Silver Cations Research Articles

Synthesis, characterization, lipophilicity and antifungal properties of three new complexes of sulfamerazine (HSMR) with Ag(I): [Ag(SMR)], [Ag2(SMR)SCN] and [Ag(SMR)o-phenanthroline

Fungal infections, whose treatment is complicated by the appearance of the resistance to antifungal drugs, remain a major problem. Consequently, there is a critical need for new antifungal agents. Complexation of the silver(I) cation with different ligands reduces adverse effects and increases its therapeutic potential and has led to a large number of promising antibacterial, antifungal and anticancer compounds. The interest in metal–sulfanilamide derivatives has been stimulated by the successful introduction of a silver-sulfadiazine complex, still in current use. Sulfamerazine (HSMR) is a long-acting sulfanilamide antibacterial agent, which can be used to treat bronchitis, prostatitis and urinary tract infections. As a continuation of our work on metal complexes of sulfa drugs, in this paper we report the synthesis, characterization by elemental analysis, thermogravimetric analyses (TG), proton nuclear magnetic resonance (1H NMR) and Fourier transformed infrared spectra (FTIR) besides principal component analysis (PCA) spectroscopies, lipophilicity and antifungal properties of three new complexes of silver(I) with HSMR, one homoleptic: [Ag(SMR)], and two heteroleptic, using as second ligand KSCN: [Ag2(SMR)SCN] and o-phenanthroline (phen): [Ag(SMR)phen], named as (1), (2) and (3) respectively. 1H NMR spectra show that the sulfonamide moiety loses its acidic proton in all complexes, in agreement with the FTIR results. Lipophilicity: log Koctanol/water values were similarly to other sulfa-metal complexes, indicating low affinity for animal fat tissues. The three complexes showed a moderate antifungal activity, especially (3) mainly against Candida albicans, Cryptococcus neoformans and the dermatophytes Microsporum gypseum, Trichophyton rubrum and T. mentagrophytes. None of the tested fungi was inhibited by the free ligand, NaSMR. Considering these results, the studied complexes could be good candidates for further pharmaceutical studies.

Using silver exchange to achieve high uptake and selectivity for propylene/propane separation in zeolite Y

Adsorptive separation of propylene and propane, an important step of polypropylene production, is more energy-efficient than distillation. However, the challenge lies in the design of an adsorbent which exhibits both high selectivity and uptake. Herein, we hypothesise that enhancing the propylene affinity of the adsorption sites while keeping a suitable pore size can address this challenge. To do so, we performed silver exchange of a commercial zeolite Y, thereby making the adsorbent design easily scalable. We characterised the adsorbent using analytical, spectroscopic and imaging tools, tested its equilibrium and dynamic sorption properties using volumetric and gravimetric techniques and compared its performance to those of state-of-the-art adsorbents as well as other silver-functionalised adsorbents. The silver-exchanged zeolite Y (Ag-Y) exhibited one of the best selectivity vs uptake performances reported so far. Ag-Y also displayed fast adsorption kinetics and reversible propylene sorption, making it a promising new benchmark for propylene/propane separation. Synchrotron-based pair distribution function analyses identified the silver cations’ location which confirmed that the silver sites are easily accessible to the adsorbates. This aspect can, in part, explain the propylene/propane separation performance observed. The overall design strategy proposed here to enhance sorption site affinity and maintain pore size could be extended to other adsorbents and support the deployment of adsorption technology for propylene/propane separation.

Targeting of Silver Cations, Silver-Cystine Complexes, Ag Nanoclusters, and Nanoparticles towards SARS-CoV-2 RNA and Recombinant Virion Proteins.

