Sandwich-type Structures Research Articles

STRAIN GAUGE TESTING OF 3D-PRINTED CFRP SANDWICH STRUCTURE

In this article, the authors present composite sandwich-type CFRP structures and a study of their properties by strain gauge testing. The paper presents the modeling of a parameterized elementary unit serving as the core of a 3D printed structure using Fused Deposition Modelling (FDM) technology. The properties of these structures with different outer layers made of pure epoxy resin and resin with 10% and 20% carbon fiber powder were then investigated. Based on the results of the strain gauge tests, material models were reconstructed for each resin layer, which can be used in computer FEM studies of more complex components. As an application example, a strength analysis of the driver's seat of a Greenpower car made with printed sandwich structures coated with carbon fiber powder resin was conducted.

Sandwich-type aptamer-based biosensors for thrombin detection.

Thrombin, a proteolytic enzyme, plays an essential role in catalyzing many blood clotting reactions. Thrombin can act as a marker for some blood-related diseases, such as leukemia, thrombosis, Alzheimer's disease and liver disease. Therefore, its diagnosis is of great importance in the fields of biological and medical research. Biosensors containing sandwich-type structures have attracted much consideration owing to their superior features such as reproducible and stable responses with easy improvement in the sensitivity of detection. Sandwich-type platforms can be designed using a pair of receptors that are able to bind to diverse locations of the same target. Herein, we investigate recent advances in the progress and applications of thrombin aptasensors containing a sandwich-type structure, in which two thrombin-binding aptamers (TBAs) identify different parts of the thrombin molecule, leading to the formation of a sandwich structure and ultimately signal detection. We also discuss the pros and cons of these approaches and outline the most logical approach in each section.

Sandwich-type channeled chemical hydrogels manufactured by 3D ink-jet printing under freezing conditions using a photochemical process for human cell cultures

This work aims to present the technology of preparing chemical hydrogels for cell culture by printing. It consists in drop-on-drop 3D ink-jet printing under freezing conditions of a solution containing a diacrylic compound (poly(ethylene glycol) diacrylate (PEGDA)) with an initiator (hydrogen peroxide) of photochemically induced polymerization and cross-linking. 3D printing takes place at a low temperature, thanks to which droplets of the solution immediately freeze, allowing the desired structures of the frozen solution to be built. UVC radiation of frozen structures at room temperature and in the presence of oxygen induces simultaneous melting and radical polymerization and cross-linking of PEGDA, resulting in the formation of a firm chemical hydrogel with the same shape as the shape of the printed frozen solution. Discussed are: (i) technological parameters, (ii) reaction mechanism, (iii) sol–gel analysis, (iv) morphology of printed hydrogels and (v) biocompatibility studies with the applied human fibroblasts, human umbilical vein endothelial cells and human neuroblastoma cells. A method for printing sandwich-type channeled structures was developed and shown in application for cell growth. This work demonstrates the feasibility of 3D printing biocompatible chemical hydrogels for cell culture and opens new possibilities for printing hybrid structures for biomedical applications.

Diffusion mechanisms between La2SiO5 and SiO2 during formation of textured lanthanum silicate oxyapatite crystals

Lanthanum silicate oxyapatite (LSO) is a promising alternative electrolyte for solid oxide fuel cell (SOFC) applications. They exhibit anisotropic oxide-ionic conduction; therefore, a preferential crystal orientation along the c-axis is necessary to achieve optimal conductivity. This study is performed to understand the reaction mechanisms involved in the formation of an LSO layer at the interface of the La2SiO5/SiO2 diffusion pair during reactive sintering at high temperatures. Different La2SiO5/SiO2 bilayers were fabricated in sandwich-type structures using silica-type quartz or La2SiO5 supports obtained via electrophoresis and uniaxial pressing, respectively. These supports were pre-sintered and then coated with a suspension of the opposite component. The diffusion pairs were isothermally heated at 1500 °C at different holding times (10, 20, and 40 h) and at 1600 °C for 10 h. Their surfaces and cross-sections were investigated via X-ray diffraction, scanning electron microscopy (SEM) observations, energy-dispersive X-ray spectroscopy (EDS), and Raman spectroscopy.Experimental results show that two different driving forces were involved in the nucleation and growth of apatite crystals. The first is the chemical potential gradient in lanthanum between the components of the layers, and the second is the electric field created to preserve the electroneutrality of the system. The predominant diffusing species in this bilayer system were La+III and O-II, and their diffusion is enhanced by the formation of a glassy phase on the silica grains through the transformation of silica-type quartz into cristobalite. The formation of a La2Si2O7 intermediate phase was detected, which is vital to the interdiffusion of species and the nucleation and growth of oriented apatite crystals. The porosity gradient observed in the cross-section after reactive diffusion suggests the possibility of achieving an SOFC half-cell device via this process.

