Trimethylindium

Trimethylindium, often abbreviated to TMI or TMIn, is the organoindium compound with the formula In(CH₃)₃. It is a colorless, pyrophoric solid. Unlike trimethylaluminium, but akin to trimethylgallium, TMI is monomeric.

Preparation

TMI is prepared by the reaction of indium trichloride with methyl lithium.

: InCl₃ + 3LiMe → Me₃In^.OEt₂ + 3LiCl

Properties

Compared to trimethylaluminium and trimethylgallium, InMe₃ is a weaker Lewis acid. It forms adducts with secondary amines and phosphines. A complex with the heterocyclic triazine ligand (PrⁱNCH₂)₃ forms a complex with 6-coordinate In, where the C-In-C angles are 114°-117° with three long bonds to the tridentate ligand with N-In-N angles of 48.6° and long In-N bonds of 278 pm.

Structure

In the gaseous state InMe₃ is monomeric, with a trigonal planar structure, and in benzene solution it is tetrameric.''CVD of compound semiconductors, Precursor Synthesis, Development and Applications'', Anthony C. Jones, Paul O'Brien, John Wiley & Sons, 2008,
In the solid state there are two polymorphs, a tetragonal phase which is obtained, for example, by sublimation and a lower density rhombohedral phase discovered in 2005, when InMe₃ re-crystallised from hexane solution.

In the tetragonal form InMe₃ is tetrameric as in benzene solution and there is bridging between tetramers to give an infinite network. Each indium atom is five coordinate, in a distorted trigonal planar configuration, the three shortest bonds (ca. 216 pm) are those in the equatorial plane, with longer axial bonds, 308 pm for the In-C bonds joining the InMe₃ units to form the tetramers and 356 pm for the In-C linking the tetramers into an infinite network.''Inorganic Chemistry'', (2d edition), Catherine E. Housecroft, Alan G. Sharpe, Pearson Education, 2005, , The solid state structures of GaMe₃ and TlMe₃ are similar. The association in the solid state accounts for the high melting point of 89°–89.8 °C compared to triethylindium which melts at −32 °C.

The rhombohedral form of InMe₃ consists of cyclic hexamers with 12 membered (InC)₆ rings in an extended chair conformation. The hexamers are interlinked into an infinite network. Indium atoms are five coordinate the equatorial In-C distances average 216.7pm almost identical to the average for the tetragonal form, and the axial bonds are 302.8pm joining the InMe₃ units into hexamers and 313.4 pm linking the hexamers to form the infinite network.

Application to microelectronics

Indium is a component of several compound semiconductors, including as InP, InAs, InN, InSb, GaInAs, InGaN, AlGaInP, AlInP, and AlInGaNP. These materials are prepared by metalorganic vapour phase epitaxy (MOVPE) and TMI is the preferred source for the indium component. High purity in TMI (99.9999% pure or greater) is essential for many of these applications. For some materials, electron mobilities are observed as high as 287,000 cm²/Vs at 77 K and 5400 cm²/Vs at 300 K, and background carrier concentration as low as 6×10¹³ cm⁻³.

Vapor pressure equation

The vapor pressure equation log P (Torr) = 10.98–3204/T (K) describes TMI within a wide range of MOVPE growth conditions.

Safety

TMI is pyrophoric.Chemistry of Materials (2000);

References

External links

Interesting research notes
by Linus Pauling in re: Trimethylindium and its structure; ''Notebook # 19, Page 049'', August 1955.

Interactive Vapor Pressure Chart for metalorganics

{{Indium compounds

Chemical vapour deposition precursors
Indium compounds
Methyl complexes
Pyrophoric materials