Background: Nanosilver possesses antiviral, antibacterial, anti-inflammatory, anti-angiogenesis, antiplatelet, and anticancer properties. The development of disinfectants, inactivated vaccines, and combined etiotropic and immunomodulation therapy against respiratory viral infections, including COVID-19, remains urgent. Aim: Our goal was to determine the SARS-CoV-2 molecular targets (genomic RNA and the structural virion proteins S and N) for silver-containing nanomaterials. Methods: SARS-CoV-2 gene cloning, purification of S2 and N recombinant proteins, viral RNA isolation from patients’ blood samples, reverse transcription with quantitative real-time PCR ((RT)2-PCR), ELISA, and multiplex immunofluorescent analysis with magnetic beads (xMAP) for detection of 17 inflammation markers. Results: Fluorescent Ag nanoclusters (NCs) less than 2 nm with a few recovered silver atoms, citrate coated Ag nanoparticles (NPs) with diameters of 20–120 nm, and nanoconjugates of 50–150 nm consisting of Ag NPs with different protein envelopes were constructed from AgNO3 and analyzed by means of transmission electron microscopy (TEM), atomic force microscopy (AFM), ultraviolet-visible light absorption, and fluorescent spectroscopy. SARS-CoV-2 RNA isolated from COVID-19 patients’ blood samples was completely cleaved with the artificial RNase complex compound Li+[Ag+2Cys2−(OH−)2(NH3)2] (Ag-2S), whereas other Ag-containing materials provided partial RNA degradation only. Treatment of the SARS-CoV-2 S2 and N recombinant antigens with AgNO3 and Ag NPs inhibited their binding with specific polyclonal antibodies, as shown by ELISA. Fluorescent Ag NCs with albumin or immunoglobulins, Ag-2S complex, and nanoconjugates of Ag NPs with protein shells had no effect on the interaction between coronavirus recombinant antigens and antibodies. Reduced production of a majority of the 17 inflammation biomarkers after treatment of three human cell lines with nanosilver was demonstrated by xMAP. Conclusion: The antiviral properties of the silver nanomaterials against SARS-CoV-2 coronavirus differed. The small-molecular-weight artificial RNase Ag-2S provided exhaustive RNA destruction but could not bind with the SARS-CoV-2 recombinant antigens. On the contrary, Ag+ ions and Ag NPs interacted with the SARS-CoV-2 recombinant antigens N and S but were less efficient at performing viral RNA cleavage. One should note that SARS-CoV-2 RNA was more stable than MS2 phage RNA. The isolated RNA of both the MS2 phage and SARS-CoV-2 were more degradable than the MS2 phage and coronavirus particles in patients’ blood, due to the protection with structural proteins. To reduce the risk of the virus resistance, a combined treatment with Ag-2S and Ag NPs could be used. To prevent cytokine storm during the early stages of respiratory infections with RNA-containing viruses, nanoconjugates of Ag NPs with surface proteins could be recommended.

Antimicrobial Properties of Amino-Acid-Derived N-Heterocyclic Carbene Silver Complexes.

Complexes {Ag[NHCMes,R]}n (R = H, 2a; Me, 2b and 2b’; iPr, 2c; iBu, 2d), were prepared by treatment of imidazolium precursor compounds [ImMes,R] (2-(3-mesityl-1H-imidazol-3-ium-1-yl)acetate, 1a, (S)-2-alkyl(3-mesityl-1H-imidazol-3-ium-1-yl)acetate, 1b–d, and (R)-2-methyl(3-mesityl-1H-imidazol-3-ium-1-yl)acetate, 1b’, with Ag2O under appropriate conditions. They were characterised by analytical, spectroscopic (IR, 1H, and 13C NMR and polarimetry), and X-ray methods (2a). In the solid state, 2a is a one-dimensional coordination polymer, in which the silver(I) cation is bonded to the carbene ligand and to the carboxylate group of a symmetry-related Ag[NHCMes,H] moiety. The coordination environment of the silver centre is well described by the DFT study of the dimeric model {Ag[NHCMes,H]}2. The antimicrobial properties of these complexes were evaluated versus Gram-negative bacteria E. coli and P. aeruginosa. From the observed MIC and MBC values (minimal inhibitory concentration and minimal bactericidal concentration, respectively), complex 2b’ showed the best antimicrobial properties (eutomer), which were significantly better than those of its enantiomeric derivative 2b (distomer). Additionally, analysis of MIC and MBC values of 2a–d reveal a clear structure–antimicrobial effect relationship. Antimicrobial activity decreases when the steric properties of the R alkyl group in {Ag[NHCMes,R]}n increase.

