Pier Luigi Bellon

The crystal and molecular structure of the potassium salt of the hexanuclear carbonyl cobaltate [Co6(CO)14]4−

Abstract The crystal and molecular structure of the potassium salt of the tetradecacarbonylkhexacobaltate tetraanion K 4 [Co 6 (CO) 14 ]·6 H 2 O has been determined by tridimensional X-ray data measured by counter methods. The structure has been refined by least squares of F down to an R factor of 0.06. The salt crystallizes in the monoclinic space group C 5 2 h = P 2 1 n with the following cell constants: a = 12.00(1) A, b = 10.04(1) A, c = 12.00(1) A, β = 90.0° (1), there are 2 formula units per cell. The structure is composed of a tetragonal body centered packing of anions with the cavities occupied by potassium ions and water molecules. The anion [Co 6 (CO) 14 ] 4− possesses an inversion center and contains an octahedral cluster of metal atoms surrounded by 14 CO groups; 6 of them are linearly bonded one for each cobalt atom and the other 8 CO groups are triply bridged on all the octahedral faces. The metal atoms cluster and its COs environment deviate from the holosymmetry OO h . When all the distortions are taken into account the anion can be assigned the symmetry S 6 . The CoCo mean distance is 2.50 A; the CoC and the CO distances are 1.70 A and 1.17 A for linearly bonded groups; for triply bridged groups the CoC distances are scattered in the range 1.89(1)–2.31(1) A, the CO mean distance is 2.21 A.

Crystal and molecular structure of (dithiformato)bis(triphenylphosphine)dicarbonylrhenium, Re(HCS2)(CO)2[P(C6H5)3]2

Abstract The addition of CS2 to hydridotris(trisphenylphosphine)dicarbonylrhenium results in the formation of (dithioformato)bis(triphenylphosphine)dicarbonylrhenium. This complex crystallizes in the triclinic space group P I ; the reduced cell has dimensions: a 10.481(15) A, b = 12.471(15)A, c = 14.878(15) A, a = 81.799(7)°, β = 74.03(7)°, > = 107.37(7)°, V = 1735 A3. The observed density is 1.63(2) g/cm3 while the computed density for Z = 2 is 1.615 g/cm3. Intensities of 2682 independent reflections, having σ(I)/I⪡0.25, were measured by counter methods using a molybdenum source and a silicon monochromator. The structure was refined by full matrix least squares to a final R factor of 0.039. The crystal consists of a packing all discrete monomeric molecules; the distorted octahedral coordination around the metal atom has approximately a C2v symmetry. The dithioformate anion chelates on rhenium atom with bonding parameters ReS 2.500(3) A and 2.532(5) A, CS 1.64(2) A and 1.68(2) A and SCS 116.7(1)°. The mean length of the two ReP bonds is 2.42 A and that of the two ReC bonds is 1.91 A. The ReCO interactions appear to be disordered and so does the chelate anion. On the basis present determination probable structures are suggested for other addition products formed from CS2 and RhIII and IrI complexes.

Stereochemistry of tetrahedral complexes of group VIII metals. Crystal and molecular structures of dinitrosylcarbonyltriphenylphosphineiron, Fe(NO)2(CO)(PPh3), and of dinitrosylbis(triphenylphosphine)iron, Fe(NO)2(PPh3)2

Single crystal X-ray structures of Fe(NO) 2 (CO)(PPh 3 ) (I) and of Fe(NO) 2 (PPh 3 ) 2 (II) have been determined. Compound (I) form triclinic crystals of space group P , with cell constants a 10.96(1)», b 10.20(1)», c 10.45(1)», α 115.84(8)°, β 117.33(8)°, γ 78.90(8)°, U 933.4» 3 , Z 2. Compound (II) forms monoclinic crystals of space group P 2/ c with cell constants a 11.70(1) », b 8.20(1) » c 17.24(2) », β 106.60(8)°, U 1584.6 » 3 , Z 2. Both cystals contain discrete molecules of distorted tetrahedral geometry. In compound (I) the CO and NO ligands are disordered; the principal bonding parameters are: FeC/N 1.709 », C/NO 1.148 », FeC/NO 177.9°, FeP 2.260(3) », C/NFeC/N 103.9° and PFeC/N 114.4°. In compound (II), which possesses C 2 symmetry, the principal bonding parameters are: N 1.650(7) », NO 1.19(1) », FeNO 178.2(7)° NFeN 123.8(4)°, FeP 2.267(2) », PFeP 111.9(1)°. These values are compared with those found in other tetrahedral complexes of Group VIII metals and discussed in terms of π metalligand interactions.

