Rate and equilibrium constants for the nucleophilic addition of MeOin methanol and in 90% MeCN-10% MeOH, of HCtCCH 2 Oand CF 3 CH 2 Oin 50% MeCN-50% water, and of OHin 50% MeCN-50% water (rate constants only) to Fischer carbene complexes of the type (CO) 5 MdC(OMe)C 6 H 4 -4-Z (M ) Cr and W) are reported. The reactions lead to the formation of tetrahedral adducts of the type (CO) 5 M h -C(OMe)(OR)C 6 H 4 -4-Z. The kinetics of general acid catalyzed loss of MeOfrom the MeOadducts by phenol, 4-bromophenol, and 3,5-dichlorophenol, to regenerate the carbene complex, were also investigated. Hammett F and F n values as well as approximate β nuc (β nuc n ) values were determined. They suggest that the transition state for alkoxide ion addition to the carbene complexes is imbalanced in the sense that charge delocalization into the CO ligands lags somewhat behind C-O bond formation. This finding is consistent with the fact that the intrinsic rate constants for these reactions are substantially lower than those for alkoxide ion addition to carboxylic esters. Another type of imbalance is seen for the transition state of the general acid catalyzed MeOdeparture from the MeOadducts. Here proton transfer to the leaving group is ahead of C-O bond cleavage.

Gas-phase acidities of CH2=C=X (X = CH2, NH, O, and S) and barriers for the identity proton transfers (X=C=CH2 + HC triple bond C-X- right harpoon over left harpoon -X-C triple bond CH + CH2=C=X) as well as geometries and charge distributions of CH2=C=X, HC triple bond C-X- and the transition states of the proton transfer were determined by ab initio methods at the MP2/6-311+G(d,p)//MP2/6-311+G(d,p) and B3LYP/6-311+G(d,p) levels of theory. The acidities were also calculated at the CCSD(T)/6-311+G(2df,p) level. A major objective of this study was to examine how the enhanced unsaturation of CH2=C=X compared to that of CH3CH=X may affect acidities, transition state imbalances, and intrinsic barriers of the identity proton transfer. The results show that the acidities are all higher while the barriers are lower than for the corresponding CH3CH=X series. The transition states are all imbalanced but less so than for the reactions of CH3CH=X.

A kinetic study of the reactions of (CO) 5 CrdC(SMe)C 6 H 4 Z (Z ) 4-CF 3 , 4-Cl, 4-F, 4-Me, 4-MeO, and 4-Me 2 N) with n-butylamine, 2-chloroethylamine, piperidine, and 1-(2-hydroxyethyl)piperazine in 50% MeCN-50% water (v/v) at 25 °C is reported. In all cases the nucleophilic attachment of the amine to the carbene complex is rate limiting, which contrasts with the previously reported aminolysis of the methoxy analogues ((CO) 5 CrdC(OMe)C 6 H 4 Z), where leaving group departure is rate limiting in most cases. The substituent effect on the nucleophilic attachment step is much weaker (average F ) 0.55) than for the methoxy analogue (F ) 2.03). It is shown that the small F value for (CO) 5 CrdC(SMe)C 6 H 4 Z is "normal" and reflects the near cancellation of the substituent effects on the partial positive and partial negative charges at the transition state. These findings provide evidence that the high F value for (CO) 5 CrdC(OMe)C 6 H 4 Z is "abnormal" and the result of the strong π-donor effect of the methoxy group on the reactant carbene complex.

Gas-phase acidities of CH3Y (Y: NO, C identical to CH, CH=NH, and CH=S), barriers to the identity proton-transfer CH3Y + CH2=Y- reversible CH2=Y- + CH3Y, as well as geometries and charge distributions of CH3Y, CH2=Y- and the transition states of the proton transfers were determined by ab initio methods at the MP2/6-311 + G(d,p)//MP2/6-311 + G(d,p), B3LYP/6-311 + G(d,p), and BPW-91/6-311 + G-(d,p) levels of theory. The acidities were also calculated at the CCSD(T)/6-311 + G(2df,2p) level. To make more meaningful comparisons, the same quantities for previously studied systems (Y: H, CH=CH2, CH=O, CN, NO2) were recalculated at the levels used in the present work. The geometric parameters as well as the group charges indicate that the transition states for all the reactions are imbalanced, although there is no correlation between the degree of imbalance and the pi-acceptor strength of the Y group. Based on multi-parameter correlations with the field (sigma F), resonance (sigma R), and polarizability effect (sigma alpha) substituent constants, the contributions of each of these effects to the acidities and barriers were evaluated. For the Y groups whose sigma F, sigma R, and sigma alpha are unknown (CH=NH, CH=S, C identical to CH), a method for estimating these substituent constants is proposed. The barriers for the CH3Y/CH2=Y- systems are all lower than for the CH4/CH3- system; this contrasts with the situation in solution where the Y groups lead to an increase in the barrier. The reasons for this reversal are analyzed. We also make an attempt to clarify the issue as to why the transition states of these reactions are imbalanced, a question which continues to draw attention in the literature.

A kinetic study of the hydrolysis of (CO) 5 MdC(SMe)Ph (M ) Cr and W) in 50% MeCN-50% water (v/v) at 25°C over a pH range from 1.7 to 14.2 is reported. The reaction occurs in two stages: the first is formation of (CO) 5 MdC(O -)Ph or (CO) 5 MdC(OH)Ph while the second stage leads to the formation of PhCHdO and (CO) 5 MOH -. This paper is concerned with the first stage. The rate-pH profiles are complex and consistent with a mechanism (Scheme 1) that involves water/OH -addition to the substrate to form a tetrahedral intermediate (T OH -), followed by product formation via five potential pathways whose relative importance depends on the pH. A more limited study of the reactions of (CO) 5 MdC(SCH 2 CH 2 CH 3 )Ph (M ) Cr and W) and (CO) 5 MdC(SMe)C 6 H 4 -3-Cl (M ) Cr and W) with OH -is also reported. The main focus of the discussion is aimed at understanding the reactivity differences between (CO) 5 MdC(SMe)Ph and the corresponding methoxy analogues studied earlier. This understanding is in large measure based on an analysis of how the intrinsic rate constants are affected by the interplay of steric, inductive, and π-donor effects and the potential imbalances of these effects at the transition state. It is also shown that the much lower sensitivity to H + -catalysis of RS -compared to RO -leaving group departure from the tetrahedral intermediate is responsible for the more complex rate-pH profile for the hydrolysis of (CO) 5 MdC(SMe)Ph than for the hydrolysis of the methoxy analogue. IntroductionA kinetic study of the hydrolysis of the Fischer type transition metal carbene complexes 1-M, 2 2-M 2 , and 3-Cr in 50%