Restoration of antibacterial activity of inactive antibiotics via combined treatment with a cyanographene/Ag nanohybrid

The number of antibiotic-resistant bacterial strains is increasing due to the excessive and inappropriate use of antibiotics, which are therefore becoming ineffective. Here, we report an effective way of enhancing and restoring the antibacterial activity of inactive antibiotics by applying them together with a cyanographene/Ag nanohybrid, a nanomaterial that is applied for the first time for restoring the antibacterial activity of antibiotics. The cyanographene/Ag nanohybrid was synthesized by chemical reduction of a precursor material in which silver cations are coordinated on a cyanographene sheet. The antibacterial efficiency of the combined treatment was evaluated by determining fractional inhibitory concentrations (FIC) for antibiotics with different modes of action (gentamicin, ceftazidime, ciprofloxacin, and colistin) against the strains Escherichia coli, Pseudomonas aeruginosa, and Enterobacter kobei with different resistance mechanisms. Synergistic and partial synergistic effects against multiresistant strains were demonstrated for all of these antibiotics except ciprofloxacin, which exhibited an additive effect. The lowest average FICs equal to 0.29 and 0.39 were obtained for colistin against E. kobei and for gentamicin against E. coli, respectively. More importantly, we have experimentally confirmed for the first time, that interaction between the antibiotic's mode of action and the mechanism of bacterial resistance strongly influenced the combined treatment’s efficacy.

Synthesis of Metal-Loaded Carboxylated Biopolymers with Antibacterial Activity through Metal Subnanoparticle Incorporation.

Carboxymethyl starch (CMS) and carboxymethyl cellulose (CMC) loaded by highly dispersed metal subnanoparticles (MSNPs) showed antibacterial activity against E. coli and B. subtilis strains. Copper and silver were found to act in both cationic and zero-valence forms. The antibacterial activity depends on the metal species content but only up to a certain level. Silver cation (Ag+) showed higher antibacterial activity as compared to Ag0, which was, however, more effective than Cu0, due to weaker retention. The number of carboxyl groups of the biopolymers was found to govern the material dispersion in aqueous media, the metal retention strength and dispersion in the host-matrices. Cation and metal retention in both biopolymers was found to involve interactions with the oxygen atoms of both hydroxyl and carboxyl groups. There exists a ternary interdependence between the Zeta potential (ZP), pH induced by the biocidal agent and its particle size (PS). This interdependence is a key factor in the exchange processes with the surrounding species, including bacteria. Clay mineral incorporation was found to mitigate material dispersion, due to detrimental competitive clay:polymer interaction. This knowledge advancement opens promising prospects for manufacturing metal-loaded materials for biomedical applications.

From Isostructurality to Structural Diversity of Ag(I) Coordination Complexes Formed with Imidazole Based Bipodal Ligands, and Disclosure of a Unique Topology.

Two Ag(I) complexes with 1,3-bis(imidazol-1-ylmethyl)benzene (bib) and counterions BF4− (1) and PF6− (2) were synthesized in order to check their behavior in forming molecular/crystal structures. This allows comparison with the final products of analogous syntheses performed with similar bidentate ligands containing methyl substituents on the benzene ring, namely 1,3-bis(imidazol-1-ylmethyl)-5-methylbenzene (bimb) and 1,3-bis(imidazol-1-ylmethyl)-2,4,6-trimethylbenzene (bitmb). The Ag(I) complexes obtained with the methylated ligands mentioned above form isostructural pairs of waved 1D chains or dinuclear boxes, of general formula {[Ag(bimb)]X}n and [Ag2(btmb)2]X2, respectively (X = BF4−, PF6−), under the same reaction conditions. SCXRD analyses of 1 and 2 revealed the formation of polymeric coordination compounds of formula {[Ag2(bib)3](BF4)2}n and {[Ag(bib)]PF6}n, respectively, different from those observed for bimb. The 3D coordination polymer 1 forms a unique 5,5-c net of 5,5T188 topological type, observed for the very first time for a coordination compound, with silver cations adopting a trigonal geometry, whereas 2 shows the presence of 1D single-stranded cationic helices with linear coordination of the metal centers. Interestingly, these complexes differ not only from the mentioned isostructural pairs of related Ag(I) complexes, but also from the isostructural pair of compounds obtained as the final product when reacting bib and bimb with the larger counterion CF3SO3−. Hirshfeld surface analyses indicate a higher contribution of F⋯H intermolecular contacts in 2 than in 1, with H⋯H contacts being dominant in the latter.

A novel label-free colorimetric polyA aptasensing approach based on cationic polymer and silver nanoparticles for detection of tobramycin in milk

In this study, a novel colorimetric bioassay method was developed for the sensitive determination of tobramycin (TOB). To detect TOB, silver nanoparticles (AgNPs) were decorated with TOB-specific aptamers (apt), and positively charged poly diallyl dimethyl ammonium chloride (PDDA) was used. As long as tobramycin is not present in the assay system, PDDA can coalesce with the aptamer, and AgNPs would remain stable (λmax = 400 nm) in the dispersed system against PDDA-induced aggregation. When TOB is added, aptamer can bind to the compound, which leads to release of PDDA and subsequent aggregation of AgNPs (λmax = 540 nm). This remarkable change, as a colorimetric analytics signal, can be used for quantitative analysis of TOB. TOB can be detected by this highly sensitive colorimetric aptasensor with a limit of detection (LOD) of 70 pM. Furthermore, TOB can be detected with the naked eye at concentrations above 1 nM.

