Overcoming Resistance of Caco-2 Cells to 5-Fluorouracil through Diruthenium Complex Encapsulation in PMMA Nanoparticles.

Drug resistance, one of the main drawbacks in cancer chemotherapy, can be tackled by employing a combination of drugs that target different biological processes in the cell, enhancing the therapeutic efficacy. Herein, we report the synthesis and characterization of a new paddlewheel diruthenium complex that includes 5-fluorouracil (5-FU), a commonly used anticancer drug. This drug was functionalized with a carboxylate group to take advantage of the previously demonstrated release capacity of carboxylate ligands from the diruthenium core. The resulting hydrophobic complex, [Ru2Cl(DPhF)3(5-FUA)] (Ru-5-FUA) (DPhF = N,N'-diphenylformamidinate; 5-FUA = 5-fluorouracil-1-acetate) was subsequently entrapped in poly(methyl methacrylate) (PMMA) nanoparticles (PMMA@Ru-5-FUA) via a reprecipitation method to be transported in biological media. The optimized encapsulation procedure yielded particles with an average size of 81.2 nm, a PDI of 0.11, and a zeta potential of 29.2 mV. The cytotoxicity of the particles was tested in vitro using the human colon carcinoma cell line Caco-2. The IC50 (half maximal inhibitory concentration) of PMMA@Ru-5-FUA (6.08 µM) was just slightly lower than that found for the drug 5-FU (7.64 µM). Most importantly, while cells seemed to have developed drug resistance against 5-FU, PMMA@Ru-5-FUA showed an almost complete lethality at ∼30 µM. Conversely, an analogous diruthenium complex devoid of the 5-FU moiety, [Ru2Cl(DPhF)3(O2CCH3)] (PMMA@RuA), displayed a reduced cytotoxicity at equivalent concentrations. These findings highlight the effect of combining the anticancer properties of 5-FU with those of diruthenium species. This suggests that the distinct modes of action of the two chemical species are crucial for overcoming drug resistance.

A Diruthenium Metallodrug as a Potent Inhibitor of Amyloid-ß Aggregation: Synergism of Mechanisms of Action.

The physical and chemical properties of paddlewheel diruthenium compounds are highly dependent on the nature of the ligands surrounding the bimetallic core. Herein, we compare the ability of two diruthenium compounds, [Ru2Cl(D-p-FPhF)(O2CCH3)3]·H2O (1) (D-p-FPhF- = N,N'-bis(4-fluorophenyl)formamidinate) and K3[Ru2(O2CO)4]·3H2O (2), to act as inhibitors of amyloid aggregation of the Aß1-42 peptide and its peculiar fragments, Aß1-16 and Aß21-40. A wide range of biophysical techniques has been used to determine the inhibition capacity against aggregation and the possible mechanism of action of these compounds (Thioflavin T fluorescence and autofluorescence assays, UV-vis absorption spectroscopy, circular dichroism, nuclear magnetic resonance, mass spectrometry, and electron scanning microscopy). Data show that the most effective inhibitory effect is shown for compound 1. This compound inhibits fiber formation and completely abolishes the cytotoxicity of Aß1-42. The antiaggregatory capacity of this complex can be explained by a binding mechanism of the dimetallic units to the peptide chain along with π-π interactions between the formamidinate ligand and the aromatic side chains. The results suggest the potential use of paddlewheel diruthenium complexes as neurodrugs and confirm the importance of the steric and charge effects on the properties of diruthenium compounds.

Comparative Analysis of the Inhibitory Mechanism of Aß1-42 Aggregation by Diruthenium Complexes.

There is a growing interest in the search for metal-based therapeutics for protein misfolding disorders such as Alzheimer's disease (AD). A novel and largely unexplored class of metallodrugs is constituted by paddlewheel diruthenium complexes, which exhibit unusual water solubility and stability and unique coordination modes to proteins. Here, we investigate the ability of the complexes [Ru2Cl(DPhF)(O2CCH3)3]·H2O (1), [Ru2Cl(DPhF)2(O2CCH3)2]·H2O (2), and K2[Ru2(DPhF)(CO3)3]·3H2O (3) (DPhF- = N,N'-diphenylformamidinate) to interfere with the amyloid aggregation of the Aß1-42 peptide. These compounds differ in charge and steric hindrance due to the coordination of a different number of bulky ligands. The mechanisms of action of the three complexes were studied by employing a plethora of physicochemical and biophysical techniques as well as cellular assays. All these studies converge on different mechanisms of inhibition of amyloid fibrillation: complexes 1 and 2 show a clear inhibitory effect due to an exchange ligand process in the Ru2 unit aided by aromatic interactions. Complex 3 shows no inhibition of aggregation, probably due to its negative charge in solution. This study demonstrates that slight variations in the ligands surrounding the bimetallic core can modulate the amyloid aggregation inhibition and supports the use of paddlewheel diruthenium complexes as promising therapeutics for Alzheimer's disease.

