In coordination chemistry, the term scorpionate ligand refers to a tridentate (three-donor-site) ligand which would bind to a metal in a fac manner. The most popular class of scorpionates are the hydrotris(pyrazolyl)borates or Tp ligands. These were also the first to become popular. These ligands first appeared in journals in 1966 from the then little-known DuPont chemist of Ukrainian descent, Swiatoslaw Trofimenko. Trofimenko called this discovery "a new and fertile field of remarkable scope". The term scorpionate comes from the fact that the ligand can bind a metal with two donor sites like the pincers of a scorpion; the third and final donor site reaches over the plane formed by the metal and the other two donor atoms to bind to the metal. The binding can be thought of as being like a scorpion grabbing the metal with two pincers before stinging it. While many scorpionate ligands are of the Tp class, many other scorpionate ligands are known. For example, the Tm and tripodal phosphine classes have an equally good claim to be scorpionate ligands. Many of the scorpionate ligands have a central boron atom which bears a total of four groups, but it is possible to create ligands which use other central atoms. Homoscorpionates vs. Heteroscorpionates Trofimenko's initial work in the field was with the homoscorpionates where three pyrazolyl groups are attached to a boron. Since this work a range of ligands have been reported where more than one type of metal binding group is attached to the central atom; these are the heteroscorpionates. Many other chemists continue to explore the possibilities of scorpionate ligand alternatives, such as: utilizing pyrrole, imidazole, or indole compounds in place of the pyrazole rings²; the possibility of tripodal heptadentate ligands such as N4O3 from the ligand tris[6-((2-N,N-diethylcarbamoyl)pyridyl)methyl]amine³; Sulfur donor groups such as those found in the Tm ligand or oxygen donor groups. changing the ligands to alter the type of molecular encapsulation needed to metals; for very different applications, "heteroscorpionate ligands" have been examined of hybrid scorpionate/cyclopentadienyl-lithium compounds such as [Li(2,2-bis(3,5-dimethyl pyrazol-1-yl)1,1-diphenylethylcyclopentadienyl(THF)] which catalyzes olefin polymerization. Isolobality Since the work by Wilkinson and others on ferrocene a vast amount of work has been done on cyclopentadienyl complexes. It was soon understood by many organometallic chemists that a Cp ligand is isolobal to Tp. As many insights into chemistry can be obtained by the study of a series of closely related compounds (where only