Chemically-invariant percolation in silver thioarsenate glasses and two ion-transport regimes over 5 orders of magnitude in Ag content

Ionic conductivity σi measurements of AgY−As2S3 (Y = Br, I) glasses, covering 13 orders of magnitude in σi(x) over 5 orders of magnitude in silver content x, confirm two drastically different ion transport regimes in silver halide thioarsenate glasses. As expected, the ionic conductivity in the critical percolation domain, 20 ppm < x ≤ 2 at.% Ag, follows a power-law dependence, σi(x,T)∝xT0/T, where the critical temperature T0 is related to the connectivity of the glassy host. Moreover, we show that σi(x,T) is chemically invariant in the critical percolation domain. Three silver thioarsenate glass families, AgY- and Ag2S-As2S3, reveal identical ionic conductivity within experimental uncertainty over three orders of magnitude in silver contentx. The ionic conductivity diverges in the modifier-controlled region, x > 7-10 at.% Ag, and the difference in σi(x) between AgI- and Ag2S-As2S3 glasses approaches 4 orders of magnitude. Random distribution of silver in the critical percolation domain, shown by DFT modeling of neutron and high-energy x-ray diffraction data, is a key of the observed conductivity invariance. When silver cation leaves the residence site and travels throughout the glass network, characterized by the average Ag-Ag separation distance of 12 Å or more, the memory of its original chemical form (sulfide or halide) vanishes rapidly with increasing the mean square displacement. A non-random silver distribution in the modifier controlled region implying formation of preferential conduction pathways via direct contacts of edge- and corner-sharing silver chalcogenide or chalcohalide polyhedra rules out this possibility. Chemically-invariant ionic conductivity seems to be a common feature of any disordered system with random distribution of mobile ions having similar size of charge carriers.

Reactive organic radical-doped Ag(I)-based coordination compounds for highly efficient antibacterial wound therapy

Antibiotics, being critical antimicrobial agents, have been widely used for treating bacterial infections. However, prolonged use of antibiotics can induce drug resistance resulting in “superbug” that threatens human health. Therefore, developing antibiotic-free materials with intrinsic antibacterial properties is the key to the “superbug” challenge. In this study, two highly efficient metal-organic frameworks (MOFs) were successfully assembled through synergistic use of the antibacterial properties of reactive organic radicals and silver (Ag) cations. These hybrid Ag-based materials possessed radical-doped characteristics, continuously releasing Ag+, which significantly inhibited the growth of four common Gram-negative and Gram-positive human pathogens (Escherichia coli, Pseudomonas aeruginosa, Bacillus subtilis, and Staphylococcus aureus), and particularly two multi-drug-resistance bacteria (MRSA and MDR-PA). Furthermore, in vivo assays indicated that the synergistic antibacterial effect of these compounds could significantly accelerate the healing rate of infected wounds in mice. Blood biochemistry and histological analyses of main organs in treated mice also exhibited negligible cytotoxicity. This study unveiled the promising potential of Ag-MOFs for anti-infective therapies and future clinical applications.

An ab-initio study of electronic and optical properties of RhXO3 (X = Ga, Ag) perovskites

The ab-initio computations were performed to study the electronic and optoelectronic properties of perovskites using WIEN2k code. The has band gap of 2.29 eV, and the behavior of is metallic. The sub-TDOS of the studied materials revealed that rhodium and oxygen atoms have significant contributions in the valence band and conduction band formation of both materials. The silver cation is responsible for the reasonable peaks appearing at the Fermi level of which demonstrated the conducting nature of The dielectric functions, optical conductivity, energy loss function, absorption coefficient, refractive index, extinction coefficient, and reflectivity are computed for these materials to understand the optical behavior of the studied materials. The analysis of optical properties ensure the is a promising material for optoelectronics while has metallic nature.

