange of the carbonyls, and with axialradial exchange at the unique osmium atom. The phosphine derivatives III-V, all show a three stage exchange process: (1) a localized axial-radial exchange at the unsubstituted osmium atoms; (2) a semibridging terminal carbonyl exchange at the phosphine substituted osmium coupled with a restricted oscillation of the alkyne, pivoted on this osmium atom; (3) free motion of the alkyne and averaging of all the carbonyl groups. The relationship between the differences in the observed dynamic processes are understood by a comparison of the solid state structures of I and III which are also reported. Compound I crystallizes in the triclinic space group P1, with unit cell parameters a 9.292(2), b 15.340(3), c 8.391(2) Å, α 91.27(2), β 116.70 (1), γ 105.24(2)°, V 1017.3(4) ų, and Z = 2. Compound III belongs to the monoclinic space group P2₁/c, with a 14.271(4), b 11.370(2), c 21.192(5) Å, β 104.43(2)³, V 3330(1) ų, and Z = 4. The structures were refined by full matrix least squares to R_F = 0.044, R_wF = 0.058 for I, and R_F = 0.033, R_wF = 0.041 for III, respectively.