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B. The Reaction Barrier — Solvent Molecule Rearrangement

B. The Reaction Barrier — Solvent Molecule Rearrangement

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Bond Activation Reactions



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Figure 13 Ultrafast kinetics of Á5 -CpMn(CO)3 in neat room-temperature triethylsilane after (a) 295 nm; and (b) 325 nm photolysis. In each panel, the transients

are normalized against that of the singlet Á5 -CpMn(CO)2 Et3 SiH to demonstrate a

reduced singlet-to-triplet ratio when excited at a longer (325 nm) wavelength. The

time constants for the exponential fits are also shown in the plots. (Adapted from

Refs. 45 and 46.)



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Yang and Harris



Figure 14 Schematic representation of solvation/spin-crossover process. The

short horizontal bars in the unsolvated triplet surface denote vibrational levels.



The apparent free-energy barriers estimated from these time constants are

8.25 and 10.41 kcal/mol for the Mn and Re reactions, respectively.

The fact that no other intermediates are present in these

reactions suggests that the rate-limiting step is the isomerization

from the ethyl-solvate Á5 -CpM(CO)2 Et3 SiH to the final product Á5 CpM(CO)2 H SiEt3 . The isomerization mechanism can be ascribed to a

dissociative intermolecular process. Evidence that substantiates this picture

includes an enthalpy of activation H‡ act comparable to that of dissociating

an organometallic-alkane complex H‡ diss . The reaction of Á5 -CpMn(CO)2

with Et3 SiH shows a H‡ act ³ 7.9 kcal/mol (42), very close to that

of metal-heptane complexation energy H‡ diss ³ 8 9 kcal/mol (51,52).

Since for a dissociative process the H‡ diss is expected to be a dominating

factor in the isomerization rate, the measured G‡ difference for different

metals should reflect the difference in the enthalpy of complexation. An

MP2 theoretical calculation shows that the Á5 -CpM(CO)2 . . . H3 CCH3

binding energy for M D Mn is 3.5 kcal/mol less than that for M D Re (45).

This is in good agreement with the measured 2.16 kcal/mol difference in

G‡ , thereby providing additional support for the picture that the apparent

rate-limiting step is most likely composed of a dissociative process.



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Bond Activation Reactions



99



Figure 15 Nanosecond kinetics (solid lines) of (a) Á5 -CpMn(CO)3 and

(b) Á5 -CpRe(CO)3 in neat room-temperature triethylsilane after 295 nm photolysis

for the ethyl-solvate intermediates (the decaying traces) and the final products (the

rising traces). (Adapted from Ref. 45.)



C. The Reaction Mechanism — Resolving a Convolved

Chemical Reaction



A comprehensive reaction mechanism composed of the above-discussed

elementary reaction steps is illustrated in Fig. 16. The proposed scheme



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Yang and Harris



Figure 16 A proposed reaction mechanism for the silane Si–H bond activation

by the Á5 -CpM(CO)3 (M D Mn, Re) covering the ultrafast dynamics to nanosecond

kinetics.



allows for a direct experimental assessment of the Si–H bond-breaking

step. The ¾4.4 ps time scale of that step is indicative of a very small

energy barrier, thus providing experimental support for a previous theoretical prediction put forth by Musaev and Morokuma (8). The marked

difference in the bond-breaking barriers for the isoelectronic C–H and

Si–H bond activation reflects their distinct bonding characteristics. The

d orbitals on the Si atom make it easier for the Si–H bond to interact

with the transition metal center both energetically and spatially.

Figure 16 also demonstrates the complexity of a chemical reaction

in the liquid phase. A photochemical reaction such as those discussed in

this section can easily span several orders of magnitude in time from its

initiation to completion, as results of the intricate dynamical processes in



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