Lability of the first solvation shell of silver cations in liquid ammonia: A quantum mechanical charge field molecular dynamics simulation study

The structural and dynamical properties of solvated silver cations (Ag+) in liquid ammonia have been investigated using quantum mechanical charge field molecular dynamics simulation. The first solvation shell was labile with 61 successful ligand exchange events during simulation for 200 ps. The average distance of Ag+ and ammonia molecules in the first solvation shell was 2.5 Å. Four to five ammonia molecules solvated Ag+ in the first solvation shell. The mean residence time (MRT) of the ammonia ligand in the first solvation shell was 15 ps. The second solvation shell showed a more labile structure, whose MRT of ammonia was 1.57 ps. Compared with a previous study of hydrated Ag+ in an aqueous solution, the solvation shell for Ag+ in liquid ammonia was less labile.

Self-Assembled Cage for In Situ Detecting Silver Cation in Water.

Here, a capsule-shaped cage comprising three monocationic arms was efficiently self-assembled by condensing a triscationic trisaldehyde and a trisamino linkage in water. Multivalence endows the cage with thermodynamic stability in water. Despite its triscationic nature, the cage is able to use its trisimino residue to coordinate a silver cation. As a comparison, other cations lead to cage decomposition or no coordination. The cage and Ag+-coordinated complex were both characterized and confirmed by NMR spectroscopy, mass spectrometry, and theoretical calculations. The metal-ligand complex exhibits a pale-yellow color that can be detectable by the naked eye. The Ag+-coordinated complex undergoes decoordination upon the addition of NaCl, during which the cage containing imine bonds remains intact. Such stability implies that the cage might be potentially employed in silver detection and mining.

Strongly Electron-Donating Triazolylidene Ligands: Cationic Metal Carbonyl Complexes of 1-Methyl-1,2,3-triazole as Triazolium Surrogates.

A new strategy for tuning the electronic properties of 1,2,3-triazol-5-ylidene metal complexes is reported using {Mo(η3-C4H7)(bipy)(CO)2} or {Re(bipy)(CO)3} fragments as substituents at the triazole N3 atom. The reaction of cationic molybdenum(II) and rhenium(I) 1-methyl-1,2,3-triazole compounds with the strong base KN(SiMe3)2 in the presence of electrophilic metal fragments, such as AgOTf (OTf = trifluoromethanesufonate) or [CuCl(IPr)] [IPr = 2,6-(diisopropyl)phenylimidazol-2-ylidene] affords a new type of 1,2,3-triazol-5-ylidene complexes. For silver(I) cationic bis(triazolylidene), [Ag(tzNHCM)2]OTf (M = [Mo], 2; [Re], 4), complexes are obtained, whereas in the case of Cu(I) mixed normal/mesoionic NHC, [Cu(IPr)(tzNHCM)]OTf (M = [Mo], 7; [Re], 8) complexes are formed. This special type of mesoionic N-heterocyclic carbenes bear a metal fragment at the N3 atom of the 1,2,3-triazole moiety, showing notable enhancement of the carbene electron donor ability compared to conventional alkyl-substituted analogues. Transmetalation from cationic silver bis(triazolylidene) complexes 2 and 4, prepared using this methodology, has proven to be very efficient toward [M'Cl(cod)]2 (M' = Rh, Ir; cod = 1,5-cyclooctadiene), affording the corresponding cationic bis(triazolylidene) [M'(cod)(tzNHCM)2]OTf (9-12) complexes. A subsequent reaction with CO(g) easily produces substitution of the diene ligand, affording the corresponding cis-dicarbonyl [M'(CO)2(tzNHCM)2]OTf (13-16) compounds.

Nanosized titanium dioxide elevates toxicity of cationic metals species for Daphnia – have aging and natural organic matter an unexpected impact?

In aquatic ecosystems, nanosized titanium dioxide particles (nTiO2) likely interact with natural organic matter (NOM) and may alter the ecotoxicity of co-occurring metals. The magnitude of changes in toxicity may be modulated by the duration of interactions (i.e. aging) between these factors. As those interactions are hardly addressed in literature, the present study aimed at assessing the impact of aging durations (0, 1, 3 and 6 days) on metals with mainly cationic (silver (Ag), cadmium (Cd)) or anionic (arsenic (As)) toxic ions in combination with three nTiO2 levels (0.0, 0.6 and 3.0 mg/L) and two NOM levels (0 versus 8 mg TOC/L). The interaction of these factors was additionally investigated for two aging scenarios: in one scenario nTiO2 were aged together with one of the metals, while in other scenario metals were added to aged nTiO2. Subsequently, their combined acute effects on Daphnia magna were assessed. The results uncovered that nTiO2 elevate the toxicity of metals with mainly cationic species (i.e. Ag+ and Cd2+) with the effect size depending on their valence electron. Contrary, nTiO2 have no impact on the metal with mainly anionic species (i.e. HAsO4 2−). Furthermore, NOM reduced metal toxicity only for Ag and aging duration had a limited impact on the test outcome suggesting that relevant interactions between metal and nTiO2 occur rather quick (below 24 h). These findings suggest that the charge of metals’ most toxic species is the determining factor for its interaction with nanoparticles and the resulting ecotoxicological effect assessment.

