Carbonyl exchange, bridge-terminal

B. E. Hanson, M. J. Sullivan, R. J. Davis, Direct Evidence for Bridge Terminal Carbonyl Exchange in Solid Dicobalt Octacarbonyl by Variable-Temperature Magic Angle Spinning C-13 NMR-Spectroscopy. J. Am. Chem. Soc. 1984, 106, 251-253. 

The phenomenon of bridge-terminal carbonyl exchange can occur in any dimetal system that can exist in CO-bridged and metal-metal bonded forms, such as COjfCO), ... 

Structure correlation plots for the bridging— terminal carbonyl exchange. The plots have been adopted from References [27,28]. 

From the values of T,M and B calculated in this way we may note that for both iron and cobalt M 0.68 T and that B 0.5 T. The latter approximation is intuitively consistent with the observations22 of bridge-terminal exchange of coordinated CO. If we now assume that these empirical approximations apply to other polynuclear carbonyls, it is possible to extract the T and M- values for metals other than iron and cobalt, despite the limited AHo data which are available. 

Figure 22 EXSY spectrum showing stereospecific exchange correlations between bridging and terminal carbonyls in [Rli6(CO)i5 P(4-(MeO)C6H4)3 ], inCDClj at 274 K = 0.02 s...

Fig. 2. Proposed synchronous mechanism for the exchange of the bridging and terminal carbonyl ligands of (CsHsljRhjtCOl L derivatives.

One process which can explain the observed variable temperature NMR behavior involves a net rotation of the bridging methylene unit about the M-C-M axis by 180° to regenerate an equivalent conformer wherein P3rrazoles A and A, and B and B, have exchanged p3rrazoles C and C are imchanged. As the methylene rotates freely about the M-C-M axis, the four off-axis pyrazoles all become equivalent, as do all four terminal carbonyls the pyrazoles along the axis remain distinct. 

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