Effect of Equatorial Ligand Substitution on the Reactivity with Proteins of Paddlewheel Diruthenium Complexes: Structural Studies.

The paddlewheel [Ru2Cl(O2CCH3)4] complex was previously reported to react with the model protein hen egg white lysozyme (HEWL), forming adducts with two diruthenium moieties bound to Asp101 and Asp119 side chains upon the release of one acetate. To study the effect of the equatorial ligands on the reactivity with proteins of diruthenium compounds, X-ray structures of the adducts formed when HEWL reacts with [Ru2Cl(D-p-FPhF)(O2CCH3)3] [D-p-FPhF = N,N'-bis(4-fluorophenyl)formamidinate] under different conditions were solved. [Ru2Cl(D-p-FPhF)(O2CCH3)3] is bonded through their equatorial positions to the Asp side chains. Protein binding occurs cis or trans to D-p-FPhF. Lys or Arg side chains or even main-chain carbonyl groups can coordinate to the diruthenium core at the axial site. Data help to understand the reactivity of paddlewheel diruthenium complexes with proteins, providing useful information for the design of new artificial diruthenium-containing metalloenzymes with potential applications in the fields of catalysis, biomedicine, and biotechnology.

pH- and Time-Dependent Release of Phytohormones from Diruthenium Complexes.

The controlled release of functionally active compounds is important in a variety of applications. Here, we have synthesized, characterized, and studied the magnetic properties of three novel metal-metal-bonded tris(formamidinato) Ru25+ complexes. We have used different auxin-related hormones, indole-3-acetate (IAA), 2,4-dichlorophenoxyacetate (2,4-D), and 1-naphthaleneacetate (NAA), to generate [Ru2Cl(µ-DPhF)3(µ-IAA)] (RuIAA), [Ru2Cl(µ-DPhF)3(µ-2,4-D)] (Ru2,4-D), and [Ru2Cl(µ-DPhF)3(µ-NAA)] (RuNAA) (DPhF = N,N'-diphenylformamidinate). The crystal structures of RuIAA, RuIAA·THF, Ru2,4-D·CH2Cl2, and RuNAA·0.5THF have been determined by single-crystal X-ray diffraction. To assess the releasing capacity of the bound hormone, we have employed a biological assay that relied on Arabidopsis thaliana plants expressing an auxin reporter gene and we demonstrate that the release of the phytohormones from RuIAA, Ru2,4-D, and RuNAA is pH- and time-dependent. These studies serve as a proof of concept showing the potential of these types of compounds as biological molecule carriers.

Fingerprinting the junctions of RNA structure by an open-paddlewheel diruthenium compound.

RNA function is determined by its structural organization. The RNA structure consists of the combination of distinct secondary structure motifs connected by junctions that play an essential role in RNA folding. Selective 2'-hydroxyl acylation analyzed by primer extension (SHAPE) probing is an established methodology to analyze the secondary structure of long RNA molecules in solution, which provides accurate data about unpaired nucleotides. However, the residues located at the junctions of RNA structures usually remain undetected. Here we report an RNA probing method based on the use of a novel open-paddlewheel diruthenium (OPW-Ru) compound [Ru2Cl2(µ-DPhF)3(DMSO)] (DPhF = N,N'-diphenylformamidinate). This compound has four potential coordination sites in a singular disposition to establish covalent bonds with substrates. As a proof of concept, we have analyzed the reactivity of OPW-Ru toward RNA using two viral internal ribosome entry site (IRES) elements whose function depends on the structural organization of the molecule. Our study suggests that the compound OPW-Ru preferentially attacks at positions located one or two nucleotides away from junctions or bulges of the RNA structure. The OPW-Ru fingerprinting data differ from that obtained by other chemical reagents and provides new information about RNA structure features.

Crystal structure of poly[[trans-di-aqua-bis-[µ2-trans-4,4'-(diazenedi-yl)dipyridine]-nickel(II)] diiodide ethanol disolvate].