Argentophilic pyridinic nitrogen for embedding lithiophilic silver nanoparticles in a three-dimensional carbon scaffold for reversible lithium plating/stripping

Ten times heavy silver loading into a 3D scaffold via a strong Ag+–pN interaction between silver cations and argentophilic pyridinic nitrogen of melamine provides a strong lithiophilicity, largely improving lithium plating/stripping reversibility.

The catenation of a singlet diradical dication and modulation of diradical character by metal coordination.

A singlet bis(triarylamine) diradical dication and its zigzag 1D magnetic chain catenated by silver cations were isolated and characterized by single-crystal X-ray crystallography, EPR spectroscopy, SQUID measurements, cyclic voltammetry, and UV-Vis-NIR absorption spectroscopy in conjunction with DFT calculations. The ΔEOS-T has significantly changed due to the coordination of carbonyls (Lewis base) with Ag+ (Lewis acid), revealing that the diradical character can be modulated by Lewis acid-base coordination and interchain intermolecular coupling interactions.

First Principle Investigation on Electronic Properties of Cationic and Anionic CO-Alloyed Cu2ZnSnS4 Kesterite Material

The primary goal of kesterite alloying is to allow for fine tweaking of the material's characteristics for advanced device engineering. Additionally, it is seen as a viable solution to inherent kesterite absorber difficulties such as the Cu/Zn disorder or Sn multivalency. The most interesting alloying elements for kesterite are Ag replacing Cu, Cd replacing Zn, and Ge replacing Sn for cationic substitution, as well as Se replacing S for anionic substitution. This research work investigates the effect of alloying CZTS with Silver (Ag) (Cation) and Selenium (Se) (Anion) theoretically using Density Functional Theory (DFT). The compounds were found to exhibit indirect bandgap characteristics, with conduction band minima (CBM) and valence band maxima (VBM) located between the N and gamma points of the Brillouin zone for pure kesterite Cu2ZnSnS4 and between the N and A points for alloyed Ag2ZnSnSe4 respectively. The bandgap of around 1.22 eV and 0.78 eV were recorded for the pure and alloyed kesterite materials. From the obtained results, a shrink in bandgap was observed due to the presence of heavy anion (Se) and cation (Ag) alloying at the same time. It was also found that the contribution of different atomic orbitals to the formation of the valence and conduction bands is approximately identical for pure and alloyed materials.

Do 2-coordinate iodine(I) and silver(I) complexes form nucleophilic iodonium interactions (NIIs) in solution?

The interaction of a [bis(pyridine)iodine(I)]+ cation with a [bis(pyridine)silver(I)]+ cation, in which an iodonium ion acts as a nucleophile by transferring electron density to the silver(I) cation, is reinvestigated herein. No measurable interaction is observed between the cationic species in solution by NMR; DFT reveals that if there is an attractive interaction between these complexes in solution, it is dominantly the π-π interaction of pyridines.

Electron counting in cationic and anionic silver clusters doped with a 3d transition-metal atom: endo- vs. exohedral geometry.

Electron counting is a concept that often governs properties of molecules, clusters, and complexes. Here we explore silver clusters doped with a transition-metal atom, where it has been an issue whether or not 3d electrons delocalize to participate in electron counting. The experiment is performed on AgNM+/- (M = Sc-Ni) clusters to examine their stability through chemical reactivity, enabling systematic control of the number of valence electrons by the cluster size, the charge state, and the transition-metal element across the periodic table. It is revealed for 18-valence-electron clusters that 3d electrons participate in electron counting to show exceptional stability only when the transition-metal atom is endohedrally doped, except for Cr and Mn doping that forces 3d electrons to localize. We thus present new entries for superatomic metal clusters as well as a geometric factor that regulates the behavior of 3d electrons in the nanoscale regime.