Thiomersal and commonly known in the US as thimerosal, is an organomercury

Organomercury

Organomercury refers to the group of organometallic compounds that contain mercury. Typically the Hg-C bond is stable toward air and moisture but sensitive to light. Important organomercury compounds are the methylmercury cation, CH3Hg+; ethylmercury cation, C2H5Hg+; dimethylmercury, 2Hg,...

 compound. This compound is a well established antiseptic

Antiseptic

Antiseptics are antimicrobial substances that are applied to living tissue/skin to reduce the possibility of infection, sepsis, or putrefaction...

 and antifungal agent.

The pharmaceutical corporation Eli Lilly and Company

Eli Lilly and Company

Eli Lilly and Company is a global pharmaceutical company. Eli Lilly's global headquarters is located in Indianapolis, Indiana, in the United States...

 gave thiomersal the trade name Merthiolate and it has been used as a preservative

Preservative

A preservative is a naturally occurring or synthetically produced substance that is added to products such as foods, pharmaceuticals, paints, biological samples, wood, etc. to prevent decomposition by microbial growth or by undesirable chemical changes....

 in vaccine

Vaccine

A vaccine is a biological preparation that improves immunity to a particular disease. A vaccine typically contains an agent that resembles a disease-causing microorganism, and is often made from weakened or killed forms of the microbe or its toxins...

s, immunoglobulin preparations, skin test antigens, antivenin

Antivenin

Antivenom is a biological product used in the treatment of venomous bites or stings. Antivenom is created by milking venom from the desired snake, spider or insect. The venom is then diluted and injected into a horse, sheep or goat...

s, ophthalmic

Ophthalmology

Ophthalmology is the branch of medicine that deals with the anatomy, physiology and diseases of the eye. An ophthalmologist is a specialist in medical and surgical eye problems...

 and nasal products, and tattoo

Tattoo

A tattoo is made by inserting indelible ink into the dermis layer of the skin to change the pigment. Tattoos on humans are a type of body modification, and tattoos on other animals are most commonly used for identification purposes...

 inks. Its use as a vaccine preservative is controversial

Thiomersal controversy

The thiomersal controversy describes claims that vaccines containing the mercury-based preservative thiomersal contribute to the development of autism and other brain development disorders...

, and it is being phased out from routine childhood vaccines in the United States, the European Union

European Union

The European Union is an economic and political union of 27 independent member states which are located primarily in Europe. The EU traces its origins from the European Coal and Steel Community and the European Economic Community , formed by six countries in 1958...

, and a few other countries.
Thiomersal features mercury(II) with a coordination number 2, i.e. two ligands are attached to Hg, the thiolate and the ethyl group.