Homometallic Clusters Research Articles

Isolable Tetragold(0) Clusters with Polarity-Tunable exo-Au-Au Bond via Intramolecular σ-Aromatization.

Intramolecular π-aromatization is a trait of many organic compounds that enhances the stability of their structures and polarizes related C-C π bonds. In contrast, rare study is focused on this phenomenon in metal clusters. Many existing homometallic clusters exhibit aromaticity, often characterized by nonpolar metal-metal bonds and a high degree of symmetry. However, synthesizing low-symmetric homometallic clusters with high-polar metal-metal bonds is challenging due to their limited thermodynamic stability. Herein, we report a facile strategy for the synthesis of [Au(μ2 -ER2 )]3 -AuPMe3 (E=Ge, Sn; R2 =1,1,4,4-tetrakis(trimethylsilyl)butane-1,4-diyl) clusters and reveal a novel stabilization mode, intramolecular σ-aromatization. Our electronic structure analyses show that these low-symmetric clusters possess a ten-electron σ-aromatic system, which is achieved via intramolecular σ-aromatization. Moreover, the strength of σ-aromaticity gives rise to a polarity-tunable exo-Au-Au bond.

Isolable Tetragold(0) Clusters with Polarity‐Tunable exo‐Au−Au Bond via Intramolecular σ‐Aromatization

AbstractIntramolecular π‐aromatization is a trait of many organic compounds that enhances the stability of their structures and polarizes related C−C π bonds. In contrast, rare study is focused on this phenomenon in metal clusters. Many existing homometallic clusters exhibit aromaticity, often characterized by nonpolar metal‐metal bonds and a high degree of symmetry. However, synthesizing low‐symmetric homometallic clusters with high‐polar metal‐metal bonds is challenging due to their limited thermodynamic stability. Herein, we report a facile strategy for the synthesis of [Au(μ2‐ER2)]3−AuPMe3 (E=Ge, Sn; R2=1,1,4,4‐tetrakis(trimethylsilyl)butane‐1,4‐diyl) clusters and reveal a novel stabilization mode, intramolecular σ‐aromatization. Our electronic structure analyses show that these low‐symmetric clusters possess a ten‐electron σ‐aromatic system, which is achieved via intramolecular σ‐aromatization. Moreover, the strength of σ‐aromaticity gives rise to a polarity‐tunable exo‐Au−Au bond.

Mercurophilic interactions in heterometallic Ru-Hg carbonyl clusters

The reaction of [HRu3(CO)11]– (1) with 0.35 mol equivalents of Hg(CH3CO2)2 afforded [HgRu6(CO)22]2– (3), whereas a mixture of 3 and [Hg2Ru7(CO)26]2– (4) was obtained using 0.5 mol equivalents of Hg(CH3CO2)2 per mole of 1. A few crystals of [Ru3(CO)10(CH3COO)]– (7) were obtained as side products of the latter reaction. The reaction of 1 with one mole equivalent of Hg(CH3CO2)2 or HgCl2 afforded [Hg3Ru8(CO)30]2– (5) as the major product. By employing HgCl2, formation of 5 was accompanied by traces of the new homometallic cluster [Ru2Cl4(CO)5]2– (8). [HRu4(CO)12]3– (2) reacted with one mole equivalent of HgCl2 resulting in [Hg4Ru10(CO)32]4– (6). The molecular structures of the new clusters 3–8 were determined by single crystal X-ray diffraction (SC-XRD) as their [NEt4]2[3], [NEt4]2[4]·0.5CH2Cl2, [NEt4]2[5], [NEt4]4[6], [NEt4][7], [NEt4]2[8]·0.5CH3CN salts. Heterometallic Ru-Hg clusters 3–6 contain [Hg]2+, [Hg2]4+, [Hg3]6+, and [Hg4]8+ cores stabilized by [Ru(CO)4]2–, [Ru3CO)11]2– and [Ru4(CO)12]4– fragments. Metal-metal bonds were investigated by DFT calculations and atoms-in-molecules (AIM) analyses.

