Commonly abbreviated to CpIn, it is a cyclopentadienyl complex with a half-sandwich structure.

CpIn can be readily prepared by reacting indium(I) chloride with cyclopentadienyllithium:[2] InCp reacts with BF3, BCl3, BBr3, BI3 and trimethylborane B(CH3)3 to form adducts,[3] e.g.: In these adducts the bonding of the Cp ligand to the indium atom changes from η5 (π complexing) to η1 (σ bonding).

Salts containing the InX2− anion containing indium in the +1 oxidation state have been prepared from cyclopentadieneindium; for example:[4] Solid CpIn is polymeric consisting of zigzag chains of alternating indium atoms and C5H5 units.

[5] In the CpIn monomer present in the vapour phase the indium atom sits on the central axis of the aromatic cyclopentadienyl anion, C5H5−.

Bonding studies have shown that the aromatic ring electrons of the cyclopentadienyl anion interact with the indium 5s and 5p atomic orbitals, and that the lone pair on the indium atom is a dominant feature.