Four different (β-diketonate)3V complexes [β-diketonate = 2,4-pentanedione (acac), 2-acetylcyclohexanone (Cy-acac), 2,2,6,6-tetramethyl-3,5-heptanedione (t-Bu-acac), and 1,1,1,5,5,5-hexafluoro-2,4-pentanedione (F-acac)] have been prepared and tested as catalysts for propylene−ethylene copolymerization with the aim of gaining insights into the structure of the active species. Data on polymer composition and catalyst activity indicate that one of the roles of the Al cocatalyst is to trigger a major ligand scrambling around the transition metal. Attempts to isolate the catalytically active species afforded a compound formulated as {[(β-diketonate)AlCl2][VCl2][(β-diketonate)2AlCl]}. This species, which displayed only a minor catalytic activity, arises from a parasite process (catalyst deactivation). The formulation was supported by chemical degradation experiments with THF, which afforded a mixture of [V2Cl3(THF)6][AlCl4] and [(acac)2Al(THF)2][Al Cl4] (5). Two unprecedented V(II) complexes (R-acac)2V(TMEDA) [R = H(7a), Cy(7b), t-Bu (7c)] have been prepared and reacted with halocarbons to model the reactivation process. The results indicated that the primary role of reactivating substances, commonly employed in the industrial processes, is to reoxidize V(II) to the trivalent state. The reaction formed the catalyst precursor (R-acac)2VCl(TMEDA), which was characterized on the basis of analytical and spectroscopic data. In agreement with this proposal, a trapping experiment carried out with ZnCl2 or oxidation with CuCl allowed the isolation and characterization of (t-Bu-acac)2V(TMEDA)][X] [X = ZnCl4 - (8), CuCl2 - (9)]. The structures of 5, 7a, 8, and 9 have been elucidated by X-ray diffraction. Crystal data are as follows. 5: triclinic space group P1̄, a = 8.916(3) Å, b = 9.802(3) Å, c = 15.390(5) Å, α = 88.156(4)°, β = 86.041(4)°, γ = 82.710(4)°, Z = 2. 7a: triclinic space group P1̄, a = 7.896(1) Å, b = 10.032(2) Å, c = 13.134(2) Å, α = 75.265(2)°, β = 88.520(2)°, γ = 71.445(2)°, Z = 2. 8: orthorhombic space group Pbcn, a = 18.723(7) Å, b = 19.675(7) Å, c = 18.836(7) Å, Z = 4. 9: monoclinic space group C2/c, a = 13.676(5) Å, b = 19.521(3) Å, c = 13.206(3) Å, β = 98.47(3)°, Z = 4.

Reject analysis is an accepted standard of practice for quality assurance in conventional radiology. The need for reject analysis has been challenged by the introduction of computed radiography (CR) because of low reported reject rates and because criteria for improperly exposed images were lacking. Most CR systems include quality control (QC) workstations that are capable of modifying the appearance of images before release, and also of deleting poor images before they are analyzed. Texas Children's Hospital has been using computed radiography since October 1995, and now conducts essentially filmless imaging operations using a large-scale picture archival and communications system (PACS) with fourteen CR units. The QC workstation is a key element of our CR operation; however, the extensive software tools of the workstation are limited in terms of avoiding repeated examinations. Neither the QC workstation nor the PACS itself is designed to support reject analysis, so our task was to design a system that accommodates identification, isolation, and archiving of repeated examinations, making use of our electronic imaging systems. We had already developed transcription codes for our radiologist's examination critique, so we adopted these as codes for rejected images. The technologist at the QC workstation appends the critique code to patient demographic information, modifies other fields to indicate that the image is rejected, and archives as usual. Modified routing tables prevent the release of rejected images but ensure they are available for review. Our frequency and reasons for repeated examinations are comparable to other reports of reject analysis in the literature. The most frequent cause of a repeated examination is mis-positioning. The process of developing the method for capturing repeat, collecting the data, and analyzing it is only one-half of the battle; to achieve an improvement in services, it is necessary to feed back the results to management and staff and to implement training as indicated. It is our intention to share our results with PACS and CR vendors in the hope that they will incorporate some mechanisms for reject analysis into the design of their systems.

SmCl 3 (THF) 3 (THF ) tetrahydrofuran) reacts with anionic dialkylamides R 2 N -[R ) Cy (cyclohexyl), i-Pr (isopropyl), Ph (phenyl)] to give different products, depending on the nature of the R substituents. Reaction with Cy 2 NLi in a 1:2 molar ratio formed [(Cy 2 N) 2 Sm(µ-Cl)(THF)] 2 (1) in 80% yield, whereas reaction with (i-Pr) 2 NLi under similar conditions gave [(i-Pr 2 N) 2 SmCl 3 (Li(TMEDA)) 2 ] (2). Partial loss of THF from complex 1 reorganized the molecule into the tetranuclear (Cy 2 N) 6 Sm 4 Cl 6 (THF) 2 (3). Attempts to reduce complex 1 with a number of reagents gave [(Cy 2 N) 3 SmTHF]‚toluene (5), while [(Cy 2 N) 4 SmLi(THF)] (4) was isolated upon alkylation reactions carried out with either NpLi or NfLi [Np ) CH 2 C(CH 3 ) 3 ; Nf ) CH 2 C(CH 3 ) 2 Ph]. Direct synthesis of Sm(II) amides from SmI 2 (THF) 2 starting material was successful only in the case of diphenylamide anion (Ph 2 N -). Depending on the stoichiometry, -ate (Ph 2 N) 4 Sm[Na(TMEDA)] 2 (6) or neutral [(Ph 2 N) 2 Sm(THF) 4 ]‚THF (7) was obtained. The crystal structures of 1-7 were demonstrated by X-ray diffraction analysis. Crystal data are as follows. 1: C 56 H 105 N 4 O 2 Sm 2 Cl 2 , triclinic,

] complex reported as a byproduct of the reaction between (acac) 3 V and AlCl 3 (THF) 3 (right-hand column, 14th line from the top), we wish to acknowledge that this complex was previously isolated upon reaction of Cl 2 Al(acac) with 2 equiv of THF in methylene chloride (Lewinski, J.; Pasynkiewicz, S. Inorg. Chim. Acta 1987, 130, 23-27) and correctly formulated on the basis of spectroscopic characterization. Several other relevant aluminum acetylacetonate complexes have been described and characterized by NMR spectroscopy and elemental analysis (Lewinski, J.; Pasynkiewicz, A.

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