The dipteran sperm tail: ultrastructural characteristics and phylogenetic considerations

The sperm tail from representatives of several families of Diptera has been examined by high resolution electron microscopy and a computer analysis that improved the visualization of recorded patterns. A considerable variability in sperm tail structure is found within Diptera, and is actually greater than that of any other insect order. The ‘generalized insect sperm axoneme’. which is characterized as a 9+9+2 axoneme and by the accessory microtubules having 16 protofilaments, was found only in some dipterans; these are members of Mycetophilidae. From this fact we conclude that Mycetophilidae is likely to be the most primitive extant dipteran group. Another mycetophilid, Boletina, was seen to have accessory tubules with 15 protofilaments as have members of families Dixidae, Chironomidae, Culicidae, and Bibionidae. The last two families have spermatozoa of a type designated as 9+9+‘1’ there is a central rod rather than two microtubules. We regard this 9+9+‘1’pattern with 15 protofilaments to represent a synapomorphic feature. Representatives of the neatoceran families Tipulidae and Trichoceridae have accessory tubules with 13 protofilaments as do examined members of several brachyceran families. Brachycera is hence likely to be derived from the vicinity of the tipulid family. The intertubular material is small in Mycetophilidae and most nematoceran groups, whereas in Tipulidae and Brachycera it is enlarged; here it bridges the space between the accessory tubules and contains various inclusions.

Stereochemistry of tetrahedral complexes of group VIII metals : Crystal and molecular structures of Co(CO)2(NO)(PPh3) and of Co(CO)(NO)(PPh3)2

Abstract The crystal structures of Co(CO)2(NO)(PPh3)(I) and of Co(CO)(NO)(PPh3)2 (II) have been determined, by counter data, in order to provide further information on the stereochemistry of tetrahedral complexes of Group VIII metals. Compound (I) crystallizes in the triclinic space group P 1 with cell dimensions: a = 11.057(15), b = 10.251(15), c = 10.495(15) A, α = 115.77(7)°, β = 117.60(7)°, γ = 79.04(7)° and Z = 2. Compound (II) crystallizes in the monoclinic space group P2/c. The unit cell has dimensions: a = 11.734(15), b = 8.235(12), c = 17.236(18) A, β = 106.33(7)°, Z = 2. The structures were refined by full matrix less squares using 2516 reflections for compound (I) and 1352 reflections for compound (II). The final values of the reliability index R were 0.060 and 0.059, respectively. The crystals consist of packings of discrete monomeric molecules. In both species the cobalt atoms display distorted tetrahedral geometries and the carbonyl and nitrosyl groups are indistinguishable because of disorder. The CoP and CoC/N distances are 2.224(3) and 1.738(5), 1.762(7), 1.720(7) A in Co(CO)2(NO)(PPh3) and 2.230(3) and 1.718(8) A in Co(CO)(NO)(PPh3)2. The angles at the metals deviate from the ideal tetrahedral value, the larger deviation being that of the (C/N)Co(C/N) angle, which has a value of 120.0(4)° in Co(CO)(NO)(PPh3)2. The enlargment of the (C/N)Co(C/N) angles is explained in terms of π repulsions between ligand orbitals, whereas the co-ordination geometry of the phosphine ligands is shown to be dominated by interactions with phenyl hydrogens.

The sperm tail of a gall midge and its microtubular arrangement studied by two strategies of image analysis (cecidomyiidae, diptera, insecta)

Abstract Spermatozoa of the gall midge, Asphondylia ruebsaamen, were found to have an axoneme with about 2500 microtubular doublets, each carrying a single dynein arm that corresponds to the outer dynein arm of conventional flagella. The doublets form a tightly wound double spiral, in which doublets of the inner spiral and outer one are joined pairwise. Whereas most axonemes had their doublets in a curved spiral, some axonemes showed large blocks of doublets lying in a rather regular crystalline arrangement. The axoneme hence could appear in two states, spiral and crystalline, with intermediate forms. The doublets and their dynein arms and other connections have been subjected to a computer analysis that aims at calculating an average image from a great number of doublet pairs. Two strategies of image analysis were used. In one method the average doublet pair was obtained from a number of isolated image motifs by correlation methods; this strategy could be applied to both the spiral regions and crystalline ones and showed that there are definite differences in the angle between the two doublets of a pair and in the greater regularity of the dynein arm orientation in the crystalline state. Further, it can be seen that the shape of the two doublets in the pair is the same in the spiral regions and differs somewhat in the crystalline one. In the other strategy a filtration of noisy components was performed from the Fourier transform of periodic arrays portrayed in crystalline regions. This strategy included a determination of distortions in the lattice, and removal of these distortions by image resampling in iterated steps. The average doublet pairs appear much the same in reconstructions made with either strategy. It could be seen that the interspace between the microtubular protofilaments varied somewhat and appeared widened at those places where the dynein arm or other bridges were inserted. The lumen of the microtubules could also be seen to have slender radial strands that are particularly well resolved in the B-tubules of the doublets in the spiral state. The axoneme was seen to have one polypeptide band only in the dynein band region.