Supramolecular Host–Guest Assemblies of [M6Cl14]2–, M = Mo, W, Clusters with γ-Cyclodextrin for the Development of CLUSPOMs

Host–guest assemblies open up opportunities for developing novel functional CLUSPOM multicomponent systems based on transition metal clusters (CLUS), polyoxometalates (POMs) and macrocyclic organic ligands. In water–ethanol solution γ-cyclodextrin (γ-CD) interacts with halide metal clusters [M6Cl14]2– (M = Mo, W) to form sandwich-type structures. The supramolecular association between the clusters and CDs, however, remains weak in solution, and the interactions are not strong enough to prevent the hydrolysis of the inorganic guest. Although analysis of the resulting crystal structures reveals inclusion complexation, 1H NMR experiments in solution show no specific affinity between the two components. The luminescent properties of the host–guest compounds in comparison with the initial cluster complexes are also studied to evaluate the influence of CD.

Fabrication of ultrathin, flexible, all-in-one paper supercapacitor with high electrochemical performance based on multi-layer forming in paper sheet formation technology

In the era of miniaturization, low-cost, high mechanical stability and lightweight are the pre-requisites for the commercialization of smart-wearable supercapacitors (SCs). For this purpose, light-weight, binder-free, sustainable cellulose based thin-films with conductive polymers such as polypyrrole (PPy) have attained considerable attention. However, the delamination of the conductive materials in sandwich-type structures of SCs, particularly during the cyclic bending process at high current densities, is a great challenge for wearable SCs. To circumvent the problem of delamination of conductive materials, multi-layer forming concept that consists of three sequential steps (forming, pressing and drying) in papermaking technology has inspired us to prepare mechanically ultra-stable paper electrodes towards for wearable SCs. For this reason, we firstly adopted the multi-layer concept to design all-in-one paper flexible SCs by integrating PPy-modified cellulose fibers as electrodes, and un-modified cellulose paper as a separator. More importantly, a cocklebur like structure of PPy-modified cellulose fiber has been attained by the virtue of the strong inter-molecular hydrogen bonding between free hydroxyl groups on the surface cellulose fibers and PPy, resulting in increased PPy loading, and thus enhancing the electrochemical properties of the all-in-one paper supercapacitor. Furthermore, these hydroxyl groups facilitate the inter-layer bonding of the paper structure in the subsequent pressing and drying processes, favoring the integration of the electrodes and separator. Benefiting from the multi-layer forming concept and suitable morphology of PPy- modified cellulose, we have prepared an ultra-thin (150 μm) all-in-one paper SC with high areal specific capacitance (up to 562 mF cm−2), high energy density (up to 3.1 mWh cm−3) and high-power density (up to 414.9 mW cm−3). Moreover, the all-in-one paper SC shows excellent flexibility, with negligible specific capacitance loss by bending at 0° to 180° angles after repeating 1000 times. The proposed concept and supercapacitor fabrication process is scalable and can be readily implemented in a modern paper industry.

Studies related to changes in ionic strength due to addition of sodium chloride to an aqueous solution of cationic dye methylene blue (MB): Electronic spectra analysis for dimerization equilibria

