INTRODUCTION

 

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Metal carbonyls are formed by complexation of a transition metal atom with carbon monoxide. In some cases carbon monoxide is the only ligand bound to the metal e.g. Mo(CO)6 (CSD reference code FUBYIK) but generally the metal carbonyl contains a mix of ligands e.g. [Mo{1,2-(NH)2C6H4}(CO)2(PPh3)2] (CSD reference code PEPCES).

 

•	The carbonyl ligand is versatile and can bond to the metal atom in a number of ways including in a bridging mode e.g. refcode PALSOK. However for the purposes of this example we are interested only in terminal carbonyls, the most common way in which carbonyls bind to metals.

•	In metal carbonyls it is found that there is a complementary effect whereby the stronger (i.e. shorter) the MC bond, the weaker (i.e. longer) the corresponding CO bond.

•	This effect is known as pi back-bonding.

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The effect of pi back-bonding can be nicely illustrated by performing a search of the Cambridge Structural Database while the explanation for why pi back-bonding occurs is explained by considering the molecular orbitals (specifically the HOMO (highest occupied molecular orbital) and LUMO (lowest occupied molecular orbital)) involved in the MCO bonding.

 

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This module is divided into the following sections:

 

OBJECTIVES

GETTING STARTED

STEPS REQUIRED

ADVANCED EXCERCISES

SUMMARY OF KEY CONCEPTS