Pentamethylantimony

Pentamethylantimony is colourless.^[3] 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.^[3] 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.^[3] 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.^[2]

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.^[3] Pentamethylantimony can be stored as a liquid in clean glass at room temperature.^[4]

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.^[3]

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 ([(CH₃)₄Sb]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 [(CH₃)₄Sb]TlBr₄, [(CH₃)₄Sb][CH₃SbCl₅],^[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]