It is well-known that methylene blue (MB),a cationic dye forms dimer and higher aggregates in aqueous solutions. For specific concentration range (1∙10−6–1∙10−4 M), a D⇌2M equilibria has been postulated and well characterized by visible electronic spectroscopy. The dye molecules can lead to formation of stacking (end on end and sandwich) type structures in solution phase stabilized by weak dispersion van der Waals forces and cation-anion-cation ion pairs and triplet interactions usually referred as hydrophobic interactions. Recent literature reports indicate that excess chemical potential of small hydrophobic particles and the strength of hydrophobic pair interactions appear to be linearly related. Our observations about precipitation of dye (MB) at higher concentrations of salt like sodium chloride (NaCl) made us to study the dimerization equilibria in detail since in the Corona health disaster; use of MB to diagnose the type of Covid variants helped Scientists in a great way. Salting-out of proteins is in general used to explain the specific protein aggregation behaviour observed in Huntington's disease. For model compound like tertiary butyl-alcohol, Geiger et al. reported enhancement of hydrophobic contacts which increase with increase in salt concentration. NaCl is an essential ingredient of biological fluids, of which concentration controls the various biological life governing processes. However, such information is non-existent for aqueous solutions of dyes.We therefore reported measured optical absorbance of aqueous solution containing NaCl (0.1–1.0 M) and dye at different concentrations (1∙10−6–1∙10−4 M) at 298 K spectrophotometrically, in the wavelength range of 550–700 nm. The spectrums are compared to those obtained for binary solutions of MB in water. Analysis of the spectral data yielded the dimer dissociation constant and individual characteristic monomer and dimer spectra. Further information about the geometry and twist angle between the dipoles of the MB molecules in the dimer species has been obtained. The results are analyzed for the monomer-dimer equilibria and salting-out effect as well applications of mass action model and exciton theories are discussed.

Recent progress in advanced flexible zinc ion battery design

Flexible zinc ion batteries are a promising energy supply for flexible and wearable electronic devices due to their high theoretical capacity, superior safety, low cost, and eco-friendliness. The rational design of highly efficient flexible zinc ion batteries with high electrochemical performance, long cycling life, and excellent mechanical deformability is still a formidable challenge, limiting their widespread application in flexible electronics. This paper summarizes the recent progress in designing and fabricating flexible zinc ion batteries from three aspects: device configurations, battery constituent components, and practical applications. First, we introduce the energy storage mechanism and summarize modification strategies of constituent components, including current collector, zinc anode, cathode, and solid/gel electrolyte, revealing their positive effects on the performance of flexible zinc ion batteries. Then, we elucidate advanced device configurations for flexible zinc ion batteries such as sandwich-type, planar, and fiber-type structures, followed by discussion on the specific properties of flexible zinc ion batteries that enhance their adaptability in practical applications, such as integration with electronics, self-healing ability, resistance to extreme conditions, and responsiveness to environments. The current challenges and outlooks are also presented.

Surface-enhanced Raman scattering biosensor-based sandwich-type for facile and sensitive detection of Staphylococcus aureus

Foodborne infections caused by Staphylococcus aureus (S. aureus) represent a serious health risk to the general public. In this study, we proposed a surface-enhanced Raman spectroscopy (SERS) biosensor based on a signal-amplification sandwich-type system for detecting and quantifying S. aureus. Herein, the SERS response of SiO2-coated Au@Ag was studied using Raman signals from Au@Ag@SiO2 nanoparticles and van grafted PDMS for the target acquisition. It has been shown that SiO2 coated Au@Ag exhibited a robust and strong response to SERS. Therefore, the combination of vancomycin (Van) and polydimethylsiloxane (PDMS) allowed a more efficient capture of S. aureus, resulting in improved repeatability through simplified separation procedures. Consequently, as a result of the presence of S. aureus targets, sandwich-type conjugate structures between the capture and signal units, as well as synergistic Raman amplification, were observed. Based on optimized conditions, a good linear correlation (y = 1087.86x - 540.75, R2 = 0.9958) in a wide dynamic range of 38–3.8 × 107 cfu·mL−1 was achieved with a limit of detection of 2 cfu·mL−1. The accuracy of the sensors was evaluated using a standard addition technique, and the results were satisfactory. Finally, the overall biosensor demonstrated potential application for monitoring S. aureus with good recoveries.

Structured Thin Films Based on Synergistic Effects of MnTa2O6 Oxide and bis-Carboxy-phenyl-substituted Porphyrins, Capable to Inhibit Steel Corrosion

