Can spin state change slow organometallic reactions?
- Univ. of Delaware, Newark, DE (United States)
Diamagnetic Tp{sup i-R,Me}Co(CO){sub 2} (Tp{sup i-Pr,Me} = hydrotris(3-iso-propyl-5-methylpyrazolyl)borato) has been prepared and structurally characterized by X-ray diffraction. In solution it spontaneously loses carbon monoxide to yield paramagnetic Tp{sup i-Pr, Me}Co(CO), a complex with a triplet spin ground state. In the presence of CO, the two complexes are in equilibrium, and measurements of the temperature dependence of the equilibrium constant by variable temperature IH NMR spectroscopy have been used to determine the thermochemical parameters {Delta}H = -12.9(2) kcal and {Delta}S = -32(1) eu. The CO exchange reaction is fast on the NMR time scale at ambient temperature, but it can be frozen out below 224 K, the temperature of coalescence. At 224 K the rate constants for the dissociation and binding of CO were k{sub -1} = 6(1) x 10{sup 3} s{sup -1} and k{sub 1} = 3(1) x 10{sup 9} M{sup -1} s{sup -1}, respectively. This reaction represents an interconversion between a triplet and a singlet molecule by a typical organometallic two-electron process, and it is exceedingly fast. Based on this observation it is argued that the notion of `spin-blocking` of organometallic reactions is inappropriate. 18 refs., 4 figs., 2 tabs.
- Sponsoring Organization:
- USDOE
- OSTI ID:
- 171481
- Journal Information:
- Journal of the American Chemical Society, Vol. 117, Issue 47; Other Information: PBD: 29 Nov 1995
- Country of Publication:
- United States
- Language:
- English
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