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Carbon-13 NMR study and observation of restricted phenyl rotation in the cobalt alkyne complex Co2(CO)6(.mu.-PhC.tplbond.CH)

We have recently reported the application of I3C NMR relaxation time analysis to investigate the phenyl group internal rotation in benzylidyne- and phosphinidene-capped metal carbonyl clusters2-6 with the goal of elucidating the factors affecting electronic interactions between the aryl ring and molecular skeleton. The experimental results have ranged from unhindered phenyl group rotation in Co,(CO)g(p3-CPh) (1) to complete immobility in Co4(CO)lo(cc4-PPh)~ (2) and CpzCo~(r-CO)(CO)3(cc3-CPh) (3). The arrested ring rotation observed in the latter complex was attributed to a strong electronic interaction between the bridging carbonyl's T* orbital and the phenyl a system through the metal 2e and carbynyl 2p orbitals. The same phenomenon has been observed by Stoneet al.' in their low-temperatureNMR