The variance of icosahedral virus models is a key indicator in the structure determination: a model-free reconstruction of viruses, suitable for refractory particles

A model-free method to determine the three-dimensional structure of icosahedral viruses is described. The novel strategy is based upon the approximate principle that correct virus structures have high variance as do all other well-detailed structures, even wrong ones. The original projections of individual particles are reduced to a radius of 25 pixels and are used to compute single particle reconstruction models by assigning them 1800 different Euler triads. The variance of the models obtained from all projections is stored in maps and a decimation process is carried out. In a first stage, thresholds are adopted for the variance values, and in a second stage, carried out by correspondence analysis and classification, 30 clusters of models are sorted out. The clusters are refined to yield models contained in boxes of 64(3) voxels. The refined models with highest variance and closest similarity represent the correct solution. Once enlarged, these models can be used to align all available projections in their original scale in a customary projection-matching process. The method has proved successful in determining the structures of poliovirus, of the empty and filled capsids of L-A virus, and of a modified capsid of hepatitis B virus.

Electron tomography in conical tilt geometry. The accuracy of a direct Fourier method (DFM) and the suppression of non-tomographic noise

Abstract SPARK is a direct Fourier method (DFM) to reconstruct 3D structures from projections observed in the electron microscope. It is suitable for both single axis tilting and random conical tilting. The present article reports accuracy tests performed in reconstructing phantom structures from analytical projections in conical tilt geometry, both noise-free and corrupted with noise. Statistical reliability indices, similar to those used in X-ray crystallography, are proposed and reported for all tests. Phase agreement tests are discussed in connection with the possibility of determining the internal consistency of data without dividing projections into two sets and are presented for the reconstructions of phantoms and of a haematic pigment. The mathematical formulation of the 3D Fourier transform of an ordered stack of projections is illustrated. With conical tilt geometry, the transform is almost zero within an elliptic cone. This property is exploited to suppress “non-tomographic noise” (NTN) and to recover missing projections in a POCS-like implementation. The suppression of non-tomographic noise, which is possible even if reconstructions are done by back-projection methods, allows one to push low-pass filtering to a higher frequency and to achieve improved resolution.

Unexpected behaviour of alkoxycarbonyl ligands : The crystal and molecular structure of trans- bis(ethoxycarbonyl)bis(triphenylphosphine)platinum

Abstract The crystal and molecular structures of the title compound have been determined from three-dimensional X-ray data collected by counter methods. The crystals are triclinic, space group P 1 , with Z = 1; cell dimensions: a = 10.204(8), b = 11.602(8), c = 8.363(7)A, α = 106.04(8)°, β = 88.04(7)°, γ = 108.98(7)°. The structure, determined from 3435 reflections, has been refined by full-matrix least-squares methods down to R = 0.038. It consists of discrete centrosymmetric molecules; the Pt atom lies on an inversion centre. PtP and PtC bonds lengths are 2.302(2) and 2.059(7) A respectively. Essentially planar PtCOO moieties are present, which make an angle of 80.8° with the coordination plane. A trans configuration for a complex containing two Pt11C bonds is unexpected; alkoxycarbonyl ligands therefore show a peculiar behaviour which deserves further investigation.

Crystal and molecular structure of monoclinic di-µ-chloro-tris(triphenylphosphine)dicopper(I), Cu2Cl2(PPh3)3

The structure of di-µ-chloro-tris(triphenylphosphine)dicopper(I), Cu2Cl2(PPh3)3 has been determined from three-dimensional X-ray data collected by counter methods. The crystals are monoclinic, space-group P21/c and have Z= 4 in a unit cell of dimensions a= 19·27(2), b= 9·81(1), c= 27·02(3)A, β= 112° 5′(8′). The structure was refined to R 0·055 for 2046 independent reflections. The crystal contains discrete molecules in which two copper atoms are held together by two chlorine bridges. Two of the three phosphine ligands are bonded to one of the copper atoms and the third to the other, so that one metal atom is four-and the other three-co-ordinate. The mean values for the bonds to four- and three-co-ordinate copper are Cu–P 2·240 and 2·183, and Cu–Cl 2·436 and 2·272 A. These values and certain angular distortions found in the molecule are discussed.