coordination chemistry A coordination complex consists of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of chemical bond, bound molecules or ions, that are in turn known as ''ligands'' or complexing ...

, a spectator ligand is a

ligand In coordination chemistry, a ligand is an ion or molecule ( functional group) that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's ele ...

that does not participate in chemical reactions of the complex. Instead, spectator ligands (vs "actor ligands") occupy coordination sites. Spectator ligands tend to be of

polydentate In coordination chemistry, denticity () refers to the number of donor groups in a given ligand that bind to the central metal atom in a coordination complex. In many cases, only one atom in the ligand binds to the metal, so the denticity equals ...

, such that the M-spectator ensemble is inert kinetically. Although they do not participate in reactions of the metal, spectator ligands influence the reactivity of the metal center to which they are bound. These ligands are sometimes referred to as ancillary ligands. Several different classes of ligand exist that can be considered spectator ligands. A few examples include

trispyrazolylborate In inorganic chemistry, the trispyrazolylborate ligand, abbreviated Tp−, is an anionic tridentate and tripodal ligand. Trispyrazolylborate refers specifically to the anion B(C3N2H3)3sup>−, but the term trispyrazolylborate refers to derivativ ...

s (Tp),

cyclopentadienyl Cyclopentadienyl can refer to * Cyclopentadienyl anion, or cyclopentadienide, **Cyclopentadienyl ligand A cyclopentadienyl complex is a coordination complex of a metal and cyclopentadienyl anion, cyclopentadienyl groups (, abbreviated as Cp−) ...

ligands (Cp), and many chelating diphosphines such as

1,2-bis(diphenylphosphino)ethane 1,2-Bis(diphenylphosphino)ethane (dppe) is an organophosphorus compound with the formula (PhPCH) (Ph = phenyl). It is a commonly used bidentate ligand in coordination chemistry. It is a white solid that is soluble in organic solvents. Preparation ...

ligands (dppe). Varying the substituents on the spectator ligands greatly influences the solubility, stability, electronic, and steric properties of the metal complex. In the area of

platinum-based antineoplastic Platinum-based antineoplastic drugs (informally called platins) are chemotherapeutic agents used to treat cancer. They are coordination complexes of platinum. These drugs are used to treat almost half of people receiving chemotherapy for cancer. I ...

agents, spectator (and nonspectator) ligands greatly affect efficacy.Timothy C. Johnstone, Kogularamanan Suntharalingam, Stephen J. Lippard "The Next Generation of Platinum Drugs: Targeted Pt(II) Agents, Nanoparticle Delivery, and Pt(IV) Prodrugs" Chem. Rev., 2016, volume 116, pp 3436–3486.

References

{{reflist Coordination chemistry Organometallic chemistry