Covering steel surfaces with suitable materials with the capacity to protect against corrosion represents a challenge for both research and industry, as steel, due to its paramount utility, is the most recycled material. This study presents the realization of new sandwich type materials based on 5,10-(4-carboxy-phenyl)-15,20-(4-phenoxy-phenyl)-porphyrin or 5,15-(4-carboxy-phenyl)-10,20-diphenylporphyrin and MnTa2O6 designed to improve corrosion inhibition of steel in aggressive media. The thin films, designed as single- or sandwich-type structures were obtained on carbon steel through the drop-casting technique. Morphological investigations of thin films were carried out by field emission-scanning electron microscopy (SEM) and atomic force microscopy (AFM). The inhibition of a steel corrosion process was evaluated in an aggressive environment of 0.1 M HCl by performing electrochemical investigations such as open circuit potential (OCP) and the potentiodynamic polarization technique. The influence of variations in the cathodic Tafel slopes βc and anodic Tafel slopes βa over the corrosion rates was discussed. The best corrosion inhibition efficiency of 91.76% was realized by the steel electrode covered with sandwich-type layers of 5,15-(4-carboxy-phenyl)-10,20-diphenylporphyrin on the bottom layer and MnTa2O6 on the top. The effect of location of the COOH groups in the cis or trans position on the tetrapyrrolic ring was also discussed to understand the corrosion inhibition mechanism.

Experimental studies on the interaction mechanism of landslide stabilizing piles and sandwich-type bedrock

The mechanical properties of stabilizing piles are directly affected by the spatial structure of the bedrock of landslides. To effectively reinforce landslides with a sandwich-type bedrock, the interaction mechanism between manually excavated stabilizing piles and the different types of bedrock must be investigated. The research presents experimental models of manually excavated stabilizing piles for two kinds of sandwich-type bedrock structures, namely hard rock–dominant interlayered bedrock (HDIB) and weak rock–dominant interlayered bedrock (WDIB). Six physical models with different bedrock structure types and dip angles θ were prepared to explore the influence of these two parameters on the pile-bedrock interaction mechanism. The mechanical properties of the pile were investigated based on the monitoring results of the earth pressure distributions against the piles and the bending moment distributions. The embedded section of the pile experiences a rotation subjected to lateral soil movement. The center point of the rotation in the HDIB is at a greater elevation than in the WDIB. The earth pressure in the HDIB appears to increase with dip angle θ but is similar to each other in the WDIB. The pile embedded in the HDIB exhibits flexible behavior, whereas the pile embedded in the WDIB exhibits rigid behavior. With the increase of dip angle θ in the WDIB material, the bending moment steadily rises followed by fluctuation. The maximum bending moment of the embedded section increases with θ. These results provide a basis for improving the design of manually excavated stabilizing piles in sandwich-type bedrock.

Investigation of physical properties of Fe2O3 and graphene-based sandwich-type electrodes for biosensor technology

The usage of composite materials in which graphene combined with magnetic nanoparticles offers benefits for biomedical applications. Stabilization of nanoparticles on the electrode surface which is necessary for biosensors and other applications is still an important issue to be solved. Here the stabilization of the nanoparticles is achieved by inserting nanoparticles between two graphene layers in a sandwich structure. Furthermore, it has been theoretically predicted that sandwich-type structures prepared with metal nanoparticles between two graphene layers would have extraordinary physical properties. In this study, Fe2O3/SLG (single-layer graphene) and the sandwich-type SLG/Fe2O3/SLG electrodes were produced. Fe2O3 nanoparticles were synthesized by the sol–gel method, and graphene was produced by CVD (chemical vapor deposition) on Cu foil and then transferred onto FTO (fluorine-doped tin oxide). Fe2O3/SLG composite structure was produced by the drop-casting process. The structural, magnetic, and electrochemical properties of samples were investigated in detail. Structural analysis revealed that Fe2O3 has an α-phase with a rhombohedral crystal structure and the mean particle diameter is 128 nm. Raman and SEM analysis also confirmed the quality of SLG and the sandwich-type graphene structure. The nanoparticles have a magnetic phase transition which has Morin temperature at about T = 263 K. Also, Fe2O3 nanoparticles have shown ferromagnetic behavior at room temperature with 0.16 Am2/kg remanent magnetization and 0.203 T coercive field. This work demonstrates the effectiveness of graphene sandwich-type electrodes to eliminate the main stabilization obstacle of magnetic nanoparticles especially for biosensor applications.

