A Five‐Coordinate Homoleptic Organotitanium(III) Compound

Abstract

The term "homoleptic", as coined by Lappert for [{MR x } n ] qÀ species (q = 0, AE 1, AE 2,…), is a logical extension of the classical concept of "binary" compounds E n X m , used in traditional chemistry.[1] Both kinds of compounds are of fundamental importance: They are simple species whose stability, structure, and chemical properties are determined by a minimum number of factors: 1) the electronic and steric properties of a single type of atom X or group R, and 2) the electron configuration, charge, and size of the atom E or metal M. The latter set of factors is inextricably related to the oxidation state of a given element (E or M). In the realm of organotitanium chemistry, the five-coordinate species [Ti IVMe 5 ] À has been isolated [2] and structurally characterized.[3] To the best of our knowledge, however, no related [TiR 5 ] 2À species has yet been described containing titanium(iii), a much less studied oxidation state for this metal. In fact, the only well-established homoleptic organotitanium(iii) compounds conform to the [TiR 2 ][5] CH(SiMe 3 ) 2 ), [6] and [TiR 4 ] À (R = C 6 Cl 5 ) [7] Compound 1 is extremely air sensitive, both in solution and in the solid state. Solid 1 easily explodes under slight mechanical stress, for example, from percussion or a sharp temperature change. The IR spectrum shows only a weak absorption assignable to the X-sensitive vibration mode [8] of the C 6 F 5 group at 801 cm À1 . The crystal and molecular structures of 1 were established by X-ray diffraction. Two crystallographically independent [Ti(C 6 F 5 ) 5 ] 2À anions were found in the asymmetric unit of the crystal but, because of their similarity, we refer to only one of them (Figure 1). The Ti center is located in a heavily distorted trigonal-bipyramidal (TBPY-5) environment formed by five terminal, s-bonded C 6 F 5 ligands. The axial positions are defined by the C 6 F 5 groups with the widest interligand angle (C(1)-Ti(1)-C(7) 164.6(2)8). The sum of the interligand C eqTi-C eq angles is 359.3(3)8, that is, the equatorial plane shows no pyramidalization at the Ti atom. Although weaker MÀL ax bonds would be expected for a d n (TBPY-5)-ML 5 species with n < 5, [9] no significant difference between the average values of TiÀC ax (226.2(8) pm) and TiÀC eq (223.1(7) pm) bond lengths can be observed in 1. The mean TiÀC bond length (224.4(8) pm) is slightly longer than that observed in the homoleptic, tetrahedral anion [Ti III (C 6 Cl 5 ) 4 ] À (TiÀC 220.7(5) pm), [7] a fact that can be ascribed to the different coordination numbers in the two anionic species, as well as to their different global charges. Important angular deviations from ideal TBPY-5 geometry are observed in the Ti coordi-

Stereochemistry and Spin State in Four-Coordinate Transition Metal Compounds

“…Although we are aware of only one such homoleptic compound (Table ), it presents a structure that is quite distant from both the tetrahedron and the square, consistent with the shape of the potential energy curve corresponding to the high-spin state (Figure , right) and with the optimized geometry for the isoelectronic V II methyl complex (Table ). That the d 3 methyl compound presents a coordination geometry similar to that experimentally reported for the [Cr(C 6 Cl 5 ) 4 ] − anion is an interesting observation because the presence of two intramolecular Cl···Cr contacts may suggest an effective pseudo-octahedral coordination . Further confirmation of the theoretical expectations is provided by the mixed-ligand complex [Cr(en)(CH 2 Ph)(CHPh)], which is also found as an S = 3 / 2 square planar stereospinomer …”

“…That the d 3 methyl compound presents a coordination geometry similar to that experimentally reported for the [Cr(C 6 Cl 5 ) 4 ]anion is an interesting observation because the presence of two intramolecular Cl • • • Cr contacts may suggest an effective pseudo-octahedral coordination. 48 Further confirmation of the theoretical expectations is provided by the mixed-ligand complex [Cr(en)(CH 2 Ph)(CHPh)], which is also found as an S ) 3 / 2 square planar stereospinomer. 49 d 4 Configuration.…”

Homoleptic Organometallic Anions of Ti, Zr, Hf, and Nb

“…[TiA C H T U N G T R E N N U N G (C 6 F 5 ) 5 ] 2À has a regular trigonal-bipyramidal structure. [13]…”

Solid‐State and Solution Structure of a Hypervalent AX5 Compound: Sb(C6F5)5

“…The axial SbC bonds are significantly longer than the equatorial ones (222.7(3) vs. 214.7(3) pm average values), probably as the result of a 3‐center/4‐electron hypervalent bond in the axial direction 8. The difference between axial and equatorial EC bond lengths is less pronounced in the related open‐shell, paramagnetic organotransition‐metal derivatives [M III (C 6 F 5 ) 5 ] 2− (M III =Ti III (d 1 ),19 V III (d 2 )20), which also show roughly similar TBPY ‐5 geometries.…”