Alkene complexes stability

Thus, the relative reactivity is ptrallel to the order of rhodium/alkene complex stability and the insertion rate whidi can be estimated by the LUMO oiersy level of alkenes. Howeva, the additions to trisubstituted unsaturated ketones and esters are significantly slow. The rhodium oanplexes catalyze a similar reaction of 1-alkenylboronic acids, but all attempts at the addition of alkylboronic acids or trialkylboranes having a P-hydrogen are unsuccessful. 

The 7r-back donation stabilizes the alkene-metal 7c-bonding and therefore this is the reason why alkene complexes of the low-valent early transition metals so far isolated did not catalyze any polymerization. Some of them catalyze the oligomerization of olefins via metallocyclic mechanism [25,30,37-39]. For example, a zirconium-alkyl complex, CpZrn(CH2CH3)(7/4-butadiene)(dmpe) (dmpe = l,2-bis(dimethylphosphino)ethane) (24), catalyzed the selective dimerization of ethylene to 1-butene (Scheme I) [37, 38]. 

Terminal RCH—CH2 1-Hexene C4H9CH=CH2 is isomerized by complex 1 in accordance with the factors influencing the thermodynamic stability of cis- and trans-2 -hexene [15], At the end of the reaction, the alkyne complex 1 was recovered almost quantitatively. No alkene complexes or coupling products were obtained. The corresponding zirconocene complex 2a did not show any isomerization activity. Propene CH3CH=CH2 reacts with complex 6 with substitution of the alkyne and the formation of zirconacydopentanes as coupling products, the structures of which are non-uniform [16]. 

It should be noted that not the final stabilities, but rather the intermediates and transition states determine which isomer is formed. In the precursor alkene complex calculations show that already the respective a and (3 carbon atoms occupy the positions closest to the plane of coordination and that the respective barriers in both cases are indeed the lowest in the model studied [12],... 

Alkenes bonded to platinum(II) can be displaced by strongly coordinating ligands such as cyanide ion or tertiary phosphines. The displacement of ethylene from Zeise s salt by phosphines is a useful method of preparation of complexes trawa-PtCl2(PR3)2.711 Amines will also displace alkenes from coordination to platinum(II), but this reaction can compete with nucleophilic attack at the coordinated alkenic carbon. The stability of platinum(II) alkene complexes follows the sequence C2H4 > PhCH=CH2 > Ph2C=CH2 555 Ph(Me)C=CH2.712... 

Thermochemical data correspond with a decrease in the platinum(0)-alkene bond strength ir the sequence C2H4 > PhCH=CH2 > cis-PhCH=CHPh > frans-PhCH==CHPh. Displacemem reactions show an expanded stability order for platinum(O) complexes to be TCNE> PhCfeCH > alkenes.801 The relative weakness of alkene complexes relative to alkyne com plexes of platinum(O) is the reverse of that found with platinum(II).802... 

Cationic iron-alkene complexes also participate in an unusual cycloaddition process, wherein electron-deficient alkenes are attacked by nucleophilic o -allylic Fp complexes, generating stabilized carb-anions and cationic alkene-iron complexes. Attack of the carbanion on the alkene forming a five-membered ring completes this process (Scheme 13). Oxidative removal of the iron produces useful organic compounds.19-21... 

Complexes with alkenes and arenes are formed when the hydrocarbons are shaken with aqueous solutions of silver(I) salts. Di- or polyalkenes often give crystalline compounds with Ag+ bound to one to three double bonds. The formation of alkene complexes of varying stability may be used for the purification of alkenes, or for the separation of isomeric mixtures (e.g., 1,3-, 1,4-, and 1,5-cyclooctadienes), or of the optical isomers of a- and /3-pinene. There is very little back-bonding contribution in the formation of Ag1 rr-complexes. For example, the planar complex (hfa)Ag(Ph-C= C-Ph) contains an almost linear acetylene ligand with a C=C... 

Alkene insertions into M—CR3 bonds proceed via a similar transition state, with the CR3 moiety tilting towards the terminal alkene-C, with retention of configuration of the CR3 group. Note that in either case in the M(R)(alkene) complex two coordination sites are involved, whereas in the product only one is occupied. For this reason the stability of alkyls depends on blocking this second coordination site to prevent the reverse reaction. 

Simple alkene complexes, where one aUcenic group replaces a carbonyl in (1) or (2) are unstable, as already stated, and have been characterized only in solution. For example, the monoethylene complex Ru(CO)4() -C2H4) can be made photochemically in solution by irradiating (1) or (2) in the presence of an excess of ethylene. This compound decomposes below room temperature but its stability is enhanced in the presence of excess CO presumably CO dissociation is the initial step of the decomposition. The compound serves as an intermediate for the preparation of more stable alkene complexes from electron-deficient alkenes by simple exchange at low temperature in solution. Ru(CO)4(jj-alkene) complexes with electrophilic alkenes can also be isolated from Ru(CO)4(jj-C2H4) in this way. 

---