Donor and Acceptor Properties of Ligands

The traditional definition of a coordinate or a dative bond is the donation of a lone pair by the ligand to the metal. There are two important assumptions behind this simple description of a metal-ligand bond. 

since electrons are shared between the metal and the ligand, it is assumed that the bond is basically covalent in character. Second, since the ligand acts as an electron pair donor, it is assumed that the ligand must be a Lewis base and that the metal is the acceptor, i.e., a Lewis acid. 

The formation of stable coordination complexes such as p e(H20) J and [Co(NH3)g] + and many of their physicochemical properties can be adequately explained by an extension of this simple picture. Common Ugands of organometaUic complexes such as CO, ethylene, benzene, and cyclopentadienyl have filled orbitals that interact with empty metal orbitals in a similar fashion. 

However, these Ugands also have low-energy empty orbitals. These low-lying empty orbitals can accept electron density from a filled metal orbital of the correct symmetry and contribute to bonding. In other words, in these complexes the ligands and the metal simultaneously act as Lewis bases and Lewis acids. 

The bond order of the metal-ligand bond is equal to the number of electrons in the bonding orbitals minus that in the antibonding orbitals divided by two. Thus, the bond order is higher when there is back donation than in the case when there is none. 

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