Atomically precise rhodium nanoclusters: synthesis and characterization of the heterometallic [Rh18Sn3Cl2(CO)33]4− and [Rh7Sn4Cl10(CO)14]5− carbonyl compounds

This paper presents a deepening on the investigation of the Rh-Sn system of heterometallic carbonyl clusters. More specifically, we herein report the synthesis and isolation of the new [Rh7Sn4Cl10(CO)14]5− (1) compound and the atomically precise [Rh18Sn3Cl2(CO)33]4− (2) nanocluster. Cluster 1 can be obtained by reacting the [Rh7(CO)16]3− homometallic cluster with hydrated Sn(II) chloride, in acetone; conversely, cluster 2 derives from the previously known [Rh12Sn(CO)23Cl2]4− precursor after controlled addition of diluted sulphuric acid. Notably, only 2 has retained the recurrent Sn-centred icosahedral structural feature, while 1 shows a molecular structure based on two Rh4 tetrahedra joint by one vertex and stabilized by SnCl2 and [SnCl3]− fragments. Both species have been characterized by infrared (IR) analysis in solution, single-crystal X-ray diffraction and Electrospray Ionization Mass Spectrometry (ESI-MS).

Structure, assembly mechanism and magnetic properties of heterometallic dodecanuclear nanoclusters DyIII4MII8 (M = Ni, Co)

Two isostructural heterometallic dodecanuclear nanoclusters [Dy4Co8(μ3-OH)8(L)8(OAc)4(H2O)4]·3EtOH·3CH3CN·H2O (1) and [Dy4Ni8(μ3-OH)8(L)8(OAc)4(H2O)4]·3.5EtOH·0.5CH3CN·5H2O (2) with different assembly mechanisms are presented here.

Nanoscale Organolanthanum Clusters: Nuclearity-Directing Role of Cyclopentadienyl and Halogenido Ligands.

Tetramethylaluminato/halogenido(X) ligand exchange reactions in half‐sandwich complexes [CpRLa(AlMe4)2] are feasible in non‐coordinating solvents and provide access to large coordination clusters of the type [CpRLaX2]x. Incomplete exchange reactions generate the hexalanthanum clusters [CpR 6La6X8(AlMe4)4] (CpR=Cp*=C5Me5, X=I; CpR=Cp′=C5H4SiMe3, X=Br, I). Treatment of [Cp*La(AlMe4)2] with two equivalents Me3SiI gave the nonalanthanum cluster [Cp*LaI2]9, while the exhaustive reaction of [Cp′La(AlMe4)2] with the halogenido transfer reagents Me3GeX and Me3SiX (X=I, Br, Cl) produced a series of monocyclopentadienyl rare‐earth‐metal clusters with distinct nuclearity. Depending on the halogenido ion size the homometallic clusters [Cp′LaCl2]10 and [Cp′LaX2]12 (X=Br, I) could be isolated, whereas different crystallization techniques led to the aggregation of clusters of distinct structural motifs, including the desilylated cyclopentadienyl‐bridged cluster [(μ‐Cp)2Cp′8La8I14] and the heteroaluminato derivative [Cp′10La10Br18(AlBr2Me2)2]. The use of the Cp′ ancillary ligand facilitates cluster characterization by means of NMR spectroscopy.

A Global Optimizer for Nanoclusters.

We have developed an algorithm to automatically build the global minimum and other low-energy minima of nanoclusters. This method is implemented in PyAR (https://github.com/anooplab/pyar) program. The global optimization in PyAR involves two parts, generation of several trial geometries and gradient-based local optimization of the trial geometries. While generating the trial geometries, a Tabu list is used for storing the information of the already used trial geometries to avoid using the similar trial geometries. In this recursive algorithm, an n-sized cluster is built from the geometries of n−1 clusters. The overall procedure automatically generates many unique minimum energy geometries of clusters with size from 2 up to n using this evolutionary growth strategy. We have used our strategy on some of the well-studied clusters such as Pd, Pt, Au, and Al homometallic clusters, Ru-Pt and Au-Pt binary clusters, and Ag-Au-Pt ternary cluster. We have analyzed some of the popular parameters to characterize the clusters, such as relative energy, singlet-triplet energy difference, binding energy, second-order energy difference, and mixing energy, and compared with the reported properties.

Cu(ii)/Ni(ii)-organic frameworks constructed from the homometallic clusters by 5-(2-carboxyphenoxy)isophthalic acid and N-ligand: synthesis, structures and visible light-driven photocatalytic properties.