Improvements to Biomaterials Structure Used in Acrylic Prostheses

Achievements in the field of biomaterials have as a basis three scientific domains: chemistry, biology and physics, then the technical application or the putting up culminating with clinical achievements. Dental prostheses, regardless of their type, replaced the lack of teeth. Although, mainly, many people that lost their teeth could continue life with the help of prostheses in a way pretty decent, dental prostheses shows some disadvantages that make them even unbearable for some people. The study aims at analyzing the biological behavior of sandwich type structures that reunite the classic acrylate and a silicone layer, namely the Flexite type elastic acrylate. We used a silicone material, RUBBER 732 RTV, which is frequently used in mucous-bone support deficits, especially in oral maxillofacial surgery post interventions. The tensile strength was tested ona TEXTENSER traction test machine, in view of establishing the chemical analysis of the 2 materials used. Regarding the maximum tensile strength for sandwich-type test samples, the flexible acrylate - silicone RUBBER 732 RTV structure broke at 1125N, while for the classic acrylate - silicone material a fracture value of 950N was recorded. The structure of the two biomaterials within the sandwich type test samples decisively influences the force and tension at which the fracture occurs, as well as the fracture route, which is linear, without the displacement of fragments, both the flexible acrylate and the silicone being affected, respectively a linear route at the level of the acrylic structure, affecting the acrylate - silicone interface for the classic acrylate - silicone test samples.

Application of a microfluidic paper-based bioimmunosensor with laser-induced fluorescence detection in the determination of alpha-fetoprotein from serum of hepatopaths

As a common plasma protein, alpha-fetoprotein (AFP) is widely applied as the tumor biomarker for the diagnosis of many cancers. To develop a low cost, high sensitive and high-throughput method for the determination of AFP is significant for the disease diagnosis. In this work, an immunoassay with sandwich-type structures was performed on a paper-based chip for the analysis of AFP. AFP could be captured by the primary antibodies which were immobilized on the paper by chitosan. On the secondary antibodies, the modified initiator DNAs could trigger the hybridization chain reaction to amplify the fluorescence signals for AFP. A laser-induced fluorescence detector coupled with an interface was applied to detect the targets on the paper-based chip. Under the optimized conditions, the detection limit for AFP was 1.0 pg/mL. For every test, the sample solution consumption only was 10 μL. Finally, the method was applied to determine the AFP in serum of normal person and hepatopaths with hepatic malignant tumor, chronic hepatitis B and other suspected liver diseases. The AFP could be found from all of the samples and the results were similar to that obtained by chemiluminescence immunoassay. The recoveries for AFP ranged from 93.8% to 106%, which indicated the method was reliable. The method based on paper chip had great potential in the application of AFP determination.

Mitigation of vibrations in sandwich-type structures by a controllable constrained layer

This study presents and tests a method for semi-active control of vibrations in sandwich-type beam structures. This method adapts a strategy called prestress accumulation release. The prestress accumulation release strategy is based on structural reconfiguration: it uses short time, impulsive and localised changes of actuator properties (such as stiffness or damping), which are applied to a part of the system in the moments, when its strain energy attains a local maximum. The method has been earlier applied as a global control scheme to mitigate the fundamental vibration mode of a cantilever beam (by stiffness control) and in the task of mitigating the first four modes of a frame structure (by damping control). This study proposes a prestress accumulation release strategy and tests its effectiveness for the case of a three-layered sandwich structure, with the internal layer fabricated from a material with dissipative characteristic locally controllable through the material damping coefficient. In contrast to the earlier research, the control is applied thus at the level of material characteristics instead of a discrete set of dedicated actuators. Based on the finite element method, a numerical experiment involving a passively damped, as well as prestress accumulation release-controlled, three-layered cantilever beam excited by initial displacements was performed. The effectiveness of the approach was studied for a broad range of internal layer damping parameters. The presented results revealed a high potential of the prestress accumulation release strategy in semi-active damping of vibrations of sandwich-type structures.

Unraveling the Crystallization Kinetics of 2D Perovskites with Sandwich-Type Structure for High-Performance Photovoltaics.

2D perovskite solar cells with high stability and high efficiency have attracted significant attention. A systematical static and dynamic structure investigation is carried out to show the details of 2D morphology evolution. A dual additive approach is used, where the synergy between an alkali metal cation and a polar solvent leads to high-quality 2D perovskite films with sandwich-type structures and vertical phase segregation. Such novel structure can induce high-quality 2D slab growth and reduce internal and surface defects, resulting in a high device efficiency of 16.48% with enhanced continuous illumination stability and improved moisture (55-60%) and thermal (85°C) tolerances. Transient absorption spectra reveal the carrier migration from low n to high n species with different kinetics. An [PbI6 ]4- octagon coalescence transformation mechanism coupled with metal and organic cations wrapped is proposed. By solvent vapor annealing, a recrystallization and reorientation of the 2D perovskite slabs occurs to form an ideal structure with improved device performance and stability.

