Organometallic/lithium/magnesium compounds and diagonal relationships (burrows) Flashcards
What are organometallic compounds?
Compounds containing one or more bonds between a metal and a carbon.
What does the high charge density of organometallic compounds result in?
There is a large degree of covalent character in the bonding.
What is the general formula of organolithium compounds?
RLi, where R is an alkyl or aryl group.
How can organolithium compounds be formed?
RI + 2Li —> RLi + LiI.
What conditions must organolithium compounds be prepared under and why?
An inert atmosphere in a solvent that has been dried as they are air and water sensitive.
What are two important organolithium compounds?
Methyllithium and n-butyllithium (MeLi and BuLi).
What structure do organolithium compounds exist as?
Oligomers in solution and the solid state.
What units does solid methyllithium contain?
[MeLi]4 units.
What is the bonding in [MeLi]4?
It is mainly ionic with significant covalent character.
What are the properties and structure of dimethylmagnesium?
Its a polymeric solid that contains bridging methyl groups. It has 3-centre 2-electron bonding.
What are Grignard reagents?
Organomagnesium compounds where the general formula is RMgX (where X is a halide).
How are Grignard reagents formed?
The reaction of a halogenoalkane with magnesium using ethoxyethane (diethyl ether) or tetrahydrofuran as the solvent.
What are the requirements for Grignard reagents to be formed and what are their properties?
They are air and moisture sensitive so must be prepared under an inert atmosphere using a dry solvent. There is usually a coating of magnesium oxide on magnesium metal that acts as a kinetic barrier which can be overcome by using a small amount of iodine to activate the magnesium.
What is the structure of Grignard reagents?
They are usually tetrahedral centres with solvent molecules occupying some of the coordination sites.
What is the Schlenk redistribution?
2MgMgBr MgMe2 + MgBr2.
What are some of the properties in common between lithium and magnesium?
They both react with N2 to give nitrides, burn in air to form the normal oxides and not peroxides, the carbonates and nitrates both decompose to the oxides on heating, both ions are more heavily hydrated than others in their respective group and both form an extensive number of organometallic compounds that have a large degree of covalent character in the M-C bonds.
Why do diagonal relationships occur?
The different ways in which many atomic properties vary down groups and across periods of the periodic table e.g. atomic and ionic radii decrease down group 1 but decrease across the second period - moving diagonally, these trends cancel each other out.