Four new complexes, namely, Cu2(O-cpia)(btb)0.5·(OH) (1), Cu3(O-cpia)2(bpy)2 (2), [Ni2(O-cpia)(phen)·(OH)·H2O]·2H2O (3) and [Ni3(O-cpia)2(bpy)3·2H2O]·2H2O (4) (O-cpia = 5-(2-carboxyphenoxy)isophthalic acid, btb = 1,4-bis(1,2,4-triazol-1-yl)butane, bpy = 4,4′-bipyridine) were successfully isolated under hydrothermal conditions. The four complexes exhibit different architectures constructed from different homometallic clusters varying from mononuclear, binuclear to tetranuclear metal(ii) polyhedra as Second Building Blocks (SBUs). 1 features a 3D framework constructed from the tetranuclear clusters [Cu4(μ3-OH)2] as SBUs, linked with Cu(1)O4N and Cu(2)O5 polyhedra by O-cpia/btb mixed linkers. 2 also exhibits a 3D structure based on trinuclear clusters [Cu3(COO)4] SBUs, bridged with Cu(1)O3N2 and Cu(2)O4 polyhedra via O-cpia/bpy mixed ligands. 3 shows a 2D network consisting of tetranuclear clusters [Ni4(μ3-OH)2] SBUs, which are bridged with Ni(1)O4N2 and Ni(2)O6 through O-cpia ligands. It is worth noting that 4, with a 3D structure, is generated from the binuclear clusters [Ni2(COO)4] (Ni(1)O4N) and mononuclear metal Ni(2) cores (Ni(2)O4N2) as SBUs, and bridged by O-cpia/bpy mixed ligands. Meanwhile, the degradation of dyes (RhB) by the complexes under visible light irradiation was studied. 1–4 are semiconducting in nature, with Eg of 1.30 eV (1), 1.78 eV (2), 2.85 eV (3) and 2.14 eV (4). Cu(ii) complexes 1 and 2 are highly efficient photocatalysts for the degradation of RhB under visible light irradiation.

Structure and Thermodynamic Stability of Rhenium and Ruthenium Oxoalkoxo Derivatives MxN4 – xO6(OMе)10 (M, N = Re, Ru; x = 4–0)

Quantum-chemical calculations of the structure and relative thermodynamic stability of tetranuclear clusters MxN4 – xO6(OMе)10 (M, N = Re, Ru; x = 4–0) including geometric isomers of bimetallic clusters are performed to establish the possibility of replacing Re atoms by Ru atoms while maintaining a cyclic tetranuclear structure. As the initial model, the structure of Re4O6(OPri)10 was chosen which is an almost regular rhombus. The values of the metal–metal and metal–oxygen interatomic distances are calculated, and features of their variation under the transition from mononuclear to heteronuclear clusters and from neutral clusters to anions are discussed. Differences between the relative thermodynamic stability of homometallic and heterometallic tetranuclear clusters, including the geometric isomers of the latter, are discussed.

Reduction of C1 Substrates to Hydrocarbons by the Homometallic Precursor and Synthetic Mimic of the Nitrogenase Cofactor.

Solvent-extracted nitrogenase cofactors can reduce C1 substrates (CN-, CO and CO2) to hydrocarbons in reactions driven by a strong reductant, SmI2 (E0' = -1.55 V vs SCE). Here we show that a synthetic [Et4N]4[Fe6S9(SEt)2] cluster (designated the Fe6RHH-cluster), which mimics the homometallic [Fe8S9C] core of the nitrogenase cofactor (designated the L-cluster), is capable of conversion of C1 substrates into hydrocarbons in the same reactions. Comparison of the yields and product profiles between these homometallic clusters and their heterometallic counterparts points to possible roles of the heterometal, interstitial carbide and belt sulfur-bridged iron atoms in catalysis. More importantly, the observation that a "simplified", homometallic cofactor mimic can perform Fischer-Tropsch-like hydrocarbon synthesis suggests future biotechnological adaptability of nitrogenase-based biomimetic compounds for recycling C1 substrates into useful chemical and fuel products.

