Richard Blom

A modification of the Schomaker—Stevenson rule for prediction of single bond distances

Abstract A modification of the Schomaker—Stevenson rule: c = 8.5 pm, n = 1.4, significantly reduces the discrepancy between experimental calculated bond lengths for every polar bonds between main group elements.

The molecular structures of bis(pentamethylcyclopentadienyl)-calcium and -ytterbium in the gas phase; two bent metallocenes

Abstract Gas electron diffraction studies show that whereas the ligand rings in (η 5 -C 5 Me 5 ) 2 Mg, are essentially parallel, the thermal average structures of (η 5 -C 5 Me 5 ) 2 Ca and (η 5 -C 5 Me 5 ) 2 Yb are bent, the ring-centroid—metal—ring-centroid angles being 154(3)° and 158(4)°, respectively.

The molecular gas phase structure of Tl(5Me5)

Abstract The thermal average molecular structure of Tl(C 5 Me 5 ) has been determined by gas phase electron diffraction (GED). The molecule has a half sandwich structure of C 5 v symmetry. The Tlue5f8C bond distance is 266.3(5) pm, with a root-mean-square amplitude of vibration ( l -value) of 5.2(24) ppm. The results indicate that the methyl groups of the ring tend to decrease the thalliumue5f8carbon bond distance compared with that in the non-methylated compound.

The molecular structure of Me3TiCp in the gas phase

The thermal average molecular structure of Me3TiCp★ has been determined by gas phase electron diffraction (GED). The GED data are consistent with a molecular model in which the TiMe3 fragment has C3v symmetry and the TiCp★ fragment has C5v symmetry. No static tilt of the methyl groups attached to titanium can be detected, although a flattening of the methyl groups with Tiue5f8Cue5f8H 103.8(1.2)° is obtained. The Tiue5f8C(Me) bond distance is 210.7(5) pm, and the Tiue5f8C(Cp★) distance 238.0(5) pm.

Gasphasen- und Kristall-Struktur sowie Reaktionen von Hexahydrido(η5-pentamethylcyclopentadienyl)rhenium(VII), (η5-C5Me5)ReH6: Elektronen- und Röntgenbeugung

Abstract The hexahydrido rhenium complex (η 5 -C 5 Me 5 )ReH 6 ( 1 ) has been studied by use of gas-phase electron diffraction (Oslo), while the derivative (η 5 -C 5 Me 4 Et)ReH 6 ( 1 ′) has structurally been defined by a single-crystal X-ray diffraction study at 23°C (Munich). The data confirm an earlier proposal based on NMR data, that both compounds have the rare pentagonal-bipyramidal geometry, with the five hydrido ligands adopting equatorial positions. The hydrogen-to-rhenium distances are 140–166 pm (X-ray) and 166.5(14) pm (electron diffraction), respectively. The umbrella-type configuration of the H′ue5f8ReH 5 structural unit (angle H′ue5f8Reue5f8H 66–70°; X-ray data) is probably due to the relatively large steric bulk of the (centrically π-bonded) ring ligand. Hydridic rather than protic H-ligands are chemically and structurally in evidence. The structural data obtained independently for the gas-phase and for the crystalline state are consistent with NMR data obtained for solution; the found T , 1 values indicates that η 1 -H- rather than η 2 -H 2 ligands are present.

Molecular structures of tris(methylcyclopentadienyl)-scandium and -ytterbium as studied by gas phase electron diffraction and molecular mechanics calculations : the scandium atom is too small to accommodate three pentahapto cyclopentadienyl rings

