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

The synthesis of the phosphine substituted complexes Os3(CO)9(.mu.3-.eta.2-CH3CH2CCCH2CH3)L [L = PPh3 (I), PMe3 (II)] and Os3(CO)9(.mu.3-.eta.2-CH3CCCH3)L [L = P(OMe)3 (III)] are reported. A detailed anal. of the 1H and 13C NMR of these complexes is presented and compared with the same studies on the parent complexes Os3(CO)9(.mu.3-.eta.2-RCCR)(.mu.-CO) [R = Et(IV), Me(V)]. In the parent complexes IV and V a two stage exchange process is obsd.: (1) a low energy process involving axial radical 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 axial-radical exchange at the unique osmium atom. The phosphine derivs. I-III, 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 obsd. dynamic processes are understood by a comparison of the solid state structures of I and IV which are detected by x-ray crystallography.