Electrophilic ligand addition

The effect of metal basicity on the mode of reactivity of the metal-carbon bond in carbene complexes toward electrophilic and nucleophilic reagents was emphasized in Section II above. Reactivity studies of alkylidene ligands in d8 and d6 Ru, Os, and Ir complexes reinforce the notion that electrophilic additions to electron-rich compounds and nucleophilic additions to electron-deficient compounds are the expected patterns. Notable exceptions include addition of CO and CNR to the osmium methylene complex 47. These latter reactions can be interpreted in terms of non-innocent participation of the nitrosyl ligand. 

Addition of electrophiles to a coordinated benzenoid ligand is a characteristic reaction of 20-e complexes. [Co(C5H5BPh)2] in 59 is readily protonated at C-2, producing complex 79 (60) which alternatively can be obtained by hydride addition to cation 61 (69). Formal electrophilic addition is also observed in the reaction of [Co(C5H5BMe)2] in 58 with Phi which affords the two isomeric phenylation products 80 and 81 (60). 

The oxidation of Pt(II) complexes is thought to proceed in most cases via addition of an electrophile to the Pt(II) center (141,178-182). This process does not involve outer-sphere electron transfer. The initial product of the electrophile addition is possibly a five-coordinate Pt(IV) species, however, the observable Pt(IV) product is six-coordinate. Coordination of a sixth ligand, e.g. solvent, occurs (183). This may proceed concertedly with the addition of the electrophile, in particular if the sixth ligand is solvent, or alternatively after the addition of the electrophile, as shown in Scheme 35. 

Exposure of alkenes to a combination of 4 and trimethylsilyl isothiocyanate leads to the formation of 1,2-dithiocyanates [Eq. (8)] [28]. The reaction involves the formation of bis(thiocyanato)-A3-iodane by a ligand exchange. The decomposition of this iodane leads to the formation of thiocyanogen, which in turn undergoes the anti electrophilic addition to olefins. 

Another possibility found by TAMREAC is the electrophilic addition of the proton to the coordinated ethylene. The product, intermediate 7, is a neutral one (here also the initial coordination by a solvent ligand is necessary). According to Cramer s study, all the intermediates are anionic ones, so the chemist could discard this possibility. 

Recent structural and spectroscopic investigations of organometallic complexes bonding two carbons of an allenic ligand to one rhodium 50 72> 87,95) or platinum atom 58,87,98,132) may have some pertinence to possible bridged intermediates proposed for various electrophilic additions to allenes, and the cr-iron-jr-iron complexes derived from allene and diiron... 

Nucleophilic and electrophilic additions and oligomerization (cluster formation) have been demonstrated. Unusual coordination geometries and bonding may be adopted by the coordinated ligands. These in turn may lead to new reaction types for these ligands. 

If concentrations of carbenium ions are too low to be observed directly, they must be detected indirectly in kinetic studies of the racemiza-tion of optically active dormant species, ligand exchange and/or detailed studies of the effect of substituents, solvent and salts. Some of the most convincing and elegant work in this area was presented in Chapter 2 using primarily benzhydryl derivatives. As discussed in the next section, correlations between ionization rates and equilibrium constants, rates of solvolysis and rate constants of electrophilic addition can be interpolated and in some cases extrapolated to cationic polymerizations of alkenes to evaluate the reactivities of various active species and the dynamics of their isomerization. 

The parent TMM complex (190 R = H) undergoes photochemical ligand substitution with trifluorophosphine or trimethylamine Al-oxide assisted substitution with tertiary phosphines or t-butyl isocyanide (Scheme 5A) Trimethylamine A-oxide assisted substitution using isoprene as the incoming ligand results in C-C bond formation to afford the bis-TT-allyl complex (197). An intramolecular version of this reaction is also known.The parent complex (190 R = H) reacts with electrophiles. Addition of HCl or Br2 gives the methallyl complexes (192) and (198), respectively. Tetrafluoroethylene adds across the Fe bond to afford (199) under photochemical conditions. Complex (190) undergoes Friedel-Crafts-type acylation with... 

---