Organometallic/lithium/magnesium compounds and diagonal relationships (burrows)

Question 1

What are organometallic compounds?

Answer 1

Compounds containing one or more bonds between a metal and a carbon.

Question 2

What does the high charge density of organometallic compounds result in?

Answer 2

There is a large degree of covalent character in the bonding.

Question 3

What is the general formula of organolithium compounds?

Answer 3

RLi, where R is an alkyl or aryl group.

Question 4

How can organolithium compounds be formed?

Answer 4

RI + 2Li —> RLi + LiI.

Question 5

What conditions must organolithium compounds be prepared under and why?

Answer 5

An inert atmosphere in a solvent that has been dried as they are air and water sensitive.

Question 6

What are two important organolithium compounds?

Answer 6

Methyllithium and n-butyllithium (MeLi and BuLi).

Question 7

What structure do organolithium compounds exist as?

Answer 7

Oligomers in solution and the solid state.

Question 8

What units does solid methyllithium contain?

Answer 8

[MeLi]4 units.

Question 9

What is the bonding in [MeLi]4?

Answer 9

It is mainly ionic with significant covalent character.

Question 10

What are the properties and structure of dimethylmagnesium?

Answer 10

Its a polymeric solid that contains bridging methyl groups. It has 3-centre 2-electron bonding.

Question 11

What are Grignard reagents?

Answer 11

Organomagnesium compounds where the general formula is RMgX (where X is a halide).

Question 12

How are Grignard reagents formed?

Answer 12

The reaction of a halogenoalkane with magnesium using ethoxyethane (diethyl ether) or tetrahydrofuran as the solvent.

Question 13

What are the requirements for Grignard reagents to be formed and what are their properties?

Answer 13

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.

Question 14

What is the structure of Grignard reagents?

Answer 14

They are usually tetrahedral centres with solvent molecules occupying some of the coordination sites.

Question 15

What is the Schlenk redistribution?

Answer 15

2MgMgBr MgMe2 + MgBr2.

Question 16

What are some of the properties in common between lithium and magnesium?

Answer 16

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.

Question 17

Why do diagonal relationships occur?

Answer 17

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.