Bimetallic Fe–Cu Carbido Carbonyl Clusters Obtained from the Reactions of [Fe4C(CO)12{Cu(MeCN)}2] with N-Donor Ligands

The reaction of [Fe4C(CO)12{Cu(MeCN)}2] (1) with 3 equivalents of L–L (phen or Me2phen) affords [Cu(L–L)2][Fe4C(CO)12{Cu(L–L)}] (L–L = phen, 2; Me2phen, 3) in good yields. These are protonated by strong acids resulting in [HFe4C(CO)12{Cu(L–L)}] (L–L = phen, 4; Me2phen, 5). The reaction may be reversed with bases, resulting in the quaternary ammonium salts [NR4][Fe4C(CO)12{Cu(phen)}] (6). 4 and 5 react with a slight excess of L–L resulting in the elimination of copper in the form of [Cu(L–L)2]+ and formation of the previously reported [HFe4C(CO)12]− homometallic cluster. Conversely, the reaction of 1 with a monodentate N-ligand such as quinoline, even if used in large excess, results in the substitution product [Fe4C(CO)12{Cu(quinoline)}2] (8), which is then transformed into [Cu(Me2phen)2] [Fe4C(CO)12{Cu(quinoline)}] (9) after reaction with Me2phen. By using the anionic cluster [Fe5C(CO)14{Cu(MeCN)}]− instead of the neutral 1, only substitution has been observed by using both phen and quinoline, resulting in [Fe5C(CO)14{Cu(phen)}]− (10) and [Fe5C(CO)14{Cu(quinoline)}]− (11), respectively. Finally, the reaction of 1 with [Ru(tpy)(bpy)(N4C-C6H4-CN)]Cl affords crystals of [Fe4C(CO)12Cu2Cl{Ru(tpy)(bpy)(N4C-C6H4-CN)}] (12). All compounds 2-12 have been characterized by a combination of spectroscopic (IR, NMR) and crystallographic methods. All these clusters may be viewed as composed by a butterfly [Fe4C(CO)12]2− core bonded to Cu(I) fragments and/or H+ ions.

Zn(II)/Cd(II) based coordination polymers synthesized from a semi-flexible dicarboxylate ligand and their emission studies

Three new coordination polymers (CPs) of Zn(II)/Cd(II) have been synthesized using a dicarboxylate ligand, H2L=5-(benzylamino)isophthalic acid, under hydro(solvo)thermal conditions. This ligand readily reacts with Zn(II) and Cd(II) metal salts to afford three CPs, {[Zn8(L)6(μ3-OH)4(H2O)6]·(DMF)·(H2O)2.5}n (1), [Zn(L)]n (2) and [Cd(L)(DMF)]n (3). All the complexes have been characterized by single crystal X-ray crystallography (XRD), IR spectroscopy, elemental analysis, thermogravimetry (TGA) and X-ray powder diffraction (PXRD). Single crystal X-ray studies reveal that 1–3 contain homometallic clusters, with SBUs varying from dimeric to tetrameric, depending upon the metal salts as well as the reaction conditions. Topologically, 1 represents an sql net, whereas 2 and 3 exhibits rare ptr and fes topological nets respectively. Luminescence studies were also carried for complexes 1–3, which exhibit emissions as a pair of bands upon excitation at 405nm.

Syntheses of Ir4(CO)6(η5‐C5Me4H)2 and Ir7(μ3‐CO)3(CO)12(η5‐C5Me5) from Pentametallic Molybdenum‐Iridium Cluster Precursors

AbstractReaction of Mo2Ir3(μ‐CO)3(CO)6(η5‐C5H5)2(η5‐C5Me5) with Ir(CO)2(η5‐C5Me4H) afforded the four‐valence‐electron‐deficient butterfly cluster Ir4(CO)6(η5‐C5Me4H)2; its stability has been rationalized with the aid of density functional theory calculations, which suggest that significant additional intracluster bonding alleviates the formal electron deficiency. Reaction of MoIr4(CO)10(η5‐C5H5)(η5‐C5Me5) with [N(PPh3)2][Ir(CO)4] afforded the capped octahedral cluster Ir7(μ3‐CO)3(CO)12(η5‐C5Me5), which possesses three semi‐face‐capping CO ligands. These outcomes demonstrate that heterometallic clusters may serve as a potential source of new homometallic clusters following appropriate M–M′ cleavage.

