Reactions of Coordination Compounds



 

Learning Objective

  • Discuss the common reaction classes of coordination compounds.

Key Points

    • A coordination complex, or metal complex, consists of an atom or ion (usually metallic) and a surrounding array of molecules or anions called ligands or complexing agents.
    • A coordination compound is any molecule that contains a coordination complex.
    • The donor atom is the atom within a ligand that is bonded to the central atom or ion.
    • Coordination complexes can undergo a variety of reactions, including electron transfer, ligand exchange, and associative processes.

Terms

  • coordinationThe reaction of one or more ligands with a metal ion to form a coordination compound.
  • redoxA reversible chemical reaction in which one reaction is an oxidation and the reverse is a reduction.
  • donor atomThe atom within a ligand that is bonded to the central atom or ion within a coordination complex.

In chemistry, a coordination or metal complex consists of an atom or ion (usually metallic) and a surrounding array of bound molecules or anions known as ligands or complexing agents. Many metal-containing compounds consist of coordination complexes.

CisplatinThis complex, PtCl2(NH3)2, is an anti-tumor drug and an example of a coordination complex.

Structure of Coordination Complexes

Donor Atom

The atom within a ligand that is bonded to the central atom or ion is called the donor atom. A typical complex is bound to several donor atoms, which can be same or different elements.

Polydentate (multiple bonded) ligands consist of several donor atoms, several of which are bound to the central atom or ion. These complexes are called chelate complexes, the formation of which is called chelation, complexation, and coordination.

EDTA chelationThe EDTA molecule has six different donor atoms that form the complex.

Ligands

The ions or molecules surrounding the central atom are called ligands. These are generally bound to the central atom by a coordinate covalent bond (donating electrons from a lone electron pair into an empty metal orbital). There are also organic ligands such as alkenes whose pi (π) bonds can coordinate to empty metal orbitals. An example is ethene in the complex known as Zeise’s salt, K+[PtCl3(C2H4)].

The central atom or ion, together with all ligands, comprise the coordination sphere. The central atoms or ion and the donor atoms comprise the first coordination sphere. Coordination refers to the coordinate covalent bonds (dipolar bonds) between the ligands and the central atom.

Originally, a complex implied a reversible association of molecules, atoms, or ions through such weak chemical bonds. As applied to coordination chemistry, this meaning has evolved. Some metal complexes are formed virtually irreversibly and many are bound together by bonds that are quite strong.

Reactivity

Electron Transfers

A common reaction between coordination complexes involving ligands are electron transfers. There are two different mechanisms of electron transfer redox reactions: inner sphere or outer sphere electron transfer. In electron transfer, an electron moves from one atom to another, changing the charge on each but leaving the net charge of the system the same.

Ligand Exchange

One important indicator of reactivity is the rate of degenerate exchange of ligands. For example, the rate of interchange of the coordinate water in [M(H2O)6]n+ complexes varies over 20 orders of magnitude. Complexes where the ligands are released and rebound rapidly are classified as labile. Such labile complexes can be quite stable thermodynamically. Typically they either have low-charge (Na+), electrons in d orbitals that are antibonding with respect to the ligands (Zn2+), or lack covalency (Ln3+, where Ln is any lanthanide).

The lability of a metal complex also depends on the high-spin vs. low-spin configurations when such is possible. Thus, high-spin Fe(II) and Co(III) form labile complexes, whereas low-spin analogues are inert.

Associate Processes

Complexes that have unfilled or half-filled orbitals often show the capability to react with substrates. Most substrates have a singlet ground-state; that is, they have lone electron pairs (e.g., water, amines, ethers). These substrates need an empty orbital to be able to react with a metal center. Some substrates (e.g., molecular oxygen) have a triplet ground state. Metals with half-filled orbitals have a tendency to react with such substrates. If the ligands around the metal are carefully chosen, the metal can aid in (stoichiometric or catalytic) transformations of molecules or be used as a sensor.