Metal ions preferentially bind certain ligands. Ligands and metal ions can be ordered in many ways one ranking system focuses on ligand 'hardness' (see also hard/soft acid/base theory). Bonding is often described using the formalisms of molecular orbital theory. This description has been semi-quantified in many ways, e.g. In general, ligands are viewed as electron donors and the metals as electron acceptors, i.e., respectively, Lewis bases and Lewis acids. He resolved the first coordination complex called hexol into optical isomers, overthrowing the theory that chirality was necessarily associated with carbon compounds. The theory allows one to understand the difference between coordinated and ionic chloride in the cobalt ammine chlorides and to explain many of the previously inexplicable isomers. The first to use the term "ligand" were Alfred Werner and Carl Somiesky, in relation to silicon chemistry. He showed, among other things, that the formulas of many cobalt(III) and chromium(III) compounds can be understood if the metal has six ligands in an octahedral geometry. The key breakthrough occurred when Alfred Werner reconciled formulas and isomers. The composition of coordination complexes have been known since the early 1800s, such as Prussian blue and copper vitriol. The size of a ligand is indicated by its cone angle. Ligands are classified in many ways, including: charge, size (bulk), the identity of the coordinating atom(s), and the number of electrons donated to the metal ( denticity or hapticity). Ligand selection is a critical consideration in many practical areas, including bioinorganic and medicinal chemistry, homogeneous catalysis, and environmental chemistry.
Ligands in a complex dictate the reactivity of the central atom, including ligand substitution rates, the reactivity of the ligands themselves, and redox. Metals and metalloids are bound to ligands in almost all circumstances, although gaseous "naked" metal ions can be generated in a high vacuum. Ligands are viewed as Lewis bases, although rare cases are known to involve Lewis acidic "ligands". Furthermore, the metal–ligand bond order can range from one to three. The nature of metal–ligand bonding can range from covalent to ionic. The bonding with the metal generally involves formal donation of one or more of the ligand's electron pairs often through Lewis Bases.
In coordination chemistry, a ligand is an ion or molecule ( functional group) that binds to a central atom to form a coordination complex. Cobalt complex HCo(CO) 4 with five ligands
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