Modulation of experimental conditions towards generation of a heterometallic Na2Co4 cluster or a homometallic Co4 cluster and ligand formed in situ

In this work, we report two clusters, namely Co4(Q)8 (1), Co4Na2(Q)2(L)4 (2) (H2L = 7,7′-(ethane-1,1-diyl)diquinolin-8-ol, HQ = 8-hydroxyquinoline), where their formation is well controlled by the reactants. When NaN3 is used, compound 2 a heterometallic cluster, Na2Co4, was obtained. By contrast, without the addition of NaN3, a distinct homometallic Co4 cluster is generated. Importantly, this is accompanied by the formation of ligand L2−, which is also dependent on the addition of NaN3. Further, their magnetic properties are also discussed.

Syntheses, structures, and nonlinear optical properties of two polynuclear homometallic and heterometallic cluster compounds containing silver(I) bromide species

Interaction of [Et4N]2[WSe4] and AgBr in the presence of [Et4N]Br·xH2O in a DMF/MeCN/2-picoline solution (3 : 2 : 1) at room temperature afforded a colorless crystalline compound [Et4N]6[(AgBr)6(μ3-Br)2(μ4-Br)3]Br (1) in 21% yield and a red crystalline compound [Et4N]4[(μ5-WSe4)(AgBr)5(μ-Br)2] (2) in 48% yield. The cluster anion in 1 comprises six AgBr fragments bridged by three bromides and capped by two bromides, forming a trigonal prism structure. The cluster anion in 2 consists of five AgBr fragments coordinated to five edges of the tetrahedral [WSe4]2− along with two bridging bromides connected to each of the two pairs of symmetric silvers, exhibiting a crown-like core structure. The nonlinear optical absorption and refraction of 2 were determined to be α2 = 5.72 × 10−9 m W−1 and n2 = 3.64 × 10−9 esu, respectively.

Four novel coordination polymers of transition metals built using a semi rigid oxygen donor ligand: Crystal structures, novel topology and emission studies

Four new coordination polymers namely, [Cd3(L1)2(DMA)4]n (1), [Mn(HL1)(H2O)3]n (2), [Mn(H2L2)]n (3) and {[Co2(HL2)(L2)]·(Et4N)·(1.5H2O)}n (4) have been synthesized with semi-rigid ether bridged tricarboxylic acid ligands H3L1 and H3L2 [H3L1=5-(4-carboxy-phenoxy)-isophthalic acid and H3L2=5-(2-carboxy-phenoxy)-isophthalic acid] with various transition metal ions using hydro(solvo)thermal techniques. All the coordination polymers have been structurally characterized by IR spectroscopy, elemental analysis, powder X-ray diffraction (PXRD), thermogravimetric analysis (TGA) and X-ray single crystallography. Single crystal X-ray studies show that these coordination polymers contain homometallic clusters varying from dimeric to trimeric units except 2 which is a linear monomeric. On reacting para-isomer of the ligand H3L1 with metal ions, 2D and 3D supramolecular structures have been obtained while with the ortho-isomer H3L2 dimeric 2D and 3D framework were obtained. Topological analysis reveals that 1 and 3 are binodal (3,6)-connected kgd net, 2 is also a binodal (3,3)-connected honey-comb type hcb net, while 4 shows unprecedented (3,3,6)-connected pkb5 topological trinodal-network. Interestingly, 3 and 4 exhibits unusual coordination modes involving one of carbonyl O atom of an acid functional group. Moreover, in 4 ethereal O atom is also coordinated to one of the metal center. All complexes show high thermal stability, thus losing coordinated and lattice solvent molecules in the range, ∼100–200°C. Upon excitation at 325nm, compounds 1–4 exhibit solid-state luminescences at room temperature.

Synthesis, Structure, and Electrochemistry of the Ni–Au Carbonyl Cluster [Ni12Au(CO)24]3– and Its Relation to [Ni32Au6(CO)44]6–

A detailed study of the reaction between [Ni(6)(CO)(12)](2-) and [AuCl(4)](-) afforded the isolation of the new Ni-Au cluster [Ni(12)Au(CO)(24)](3-) as well as identifying an improved synthesis for the previously reported [Ni(32)Au(6)(CO)(44)](6-). The new [Ni(12)Au(CO)(24)](3-) cluster is composed by two [Ni(6)(CO)(12)](2-) moieties coordinated to a central Au(I) ion, as determined by X-ray diffraction. It is noteworthy that the two [Ni(6)(CO)(12)](2-) fragments display different geometries, i.e., trigonal antiprismatic (distorted octahedral) and distorted trigonal prismatic (monocapped square pyramidal). The chemical reactivity of these clusters and their electrochemical behavior have been studied. [Ni(12)Au(CO)(24)](3-) is irreversibly transformed, upon electrochemical reduction, into Au(0) and [Ni(6)(CO)(12)](2-), followed by the reversible reduction of the latter homometallic cluster. Conversely, [Ni(32)Au(6)(CO)(44)](6-) displays five reductions, with apparent features of reversibility, confirming the ability of larger metal carbonyl clusters to reversibly accept and release electrons.