Abstract The syntheses and spectroscopic properties (IR and 1H and 13C NMR spectra) of tris(methylcyclopentadienyl)-scandium and -ytterbium are described. The vapour pressure of Yb(MeCP)3 has been determined over the range 70–90°C. The gas phase electron diffraction data for M(MeCP)3 (M ue5fb Sc or Yb) have been recorded with nozzle temperatures of about 160°C. The data for M ue5fb Yb are consistent with a model containing three pentahapto cyclopentadienyl rings; Yb(η5-MeCp)3, a mean Yb to ring-centre distance of Ybue5f8Z = 236.6(6) pm, and a mean Ybue5f8C bond distance of 265.5(7) pm. The gas phase electron diffraction data for M ue5fb Sc are incompatible with models containing three η5-MeCp rings, but consistent with models containing two η5 rings and one ring with a hapticity of 2 or 3; η2/3-MeCp. The distance from Sc to the centres of the two η5-MeCp rings is 222.3(6) pm, corresponding to a mean Scue5f8 C(η5) bond distance of 253.0(6) pm. The third ring is at a greater distance from the Sc atom, SCue5f8Z(η2/3) = 252(5) pm, but is tilted, the angle between the Scue5f8Z(μ2/3) vector and the ring normal being 22(4)°. As a result two or three carbon atoms of the ring are in a position to form strong bonds to the metal atom. Molecular mechanics calculations indicate that interligand interactions in M(η5-C5H4)3 or M(η5-MeCP)3 molecules are strongly repulsive when Mue5f8Z(η5) is less than 225 pm, and that the strain is eliminated on rearrangement to M(η5-C5H5)2(η2/3-C5H5) or M(η5-MeCP)2(η 2/3-MeCp) configurations.

Semi‐Batch Polymerisations of Ethylene with Metallocene Catalysts in the Presence of Hydrogen, 3. Correlation Between Hydrogen Sensitivity and Molecular Parameters

Semi-batch ethylene polymerisations have been carried out with heterogenised Cp 2 ZrCl 2 /MAO, Ind 2 ZrCl 2 /MAO, Cp(Ind)ZrCl 2 /MAO and Cp(Flu)ZrCl 2 / MAO (MAO, methylaluminoxane) catalysts where hydrogen was added as the chain-transfer agent. Modelling of molecular weight distributions of the polymer formed gave estimates of the relative rate constant for the chain transfer to hydrogen and the rate constant for propagation, k tH2 /k p . Values of 0.7(2), 22(7), 13(3) and 35(4) were obtained for the Cp 2 , Ind 2 , (Cp,Ind) and (Cp,Flu) catalysts, respectively. The observed order of reactivity towards hydrogen has been correlated with chemical shifts from 91 Zr NMR and with the atomic charges of zirconium from DFT calculations for a series of metallocene complexes, The efficiency of hydrogen as a chain-transfer agent is larger the more electron deficient the zirconium atom in the catalyst is.

Synthesis and thermal average gas phase molecular structures of bis(pentamethylcyclopentadienyl)-strontium and -barium; the first organo-strontium and -barium structures

The thermal average gas phase molecular structures of bis(pentamethylcyclopentadienyl)-strontium, Sr(C5Me5)2, and -barium, Ba(C5Me5)2, are best described as bent sandwich structures where the mean Sr–C and Ba–C bond distances are 275.0(8) and 289.8(17)pm and the ring centroid–metal–ring centroid angles are 149(3)° and 148(6)° respectively.

The molecular structure of bis(pentamethylcyclopentadienyl)zinc determined by gas electron diffraction

Bis(pentamethylcyclopentadienyl)zinc, (Me5C5)2Zn, in the gas phase contains one η5-bonded ring with Zn–C(η5) 230(2) pm and one approximately σ-bonded ring with Zn–C(σ) 209(5) pm; the angle between the Zn–C(σ) bond and the plane defined by the σ-bonded ring is 87(3)°.

The molecular structure of (η5-cyclopentadienyl)-bis(ethene)rhodium determined by gas-phase electron diffraction

The molecular structure of [Rh(η5-C5H5)(C2H4)2] has been determined in the gas phase by electron diffraction at 473 K. The molecule has C2v local symmetry for the Rh(C2H4)2 fragment and C5v for the Rh(C5H5) fragment, with the local rotation axes coincident. The distance from the rhodium atom to the center of the cyclopentadienyl ring is 190.7(3) pm, and that from the rhodium atom to the centres of the C–C bonds of the ethene ligands is 198.0(3) pm. The C–C bond distances in the cyclopentadienyl ligands refined to 143.2(2) pm, as expected for such five-co-ordinated rings, but the C–C bonds of the ethene ligands. [145.7(7) pm] are longer than those previously reported for rhodium-ethene compounds. The angle between the lines linking the midpoints of the CC bonds of the ethene ligands to the rhodium atom is not uniquely determined by the electron diffraction data, and models with values of 91.2(13) and 111.8(12)° both fit well. The former value is preferred, as it gives a lower R factor, and is consistent with corresponding angles reported for bis(ethene) complexes in th e crystalline phase. The two ethene ligands are tilted about their CC axes away from one another by 11.7(18)°.