In the title compound, {[Ni(C10H8N4)2(H2O)2]I2·2C2H5OH} n , the complex shows an octa-hedral environment of the Ni(2+) cation in which it is located on a centre of symmetry, linked to two water mol-ecules and the pyridine-N atoms of four 4,4'-(diazenediyl)dipyridine ligands bridging Ni(2+) cations along the b- and c-axis directions, giving rise to a two-dimensional arrangement. The Ni-N bond lengths are in the range 2.109 (4)-2.186 (3) Šand the Ni-O bond length is 2.080 (3) Å. The 4,4'-(diazenedi-yl)dipyridine ligand lies on an inversion centre. An O-H⋯O hydrogen-bond inter-action is observed between water and ethanol mol-ecules. The I(-) ions can be regarded as free anions in the crystal lattice.

Personalized Assessment of Mortality Risk and Hospital Stay Duration in Hospitalized Patients with COVID-19 Treated with Remdesivir: A Machine Learning Approach.

Background: Despite advancements in vaccination, early treatments, and understanding of SARS-CoV-2, its impact remains significant worldwide. Many patients require intensive care due to severe COVID-19. Remdesivir, a key treatment option among viral RNA polymerase inhibitors, lacks comprehensive studies on factors associated with its effectiveness. Methods: We conducted a retrospective study in 2022, analyzing data from 252 hospitalized COVID-19 patients treated with remdesivir. Six machine learning algorithms were compared to predict factors influencing remdesivir's clinical benefits regarding mortality and hospital stay. Results: The extreme gradient boost (XGB) method showed the highest accuracy for both mortality (95.45%) and hospital stay (94.24%). Factors associated with worse outcomes in terms of mortality included limitations in life support, ventilatory support needs, lymphopenia, low albumin and hemoglobin levels, flu and/or coinfection, and cough. For hospital stay, factors included vaccine doses, lung density, pulmonary radiological status, comorbidities, oxygen therapy, troponin, lactate dehydrogenase levels, and asthenia. Conclusions: These findings underscore XGB's effectiveness in accurately categorizing COVID-19 patients undergoing remdesivir treatment.

Torsion Effects Beyond the δ Bond and the Role of π Metal-Ligand Interactions.

Previous studies on bimetallic paddlewheel compounds have established a direct correlation between metal-metal distance and ligand torsion angles, leading to the rule that higher torsion results in longer metal-metal bond distances. Here, the new discovery based on diarylformamidinate Ru25⁺ paddlewheel compounds [Ru2Cl(DArF)4] that show an opposite behavior is reported: higher torsions lead to shorter metal-metal distances. This discovery challenges the assumption that internal rotation solely impacts the δ bond. By combining experimental and theoretical techniques, it is demostrated that this trend is associated with previously overlooked π metal-ligand interactions. These π metal-ligand interactions are a direct consequence of the paddlewheel structure and the conjugated nature of the bidentate ligands. This findings offer far-reaching insights into the influence of equatorial ligands and their π-conjugation characteristics on the electronic properties of paddlewheel complexes. That this effect is not exclusive of diruthenium compounds but also occurs in other bimetallic cores such as ditungsten or dirhodium is demonstrated, and with other ligands showing allyl type conjugation. These results provide a novel approach for fine-tuning the properties of these compounds with significant implications for materials design.

Modulation of the magnetic properties of two-dimensional compounds [NiX2(N-N)] by tailoring their crystal structure.

A series of 2-D nickel compounds with the stoichiometry [NiCl2(N-N)] has been prepared, [N-N = pyrazine (1), 4,4'-bipyridine (2), trans-4,4'-azopyridine (3), trans-1,2-bis(4-pyridyl)ethylene (4), and 1,2-bis(4-pyridyl)ethane (5)]. The complex [NiBr2(4,4'-bpy)] (6) was also obtained for comparative reasons. Compound 2 is the ß phase of the previously reported complex [NiCl2(4,4'-bpy)]. The syntheses of complexes 1-6 were carried out using solvothermal and microwave techniques. The compounds have been characterized by elemental analysis, infrared spectroscopy, thermogravimetry, and powder X-ray diffraction. The crystal structures of compounds 1, 4, and 5 have been solved using ab initio X-ray powder diffraction methods. Compounds 1-6 show the same arrangement, and their structures are described as layers formed by [NiX2] chains linked perpendicularly by N-N ligands. The magnetic properties of the compounds are explained as a balance between the ferromagnetic interactions along the [NiX2] chains and the antiferromagnetic interactions between chains from different layers. This work demonstrates that this balance can be tuned by the length of the N-N ligand.