MnII Coordination Polymers Based on Bi-, Tri-, and Tetranuclear and Polymeric Chain Building Units: Crystal Structures and Magnetic Properties

Five new Mn(II) coordination polymers, namely [Mn(2)(tbip)(2)(bix)] (1), [Mn(3)(tbip)(3)(bix)(2)] (2), [Mn(3)(tbip)(2)(Htbip)(2)(bib)(2)]·4H(2)O (3), [Mn(4)(tbip)(4)(bbp)(2)(H(2)O)(2)] (4), and [Mn(4)(tbip)(4)(bip)]·2H(2)O (5), were prepared by hydrothermal reactions of Mn(II) acetate with H(2)tbip (5-tert-butyl isophthalic acid) in the presence of different di-imidazolyl coligands (bix =1,4-bis(imidazol-1-ylmethyl)benzene, bib =1,4-bis(imidazol) butane, bbp =1,3-bis(benzimidazol)propane, bip =1,3-bis(imidazol)propane). All complexes were characterized by elemental analysis, IR spectra, thermogravimetric analysis, single-crystal X-ray crystallography, and powder X-ray diffraction. Single crystal X-ray studies show that these coordination polymers contain homometallic clusters varying from dimeric, trimeric, and tetrameric motifs to polymeric chains depending upon the coligands used. Complex 1 has a 3D 6-connected polycatenane network with dinuclear [Mn(2)O(2)] secondary building units. Complex 2 possesses a 3D 8-connected structure with trinuclear [Mn(3)(COO)(6)] units. Complex 3 shows a 3D pcu net based on trinuclear [Mn(3)(COO)(6)] clusters as nodes. Complex 4 features a 3D 8-connected structure constructed from the distorted square-grid tetranuclear [Mn(4)(μ(2)-COO)(8)(μ(2)-H(2)O)] units. Complex 5 shows a 3D (4,5,6)-connected net containing 1D μ-O/μ-COO alternately bridged chains. Magnetic susceptibility measurements indicate that complexes 1 and 3-5 show weak antiferromagnetic interactions between the adjacent Mn(II) ions, whereas 2 is a three-spin center homometallic ferromagnetic system.

Co(II) Coordination Polymers with Co-Ligand Dependent Dinuclear to Tetranuclear Core: Spin-Canting, Weak Ferromagnetic, and Antiferromagnetic Behavior

Four new Co(II) containing coordination polymers have been synthesized using an ether bridged tricarboxylic acid ligand, o-cpiaH3 (5-(2-carboxy-phenoxy)-isophthalic acid). This ligand readily reacts with CoCl2·6H2O in the presence of different nitrogen donor ligands such as 1,10-phenanthroline (phen), 4,4′-bipyridyl (bpy), 4,4′-azopyridine (apy), and 1,4-bis(4-pyridinylmethyl)piperazine (bpmp) under hydrothermal conditions to afford three 3D and one 2D coordination polymers, {Co3(o-cpia)2(phen)(H2O)·5H2O}n(1), {Co1.5(o-cpia)(bpy)}n (2),{Co2(o-cpia)(OH)(apy)0.5}n (3), and {Co(o-cpiaH)(bpmp)0.5}n (4). Single crystal X-ray studies show that these coordination polymers contain homometallic clusters varying from dimeric to tetrameric depending upon the co-ligand used. In complexes 1 and 2, Co(II) ions form an angular and linear trimeric unit that extends along all directions to generate an overall 3D structure. In contrast, 3 forms a 3D coordination polymer containing a tetranuclear Co(II) unit. When the dista...

ChemInform Abstract: Oligomeric Group 13 Hydroxide Compounds — A Rare But Varied Class of Molecules

AbstractReview: tutorial review; 50 refs.