Lewis acidic antimony complexes with a variety of oxidation states and coordination numbers are known. Several salient examples are introduced below. shift of
OPEt3 adducts (
Gutmann–Beckett method). The chemical shift of OPEt3 is 51.0 ppm (in CH2Cl2), 51.2 ppm (in CHCl3), and 47.6 ppm (in DFB (difluorobenzene)).File:Sb(tol)Cp* IBOs.png|thumb|IBO of Sb-(η5-Cp*) bondings in [Sb(tol)(Cp*)]2+(
1). In [Sb(
tol)(
Cp*)]2+ (
1), the
η5-Cp* binding mode is confirmed using IBO analysis. In the solid state structure, the Sb-C bond distances between Sb and carbons in the Cp* ring are 2.394(4) to 2.424(4) Å, but the Sb–C bond distances with the toluene are 2.993(5) to 3.182(5) Å. This longer Sb–toluene distance implies toluene lability in solution. Sb2(
o-catecholate)2(
μ-O) (
2) had been predicted that a Lewis base would bind to two antimony centers in a bridging manner. However, it was observed that
2 binds with halide anions in various ratios (3:1, 2:1, 1:1, 1:2, 1:3). Cozzolono et al. suggested three reasons for its complex binding mode. First, rotational freedom around the bridge oxygen disrupts the Lewis base binding between two antimony centers. Second, intramolecular interactions between oxygen at catecholate and antimony competes with external Lewis base binding. Third, a high-polarity nucleophilic solvent,
dimethylsulfoxide, is required to dissolve
2 due to the solubility and the solvent is also able to bind at antimony.
3-coordinate Sb(V) [SbPh3]2+ (
4) was not isolated. Instead, its Lewis adducts, [SbPh3(OPPh3)2]2+ and [SbPh3(dmap)2(OTf)]+, were isolated. In the
trigonal bipyramidal [SbPh3(OPPh3)2]2+, two OPPh3 are located in axial positions and the Sb–O bond distance (2.102(2) Å) is similar to the sum of the covalent radii of Sb and O (2.05 Å). In the
distorted octahedral [SbPh3(dmap)2(OTf)]+, the Sb–N distance with the
dmap (2.222(2) Å) is shorter than reported N–Sb+ distances. This bond distance implies Lewis adduct formation. In addition, a reaction between dmap and [SbPh3(OPPh3)2]2+ forms [SbPh3(dmap)2(OTf)]+. The experimental results indicate that [SbPh3]2+ is the Lewis acidic counterpart of these adducts.
4-coordinate Sb(V) Tetrahedral stibonium cations also show Lewis acidity. Since [Sb(C6F5)4]+ (
5) forms an adduct with triflate, the cation can be isolated as a [Sb(C6F5)4][B(C6F5)4] salt. Short Sb–C bond distances of 2.095(2) Å and a tetrahedral space group in the crystal proves that isolated [Sb(C6F5)4]+ is completely free of external electron donors. This cationic antimony Lewis acid shows strong acidity: firstly, [Sb(C6F5)4]+ abstracts fluoride anion from weakly coordinating anions, , and secondly, the acidity measured by the Gutmann–Beckett method of [Sb(C6F5)4]+ (
5) is comparable with that of the
B(C6F5)3 adduct in
CH2Cl2 (76.6 ppm). SbPh3(Ant)+ (
6) (where Ant is 9-anthryl) was isolated as
triflate salt.
6 has a tetrahedral structure like
5. In a solid state structure of a fluoride adduct, AntPh3SbF, the incoming fluoride occupies the axial position of a trigonal bipyramidal structure, and the sterically demanding anthryl is located at the equatorial site.
5-coordinate Sb(V) Neutral Sb(V) complexes are also Lewis acids. Compounds
7,
8 and
11 share the structure of
spirocyclic stiborane. The LUMO of
8 mainly has its lobe at the antimony atoms and it renders
8 Lewis acidic. In detail, the LUMO can be assigned to as localized orbital on stiborafluorene moiety with larger nodes at the 9-position (Sb). Thus, Lewis bases bind towards
trans to biphenylene and its fluoride adducts are asymmetric:
8·F− has two enantiomers and
7·F− has two diastereomers and four enantiomers. A bisantimony complex (
9) is synthesized starting from
xanthene.
9 has
C2 symmetry and the Sb–Sb distance is 4.7805(7) Å. Both antimony(V) centers have distorted
square pyramidal geometry with the
geometry index τ5 = 0.08. The base planes of the antimony centers meet face to face and this geometry allows 1:1 binding with F−, unlike
2. == Trends in acidity ==