Steric hindrance and charge influence on the cytotoxic activity and protein binding properties of diruthenium complexes.

Paddlewheel diruthenium complexes are being used as metal-based drugs. It has been proposed that their charge and steric properties determine their selectivity towards proteins. Here, we explore these parameters using the first water-soluble diruthenium complex bearing two formamidinate ligands, [Ru2Cl(DPhF)2(O2CCH3)2], and two derivatives, [Ru2Cl(DPhF)(O2CCH3)3] and K2[Ru2(DPhF)(CO3)3] (DPhF- = N,N'-diphenylformamidinate), with one formamidinate. Their protein binding properties have been assessed employing hen egg white lysozyme (HEWL). The results confirm the relationship between the type of interaction (coordinate/non-coordinate bonds) and the charge of diruthenium complexes. The crystallization medium is also a key factor. In all cases, diruthenium species maintain the M-M bond and produce stable adducts. The antiproliferative properties of these diruthenium complexes have been evaluated on an eukaryotic cell-based model. Our data show a correlation between the number of the formamidinate ligands and the anticancer activity of the diruthenium derivatives against human epithelial carcinoma cells. Increased cytotoxicity may be related to increased steric hindrance and Ru25+ core electronic density. However, the effect of increasing the lipophilicity of diruthenium species by introducing a second N,N'-diphenylformamidinate must be also considered. This work illustrates a systematic approach to shed light on the relevant properties of diruthenium compounds to design metal-based metallodrugs and diruthenium metalloenzymes.

New insights into progressive ligand replacement from [Ru2Cl(O2CCH3)4]: synthetic strategies and variation in redox potentials and paramagnetic shifts.

The complete series of [Ru2Cl(Dp-FPhF)x(O2CCH3)4-x] (x = 1-4; Dp-FPhF- = N,N'-bis(4-fluorophenyl)formamidinate) compounds, has been prepared and characterized by a multi-technique approach, including single crystal X-ray diffraction. A careful study of the different methodologies has allowed us to prepare four compounds with good yields and without an inert atmosphere or further purification. Specifically, [Ru2Cl(Dp-FPhF)(O2CCH3)3] (1) was obtained using an ultrasound-assisted (USS) method, while [Ru2Cl(Dp-FPhF)4] (4) was prepared by microwave assisted solvothermal synthesis (MWS). The intermediate substitution products cis-[Ru2Cl(Dp-FPhF)2(O2CCH3)2] (2) and [Ru2Cl(Dp-FPhF)3(O2CCH3)] (3) have been prepared by conventional heating, controlling the molar ratio of the starting materials. ESI-MS and infrared spectroscopy were used to follow all the reactions and permitted a qualitative evaluation of the axial reactivity in this series. Magnetic and absorption measurements confirmed a high spin σ2π4δ2(π*δ*)3 electronic configuration in all cases. However, the effect of the gradual modification of the electronic density in the diruthenium core markedly affects other properties. The cyclic voltammograms of the compounds show a strong decrease in the one electron oxidation potential and an increase in the reduction potential in the series from 1 to 4. Furthermore, despite their paramagnetic nature, 1H- and 19F-NMR spectra were recorded, and a correlation between the paramagnetic shift of the signals and the substitution degree of the diruthenium species was observed. These results provide a comprehensive guide to synthesise and understand the effects of equatorial ligand substitution on the properties of Ru25+ compounds.

Ultrasound-assisted synthesis of water-soluble monosubstituted diruthenium compounds.

