Pentamethylantimony or pentamethylstiborane is an organometalllic compound containing five methyl groups bound to an antimony atom with formula Sb(CH3)5. It is an example of a hypervalent compound. The molecular shape is trigonal bipyramid.[1] Some other antimony(V) organometallic compounds include pentapropynylantimony (Sb(CCCH3)5) and pentaphenyl antimony (Sb(C6H5)5). Other known pentamethyl-pnictides include pentamethylbismuth and pentamethylarsenic.
Pentamethylantimony can be made by reacting Sb(CH3)3Br2 with two equivalents of methyl lithium.[2] Another production route is to convert trimethylstibine to the trimethyl antimony dichloride, and then replace the chlorine with methyl groups with methyl lithium.
Sb(CH3)3 + Cl2 → Sb(CH3)3Cl2
Sb(CH3)3Cl2 + 2LiCH3 → Sb(CH3)5 + 2LiCl
Pentamethylantimony is colourless.[2] At -143 °C it crystallizes in the orthorhombic system with space group Ccmm. Unit cell dimensions are a=6.630 Å b=11.004 Å c=11.090 Å. There are four formula per unit cell. Unit cell volume is 809.1 Å3.[2] The trigonal bipyramid shape has three equatorial positions for carbon, and two axial positions at the peaks of the pyramids. The length of the antimony-carbon bond is around 214 pm for equatorial methyl groups and 222 pm for the axial positions. The bond angles are 120° for ∠C-Sb-C across the equator, and 90° for ∠C-Sb-C between equator and axis.[2] The molecules rapidly change carbon atom position, so that in NMR spectrum as low as −100 °C, there is only one kind of hydrogen position.[3]
Pentamethylantimony is more stable than pentamethylbismuth, because in lower energy trimethylbismuth, the non-bonding pair of electrons is more shielded due to the f-electrons and the lanthanoid contraction. Trimethylantimony is higher in energy, and thus less is released in a decomposition of pentamethylantimony.[2] Pentamethylantimony can be stored as a liquid in clean glass at room temperature.
Pentamethylantimony melts at -19 °C. Although it decomposes when boiling is attempted and can explode, it has a high vapour pressure at 8 mmHg at 25 °C.[4]
There are two absorption bands in the ultraviolet at 2380 and 2500 Å.[4]
Pentamethylantimony reacts with methyl lithium to yield a colourless lithium hexamethylantimonate in tetrahydrofuran.[2]
Sb(CH3)5 + LiCH3 → Li(thf)Sb(CH3)6
Pentamethylantimony reacts with silsesquioxanes to yield tetramethylstibonium silsesquioxanes. eg (cyclo-C6H11)7Si7O9(OH)3 yields (cyclo-C6H11)7Si7O9(OSb(CH3)4)3. The reaction happens quickly when there are more than two OH groups.[5]
Phosphonic acids and phosphinic acids combine with pentamethylantimony to yield compounds like (CH3)4SbOP(O)Ph2, (CH3)4SbOP(O)(OH)Ph and (CH3)4SbOP(O)(OH)3, eliminating methane.[6]
Stannocene Sn(C5H5)2 combines with pentamethylantimony to produce bis(tetramethylstibonium)tetracyclopentadienylstannate ([(CH<sub>3</sub>)<sub>4</sub>Sb]2Sn(C5H5)4).[7]
Pentamethylantimony reacts with many very weak acids to form a tetramethylstibonium salt or tetramethylstibonium derivative with the acid. Such acids include water (H2O), alcohols, thiols, phenol, carboxylic acids, hydrogen fluoride, thiocyanic acid, hydrazoic acid, difluorophosphoric acid, thiophosphinic acids, and alkylsilols.[8]
With halogens, pentamethylantimony has one or two methyl groups replaced by the halogen atoms.[8] Lewis acids also react to form tetramethyl stibonium salts, including [(CH<sub>3</sub>)<sub>4</sub>Sb]TlBr4, [(CH<sub>3</sub>)<sub>4</sub>Sb][CH<sub>3</sub>SbCl<sub>5</sub>],[8]
Pentamethylantimony reacts with the surface of silica to coat it with Si-O-Sb(CH3)4 groups. Over 250 °C this decomposes to Sb(CH3) and leaves methyl groups attached to the silica surface.[9]