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Wandlungsfähig: Der Et2O‐stabilisierte Titelkomplex wurde synthetisiert und röntgenographisch charakterisiert (siehe Bild). Während die Allyl‐Liganden im Festkörper σ‐gebunden sind, hat die Verbindung in Lösung eine fluktuierende Struktur; außerdem geht sie mit BeCl2ein Schlenk‐Gleichgewicht ein. Nach DFT‐Rechnungen ohne koordinierte Base können sowohl die σ‐ als auch die π‐gebundene Form Energieminima darstellen.

The allyl anion [C3H5]–is often found in combination with other ligands in organometallic complexes. Compounds in which allyl groups are the only or principal type of ligand, however, are often coordinatively unsaturated and prone to decomposition. It is possible to increase the thermal and sometimes oxidative stability of such complexes by adding sterically bulky substituents e.g., –SiMe3 to the allyl group. Main group, transition metal, and felement compounds constructed with bulky allyl ligands display a wide range ofmonomeric, oligomeric, and polymeric structures. Some of these complexes have no counterparts with unsubstitutedallyl groups, and others are active polymerization catalysts. Research in this area has served to expand the range of structural types and bonding that can be incorporated into metal allyl chemistry.

Previous work on bis(indenyl)chromium(II) complexes substituted with bulky groups (i-Pr, t-Bu, SiMe3) found that their spin state (S 1 or 2) depended on the symmetry of the molecules. Complexes with inversion symmetry (staggered rings) were high-spin; lower symmetry compounds with twisted (gauche) ligands were low spin. The present work explores the effect of methyl group substitution on the indenyl ligand, which leads to complexes possessing either staggered or eclipsed conformations. The monosubstituted compounds (1 or 2)-MeC9H62Cr are prepared from the substituted alkali metal indenides and CrCl2in THF. X-ray diffraction results indicate that (2-MeC9H6)2Cr exists in a staggered conformation, with Cr−C (av) 2.308(7) Å, and is a high-spin species in the solid state and solution. In contrast, the monomeric (1-MeC9H6)2Cr is eclipsed in the solid state, where it exhibits spin-crossover behavior over a wide temperature range; the average Cr−C distances vary with temperature, from 2.179(9) Å at 105 K to 2.262(10) Å at 298 K. An attempt to form (4-MeC9H6)2Cr produced the dimeric, thermally unstable complex (5-indenyl‘)2(,3-indenyl‘)Cr2(-Cl) instead. Correlations between the structure and magnetic properties in bis(indenyl)chromium(II) complexes have been made with density functional theory calculations, which indicate that an eclipsed ligand conformation supports a high spin state, but not to the extent that the staggered form does.

Hydridotris(triphenylphosphine)rhodium HRh(PPh3)3, I, crystallizes out of a toluene/hexane mixture free from solvent in the space group P21/a with a=23.289(8), b=19.985(6), c=9.810(2)Å, β=97.83(2)°, V=4523(2)Å3 and Dcalcd=1.308 g cm-3 for Z=4. Molecules of I exhibit a distorted square planar geometry in which the angle subtended by the phosphines adjacent to the hydride ligand is 147.28(4)°. A difference Fourier map based on low-angle diffraction data provided evidence for the hydride ligand with a Rh-H distance of 1.63Å. In contrast to the known structure of HRh(PPh3)3 · HN(CH3)2 · THF, II, which contains solvent of crystallization, and has no ortho hydrogen rhodium separations less than 3.01Å, a phenyl ring in I has rotated so as to place a hydrogen within 2.87Å of the rhodium center.

YCl3reacts with four equivalents of t-C4H9Li to form Y(CMe3)4Li(THF)4(1) in high yield. 1reacts with HCCCMe3to form Y(CCCMe3)4Li(THF) (2) quantitatively. 2reacts with KC5Me5to form (C5Me5)2Y(μ-CCCMe3)2Li(THF) (3) which can also be prepared from (C5Me5)2YCl(THF) and LiCCCMe3. 3crystallizes from toluene at −34°C in space group Cmcmwith a= 13.363(4) Å, b= 15.918(7) Å, c= 17.453(9) Å and Dcalcd= 1.07 g cm−3for Z= 4. Least-squares refinement of the model based on 733 reflections converged to a final RF= 0.077. 3contains a (C5Me5)2Y bent metallocene unit connected to Li via two alkynide bridges. The proximity of the two CMe3groups to Li prevents coordination by more than one THF of solvation.

The metallocenes of the Group 2 and Group 14 elements display a higher degree of structural and chemical similarity than would be expected based on the differences in their electronic configurations alone. A comparison of their structures suggests that the interplanar ring angles in both are primarily determined by the size and shape of the cyclopentadienyl ligands; the two additional valence electrons in the Group 14 compounds exert little stereochemical influence. Although the two metallocene families display differences in reactivity, the presence or absence of metal valence electrons is not always a useful criterion for distinguishing between them. In Group 14 metallocenes, the electrons are not efficient donors to electrophiles, and sufficiently bulky cyclopentadienyl rings can interfere with the extent of redox reactions. Examination of main-group metallocene melting points reveals a trend based on the symmetry and flexibility of the cyclopentadienyl ring. In rigorously monomeric species, compounds with longer metal-carbon bonds have lower melting points.

Unsolvated 1,2,4-tris(trimethylsilyl)cyclopentadienyl potassium (K[Cp^3Si]) crystallizes as a base-free “super-sandwich” of alternating potassium ions and Cp' rings. The polymeric chains in the structure are not connected as in other unsolvated KCp' structures, but uniquely consist of independent columns. This type of structure has previously been observed in base-free lithium cyclopentadienyl complexes; however, K[Cp^3Si] is the first base-free potassium cyclopentadienyl complex that crystallizes without interactions between neighboring chains.

The triethylborohydride anion [HBEt₃]⁻ transfers intact from Na[HBEt₃] to a calcium center to form the monomeric organocalcium complex [Ca(HBEt₃){1,2,4-C₅(SiMe₃)₃H₂}(thf)₂], which is stable in solution and the solid state; the [HBEt₃]⁻ anion is coordinated in a multidentate fashion to the metal, which likely contributes to the failure of attempts to abstract the triethylborane moiety from the compound.

Wo bleiben die Bis(π‐indenyl)komplexe der „Nach‐Übergangselemente”︁? Nun konnte der erste derartige Komplex eines Elements der Gruppe 14 synthetisiert und charakterisiert werden: [Pb{1,3‐(SiMe3)2‐C9H5}2] ist in Lösung nicht stabil, bildet jedoch thermisch stabile Kristalle, in denen er Sandwichgeometrie mit nahezu parallelen, η5‐koordinierten Indenyl‐Fünfringen aufweist (siehe rechts).

Zwei η3-Allylliganden in anti-Konfiguration weist die Titelverbindung auf (Struktur im Kristall siehe Bild). Zwar haben die Ca-C-Bindungen etwa die Länge, die man für Cyclopentadienylkomplexe erwartet, doch ähneln die H-Atom-Positionen im Bis(trimethylsilyl)allyl-Anion eher denen anderer Hauptgruppen- und Übergangsmetall-π-Allyl-Komplexe.

Where are all the bis(p-indenyl) complexes of the “post-transition elements”?The first example of this type of complex with a Group 14 element has now been synthesized and characterized. [Pb{1,3-(SiMe3)2C9H5}2] is unstable in solution, but forms thermally stable crystals that reveal a sandwich geometry with nearly parallel, ?5-coordinated C5rings of the indenyl ligand (see structure on the right).

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.