The elusive monosubstituted diruthenium complexes [Ru2Cl(DAniF)(O2CMe)3] (1), [Ru2Cl(DPhF)(O2CMe)3] (2), [Ru2Cl(D-p-CNPhF)(O2CMe)3] (3), [Ru2Cl(D-o-TolF)(O2CMe)3] (4), [Ru2Cl(D-m-TolF)(O2CMe)3] (5), [Ru2Cl(D-p-TolF)(O2CMe)3] (6) and [Ru2Cl(p-TolA)(O2CMe)3] (7) have been synthesized using for the first time ultrasound-assisted synthesis to carry out a substitution reaction in metal-metal bonded dinuclear compounds (DAniF- = N,N'-bis(4-anisyl)formamidinate; DPhF- = N,N'-diphenylformamidinate; D-p-CNPhF- = N,N'-bis(4-cyanophenyl)formamidinate; D-o/m/p-TolF- = N,N'-bis(2/3/4-tolyl)formamidinate; p-TolA- = N-4-tolylamidate). This is a simpler and greener method than the tedious procedures described in the literature, and it has permitted to obtain water-soluble complexes with good yields in a short period of time. A synthetic study has been implemented to find the best experimental conditions to prepare compounds 1-7. Two different types of ligands, formamidinate and amidate, have been used to check the generality of the method for the preparation of monosubstituted complexes. Five new compounds (2-6) have been obtained using a formamidinate ligand, the synthesis of the previously described compound 1 has been improved, and an unprecedented monoamidate complex has been achieved (7). The crystal structures of compounds 3 and 7 have been solved by single crystal X-ray diffraction. These compounds show the typical paddlewheel structure with three acetate ligands and one formamidinate (3) or amidate (7) bridging ligand at the equatorial positions. The axial positions are occupied by the chloride ligand giving rise to one-dimensional polymer structures that were previously unknown for monosubstituted compounds.

Tuning the magnetic moment of [Ru2(DPhF)3(O2CMe)L]+ complexes (DPhF=N,N'-diphenylformamidinate): a theoretical explanation of the axial ligand influence.

The magnetic behaviour of the compounds containing the [Ru(2)(DPhF)(3)(O(2)CMe)](+) ion (DPhF(-)=N,N'-diphenylformamidinate) shows a strong dependence on the nature of the ligand bonded to the axial position. The new complexes [Ru(2)(DPhF)(3)(O(2)CMe)(OPMe(3))][BF(4)]0.5 CH(2)Cl(2) (1 0.5 CH(2)Cl(2)) and [Ru(2)(DPhF)(3)(O(2)CMe)(4-pic)][BF(4)] (2) (4-pic=4-methylpyridine) clearly display this influence. Complex 1.0.5 CH(2)Cl(2) shows a magnetic moment corresponding to a S=3/2 system affected by the common zero-field splitting (ZFS) and a weak antiferromagnetic interaction, whereas complex 2 displays an intermediate behaviour between S=3/2 and S=1/2 systems. The experimental data of complex 1 are fitted with a model that considers the ZFS effect using the Hamiltonian H(D)=S.D.S. The weak antiferromagnetic coupling is introduced as a perturbation, using the molecular field approximation. DFT calculations demonstrate that, in the [Ru(2)(O(2)CMe)(DPhF)(3)(L)](+) complexes, the energy level of the metal-metal molecular orbitals is strongly dependent on the nature of the axial ligand (L). This study reveals that the increase in the pi-acceptor character of L leads to a greater split between the pi* and delta* HOMO orbitals. The influence of the axial ligand in the relative energy between the doublet and quartet states in this type of complexes was also analysed. This study was performed on the new complexes 1.0.5 CH(2)Cl(2) and 2. The previously isolated [Ru(2)(DPhF)(3)(O(2)CMe)(OH(2))][BF(4)].0.5 CH(2)Cl(2) (3.0.5 CH(2)Cl(2)) and [Ru(2)(DPhF)(3)(O(2)CMe)(CO)][BF(4)].CH(2)Cl(2) (4.CH(2)Cl(2)) complexes were also included in this study as representative examples of spin-admixed and low-spin configurations, respectively. The [Ru(2)(DPhF)(3)(O(2)CMe)](+) (5) unit was used as a reference compound. These theoretical studies are in accordance with the different magnetic behaviour experimentally observed.

Tetracarbonatodiruthenium Fragments and Lanthanide(III) Ions as Building Blocks to Construct 2D Coordination Polymers.

