X-type ligand

In general, any organometallic complex containing L- and X-type ligands may be represented by the general formula... 

A generalized X-type ligand (1 e in the neutral ligand counting method)... 

In the fictitious dissociation that we are considering, a ligand L leaves with the two electrons that it had supplied, so the number of electrons on the metal is not changed in any way. However, an X-type ligand, which had supplied only a single electron to make the bond, leaves in its anionic form X , carrying the two electrons from the bond with it. It therefore removes an electron from the metal, that is, it oxidizes it by one unit. The result of this dissociation is therefore written ... 

The oxidation state (no) of the metal in the complex is therefore equal to the algebraic sum of the number of X-type ligands and the charge on the complex ... 

The additional electron supplied to an X-type ligand to transform it into a Lewis base comes, of course, firom the metal. The metal ligand ensemble is therefore described as an X ligand interacting with a metallic cation M+, thereby giving a purely ionic description of the metal-ligand bond. As a consequence, a complex which was written ML X in the covalent model is represented, in the ionic model, as a metallic cation of charge x bound to (i + x) Lewis bases (1.5). 

The case where it is clearest that only one orbital need be considered involves the ligand H, since it possesses only one valence orbital, This orbital, which contains one electron (an X-type ligand), can be used to form a cr —h bond by combination with a metal orbital such as the orbital (1-32). ... 

By writing a Lewis structure, show that C=N is an X-type ligand. 

The elimination of the molecule R —R (R and R are X-type ligands such as H, alkyl, halogen, etc.) is a decomposition mode that is frequently found for organometallic complexes [L MRR ] (4-52). 

The removal of two X-type ligands leads to a decrease of two units in the metal s oxidation state the electronic configuration changes from d to 4" . The metal is therefore reduced, which explains the name reductive eliniination . The opposite reaction is called oxidative addition . 

An X-function ligand is one which interacts with a metal center via a normal 2-electron covalent bond, composed of 1 electron from the metal and 1 electron from the X ligand. As such, an X-function ligand donates one electron to a metal center. Since the metal uses one electron in forming the M-X bond, each X-function ligand raises the valence of the metal center by one unit. Simple examples of X-type ligands include H and CH3, i.e. radicals. 

A methyl group is considered to "donate" two electrons and to be an "anionic, two-electron" ligand by the first classification system and is considered to "donate" one electron and to be an X-type ligand by tine second classification system. 

Covalent (X-Type) Ligands Bound Through Metal-Carbon and Metal-Hydrogen Bonds... 

CHAPTER 3 covalent (x-TYPE) LIGANDS BOUND THROUGH METAL-CARBON AND METAL-HYDROGEN BONDS... 

---