Properties
Organocopper compounds are very reactive towards oxygen and water forming copper(I) oxide and tend to be thermally unstable. Because most cuprates are salts, many are generally insoluble in nonpolar solvents. Despite these difficulties, organocopper reagents are frequently generated and consumed in situ with no attempt to isolate them. They are used very frequently in organic chemistry as alkylating reagents because they exhibit more functional group tolerance than corresponding Grignard and organolithium reagents. The electronegativity of copper is much higher than its next-door neighbour in the group 12 elements, zinc, suggesting less nucleophilicity for its carbon ligands.
Copper belongs to the group of coinage metals together with silver and gold and their chemistries have many similarities. The oxidation state can be +1 or +2 and intermediates can have oxidation state +3. Monovalent alkylcopper compounds (RCu) are polymeric but form cuprates (R2CuLi) upon treatment with organolithium compounds (RLi). These cuprates are sometimes referred to as Gilman reagents. Organocopper compounds can be stabilized by complexation to a variety of ligands such as organophosphanes (R3P), thioethers (R2S), and cyanide (CN-).
The cuprates complex form complicated aggregates both in crystalline form and in solution. Lithium dimethylcuprate is a dimer in diethyl ether forming an 8-membered ring with two lithium atoms coordinating between two methyl groups. Similarly, lithium diphenylcuprate forms a dimeric etherate, 2, in the solid state.
The first ever crystal structure was determined in 1972 by Lappert for CuCH2SiMe3. This compound is relatively stable because the bulky trimethylsilyl groups provide steric protection. It is a tetramer forming an 8-membered ring with alternating Cu-C bonds. In addition the four copper atoms form a planar Cu4 ring based on three-center two-electron bonds. The copper to copper bond length is 242 pm compared to 256 pm in bulk copper. In pentamesitylpentacopper a 5-membered copper ring is formed, similar to (2,4,6-Trimethylphenyl)gold, and pentafluorophenylcopper is a tetramer.
With carbon monoxide copper forms a non-classical metal carbonyl.
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