Counterions and Solvents in Substitution

Photolytic reductive elimination of H2 can be followed by oxidative addition of a solvent C-H bond (Eq. 4.42). 

Solvents and counterions can be coordinating and must be chosen so as not to interfere with substitution. Common solvents that are most likely to bind are MeCN, pyridine, Me2SO (dimethylsulfoxide, DMSO), and Me2NCHO (dimethylformamide, DMF). Several complexes dissolve only in such solvents, as a result of the solvent binding to the metal. DMF, Me2NCHO, bonds via the O lone pair because the N lone pair is tied up by resonance (Me2N =CH-0 ). 

DMSO can bind either via the S or the O depending on both steric and hard and soft effects. Unhindered, soft Ru(II) gives S-bound 

Tetrahydrofuran (THF), acetone, water, and ethanol are less strongly ligating and often used for late metals. Early d° complexes can react with protic solvents, however. Ketones usually bind in the 

Halocarbon solvents tend to be oxidizing and can destroy sensitive compounds. PhCF3 is a useful, less oxidizing alternative to CH2CI2. Halocarbons can form stable complexes, some of which have been crystallographically characterized, such as [IrH2(IMe)2(PPh3)2]+. 

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