Two-dimensional coordination polymers of [Pr(DMSO)2(OH2)3][Ru2(CO3)4(DMSO)(OH2)]·5H2O (Prα) and [Ln(OH2)5][Ru2(CO3)4(DMSO)]·xH2O (Ln = Sm (Smß), Gd (Gdß)) formulae have been obtained by reaction of the corresponding Ln(NO3)3·6H2O dissolved in dimethyl sulphoxide (DMSO) and K3[Ru2(CO3)4]·4H2O dissolved in water. Some DMSO molecules are coordinated to the metal atoms reducing the possibilities of connection between the [Ru2(CO3)4]3- and Ln3+ building blocks giving rise to the formation of two-dimensional networks. The size of the Ln3+ ion and the synthetic method seem to have an important influence in the type of two-dimensional structure obtained. Slow diffusion of the reagents gives rise to Prα that forms a 2D net that is built by Ln3+ ions as triconnected nodes and two types of Ru25+ units as bi- and tetraconnected nodes with (2-c)(3-c)2(4-c) stoichiometry (α structure). An analogous synthetic procedure gives Smß and Gdß that display a grid-like structure, (2-c)2(4-c)2, formed by biconnected Ln3+ ions and two types of tetraconnected Ru25+ fragments (ß structure). The magnetic properties of these compounds are basically explained as the sum of the individual contributions of diruthenium and lanthanide species, although canted ferrimagnetism or weak ferromagnetism are observed at low temperature.

Coordination capacity of cytosine, adenine and derivatives towards open-paddlewheel diruthenium compounds.

[Ru2Cl2(DPhF)3] (DPhF = diphenylformamidinate) links preferentially to the junctions of RNA (ribonucleic acid) structures, although the bonding mode is not known. In order to clarify this question the reactions between [Ru2Cl2(DPhF)3] and cytosine (Hcyto), cytidine (Hcyti), cytidine 2',3'-cyclic monophosphate sodium salt (NacCMP), adenine (Hade), adenosine (Haden) and adenosine 3',5'-cyclic monophosphate (HcAMP) have been carried out. In the resultant complexes, cyto (cytosinate), cyti (cytidinate), cCMP (cytidine 2',3'-cyclic monophosphate monoanion), ade (adeninate), aden (adenosinate) and cAMP (deprotonated adenosine 3',5'-cyclic monophosphate) are bonded to the diruthenium unit as N,N'-bridging ligands, as confirmed by the solution of the crystal structures of [RuCl(DPhF)3(cyto)] and [RuCl(DPhF)3(ade)] by X-ray diffraction. The axial positions of the diruthenium species are still available for additional interactions with other residues that could explain its preference towards RNA junctions.

Formation of a novel ferromagnetic end-to-end cyanate bridged homochiral helical copper(II) Schiff base complex via spontaneous symmetry breaking.

A homochiral helical coordination polymer of copper(II) has been synthesized using achiral precursors via spontaneous symmetry breaking and has been confirmed by single crystal X-ray diffraction and solid-state CD spectroscopy. The variable temperature magnetic measurements indicate the presence of weak ferromagnetic exchange interactions mediated by end-to-end cyanate bridges (J = +0.12 cm(-1)).

Mixed Chloride/Amine Complexes of Dimolybdenum(II,II). 2. Reactions of Mo(2)Cl(4)(NHEt(2))(4) with Monodentate and Bidentate Phosphines. New Type of Compounds Mo(2)Cl(4)(diphosphine)(4).

The complex Mo(2)Cl(4)(NHEt(2))(4) (1) undergoes facile substitution reactions of the amine ligands by phosphines to give compounds with the same core structure, Mo(2)Cl(4)(phosphine)(4), where the phosphine is PMe(3) (2), PMe(2)Ph (3), PHEt(2) (4), dmpm (bis(dimethylphosphino)methane) (5), or dmpe (1,2-bis(dimethylphosphino)ethane) (6). Complexes of the type M(2)X(4)L(4) are well-known with monodentate ligands, but there is no previous crystallographically verified example of the M(2)X(4)L(4) type of compound in which L is a potentially bidentate ligand acting as a unidentate ligand. Mo(2)Cl(4)(eta(1)-dmpm)(4) (5) can be isolated in good yield as the kinetic product of the substitution reaction at room temperature. When a solution of 5 in THF is heated, transformation into the more stable compound Mo(2)Cl(4)(&mgr;-dmpm)(2) (7) takes place. Mo(2)Cl(4)(eta(1)-dmpe)(4) (6) is stable in solution in the presence of free phosphine and can be detected by NMR. Over time it converts to a polymeric material which precipitates from the solution. For dppm (bis(diphenylphosphino)methane) (8) and dppa (bis(diphenylphosphino)amine) (9), only products of the stoichiometry Mo(2)Cl(4)(diphosphine)(2) were obtained. The crystal structures of the complexes 4 and 5 have been investigated by X-ray diffraction. The crystallographic parameters for them are as follows: for 4, orthorhombic space group Pbcn with a = 7. 6015(8) Å, b = 20.120(8) Å, c = 19.070(6) Å, and Z = 4; for 5, hexagonal space group P6(4)22 with a = 12.396(1) Å, c = 21.960(2) Å, and Z = 3. Both structures have a Mo(2)Cl(4)P(4) core where the phosphorus atoms are trans one to another on each metal center. The Mo-Mo distances of 2.137(1) Å (4) and 2.137(1) Å (5) are consistent with quadruple bonding.