Controlling Properties of Interfaces in Nonlinear Sandwich-Type Structures with a Defocusing Internal Layer

A model of a three-dimensional optical structure in which the plane-parallel boundaries have intrinsic nonlinear properties is considered. The internal layer with a finite thickness is an optically transparent medium with defocusing Kerr nonlinearity; on the outside, it is in contact with dielectric linear half-spaces. The mathematical formulation of the model reduces to the nonlinear Schrodinger equation with a positive coefficient of cubic nonlinearity and with a nonlinear self-consistent potential. It is shown analytically that the system contains a nonlinear light wave propagating along the optical layer and localized in dielectric plates. The frequencies of light-field localization in this structure are obtained and the conditions for their existence are determined for different characteristics of the media and the interfaces between them. It is shown that the light field can be localized along the layers for different signs of the nonlinear response of the interfaces between layers of the three-layer structure in the case where one of them is characterized by a focusing nonlinearity; and the other, by a defocusing one.

Designing sandwich-type single-layer graphene decorated by copper nanoparticles for enhanced sensing properties

It has already been theoretically shown that sandwich-type structures prepared by stacking metal nanoparticles in two graphene layers would have exceptional optical and electrical properties for practical applications. These sandwich structures designed by band gap engineering could lead to materials capable of being developed to meet industrial demands. In order to confirm this theoretical approach, copper nanoparticle (CuNP) islands decorated in sandwich-type single-layer graphene (SLG) have been designed and used as a non-enzymatic sensor platform for the first time. Firstly, SLG has been synthesized on copper foil using a chemical vapor deposition technique and transferred onto a fluorine-doped tin oxide surface. Next, CuNPs on this SLG have been prepared using an inert-gas condensation method based on DC magnetron sputtering. A sensor platform based on the sandwich-type hetero-structure (SLG/CuNP/SLG) has been constructed by transferring another single layer of graphene onto the prepared CuNP-decorated graphene layer. In this way, a unique sandwich structure has been obtained for further applications by stacking nanoparticles with high stability, controlled size and regular particle distribution between impurity-free, large-area single layers of graphene. The sensor properties of this sandwich structure to the saccharides have been compared with those of the single-layer sensor platform (CuNP/SLG). In accordance with the theoretical studies in the literature, it has been found that the sandwich structure greatly improves sensor properties such as limit of detection, stability and response time. More importantly, it was determined that the sandwich structure acts as a shielding layer, protecting nanoparticles from electrochemical/optical and other environmental conditions.

An Experimental Investigation of Modal Densities of Composite Honeycomb Sandwich Cylindrical Shells

Honeycomb sandwich composite cylindrical shells are widely used in aerospace structures. Experimentally observed modal densities of such shells are not reported. In this work, modal densities of a typical honeycomb sandwich composite cylinder are obtained experimentally by measuring the drive point admittance. The results are in good agreement with those estimated theoretically that incorporated transverse shear deformation. Its limitations at higher frequencies are investigated and the frequency beyond which the estimation is in error is determined. The results provide an example to prove the need for measuring the imaginary part of the driving point admittance and using it in the determination of the modal densities of honeycomb sandwich-type structures. Experiments are carried out with two boundary conditions for the cylinder and the results provide experimental evidence for the fact that the modal densities at high frequencies do not depend on the boundary conditions. At higher frequencies, it is expected that both of the face sheets vibrate independently. This frequency can be approximately estimated as the fundamental bending mode frequency of the wall of the honeycomb core. The modal density determined through the measured driving point admittance will have a sharp reduction at this frequency and this feature can be used in identifying this phenomenon. The experimental results are in very good agreement with the above results.

Ligand conformations and spin states in sandwich-type complexes of the split (3+2) five-electron donor hydrocarbon ligand bicyclo[3.2.1]octa-2,6-dien-4-yl (bcod)

Sandwich-type structures are energetically preferred for the bis(bicyclo[3.2.1]octa-2,6-dien-4-yl) complexes of the first row transition metals (bcod)2M including the experimentally known chromium and iron derivatives.