Mixed Chloride/Amine Complexes of Dimolybdenum(II,II). 4. Rotational Isomers of Mo(2)Cl(4)(R-py)(4) (R-py = 4-Picoline, 3,5-Lutidine, and 4-tert-Butylpyridine).

The quadruply bonded dimolybdenum complexes with alkyl-substituted pyridines of the formula Mo(2)Cl(4)(R-py)(4) (R-py = 4-pic (4-methylpyridine) (1), 3,5-lut (3,5-dimethylpyridine) (2), and 4-Bu(t)-py (4-tert-butylpyridine) (3)) have been prepared. Nine different compounds, in which there are 11 independent molecules, have been obtained in crystalline form, and their crystal structures have been investigated by X-ray diffraction. Three types of geometric isomers which differ by the angle of internal rotation about the Mo-Mo axis have been recognized in these structures. The eclipsed structures 1-3a display pyridine ligands opposite to each other across the metal-metal bond and have a virtual symmetry D(2)(h)(). The noncentrosymmetric D(2)(d)() structures 1-3b have each pyridine ligand opposite to a Cl atom. The molecules 3c-f have a partially staggered D(2) geometry with N-Mo-Mo-N torsion angles ranging from 10.4 degrees to 25.2 degrees. As a result of this work it is now clear that Mo(2)Cl(4)(R-py)(4) compounds do not show a preference for D(2)(h)() conformation rather than D(2)(d)() conformation. In fact, they seem unusually unrestricted in their rotational conformation, and packing forces appear to have a major influence on the conformation adopted. Further clarification of this question will require spectroscopic study of solutions.

Reactions of {[Pd(&mgr;-SC(6)F(5))(&mgr;-dppm)Pd](&mgr;-SC(6)F(5))}(4).2O(C(2)H(5))(2). Crystal Structures of the Complexes [(Ph(3)P)Pd(&mgr;-SC(6)F(5))(&mgr;-dppm)Pd(SC(6)F(5))].1.4CH(2)Cl(2) and [(Ph(3)P)Pd(&mgr;-SC(6)F(5))(&mgr;-dppm)Pd(PPh(3))]SO(3)CF(3).2CH(2)Cl(2) and ab Initio MO Calculations on the Model Systems [(H(3)P)Pd(&mgr;-H(2)PCH(2)PH(2))(&mgr;-SH)Pd(PH(3))](+) and [(H(3)P)Pd(&mgr;-H(2)PCH(2)PH(2))Pd(PH(3))](2+).

{[Pd(&mgr;-SC(6)F(5))(&mgr;-dppm)Pd](&mgr;-SC(6)F(5))}(4) reacts 1:4 with neutral ligands L to give [LPd(&mgr;-SC(6)F(5))(&mgr;-dppm)Pd(SC(6)F(5))] or 1:8 to form [LPd(&mgr;-SC(6)F(5))(&mgr;-dppm)PdL](+) (dppm = bis(diphenylphosphino)methane). These binuclear complexes retain the palladium-palladium bond and the two dissimilar bridging ligands, as demonstrated by the X-ray structural determinations carried out on [(Ph(3)P)Pd(&mgr;-SC(6)F(5))(&mgr;-dppm)Pd(SC(6)F(5))].1.4CH(2)Cl(2) and [(Ph(3)P)Pd(&mgr;-SC(6)F(5))(&mgr;-dppm)Pd(PPh(3))]SO(3)CF(3).2CH(2)Cl(2). Ab initio calculations on the model systems [(H(3)P)Pd(&mgr;-H(2)PCH(2)PH(2))(&mgr;-SH)Pd(PH(3))](+) and [(H(3)P)Pd(&mgr;-H(2)PCH(2)PH(2))Pd(PH(3))](2+) show that the metal-metal bond arises mainly from interactions between palladium sp orbitals, which also play a predominant role in the binding with the sulfur bridge.