Trinuclear Heterometallic Complexes Research Articles

Heterometallic trinuclear complexes of {η1-(Me3tacn)MoO3}2M(III) (M = Y, La, Sm, Eu, Nd, Yb) with rare earth metals based on the (Me3tacn)MoO3 metalloligand

Heterometallic trinuclear complexes of {η1-(Me3tacn)MoO3}2M(III) (M = Y, La, Sm, Eu, Nd, Yb) with rare earth metals based on the (Me3tacn)MoO3 metalloligand

Synthesis, structural diversity, supramolecular architecture, and catalytic reactions of mononuclear and heteronuclear benzamide cobalt complexes

Two NNN-type benzamide derivative ligands {N,N'-(2,2′-azenidiyl-bis(2,1-phenylene))dicyclohexanecarboxamide (H3LcyHex) and N,N'-(2,2′-azenidiyl-bis(2,1-phenylene))dicyclobutanecarboxamide (H3LcyBu)} that incorporate N-amidate donors have been synthesized and characterized by 1H NMR, 13C NMR, and FT-IR techniques. Subsequently, heterometallic trinuclear cobalt-potassium complexes, [K2(DMF)6Co(HL)2] (KCyBu and KCyHex) and mononuclear cobalt complexes with complementary tetraethylammonium ions, [Et4N]2[Co(HL)2] (ECyBu and ECyHex), both incorporating these ligands, have been synthesized. Suitable crystals for single-crystal X-ray diffraction of H3LcyHex and KCyBu have been characterized using single-crystal X-ray diffraction analysis. Intermolecular interactions play a major role in the formation of the supramolecular architecture in the crystal lattice of compounds. The supramolecular structures in H3LcyHex are formed through intermolecular CH⋅⋅⋅O, CH⋅⋅⋅π, CH⋅⋅⋅O/N, and CH⋅⋅⋅N interactions, and those in KCyBu are generated by CH⋅⋅⋅π and CH⋅⋅⋅O intermolecular interactions. The amphoteric properties exhibited by new cobalt-oxygen species produced through the reaction of cobalt complexes with molecular oxygen were examined by their electrophilic reactivity in the conversion of triphenylphosphine (PPh3) to triphenylphosphoxide (OPPh3) and their nucleophilic reactivities in the deformylation reaction of 2-phenylpropionaldehyde (2-PPA) to acetophenone (catalyst/substrate ratio, 1:100). In the catalytic activity results, it was observed that the cobalt-oxygen complexes have an amphoteric feature due to their good reactivity in both reactions.

Coordination of Al(C6F5)3 vs. B(C6F5)3 on group 6 end-on dinitrogen complexes: chemical and structural divergences.

The coordination of the Lewis superacid tris(pentafluorophenyl)alane (AlCF) to phosphine-supported, group 6 bis(dinitrogen) complexes [ML2(N2)2] is explored, with M = Cr, Mo or W and L = dppe (1,2-bis(diphenylphosphino)ethane), depe (1,2-bis(diethylphosphino)ethane), dmpe (1,2-bis(dimethylphosphino)ethane) or 2 × PMe2Ph. Akin to tris(pentafluorophenyl)borane (BCF), AlCF can form 1 : 1 adducts by coordination to one distal nitrogen of general formula trans-[ML2(N2){(μ-η1:η1-N2)Al(C6F5)3}]. The boron and aluminium adducts are structurally similar, showing a comparable level of N2 push-pull activation. A notable exception is a bent (BCF adducts) vs. linear (AlCF adducts) M-N-N-LA motif (LA = Lewis acid), explained computationally as the result of steric repulsion. A striking difference arose when the formation of two-fold adducts was conducted. While in the case of BCF the 2 : 1 Lewis pairs could be observed in equilibrium with the 1 : 1 adduct and free borane but resisted isolation, AlCF forms robust 2 : 1 adducts trans-[ML2{(μ-η1:η1-N2)Al(C6F5)3}2] that isomerise into a more stable cis configuration. These compounds could be isolated and structurally characterized, and represent the first examples of trinuclear heterometallic complexes formed by Lewis acid-base interaction exhibiting p and d elements. Calculations also demonstrate that from the bare complex to the two-fold aluminium adduct, substantial decrease of the HOMO-LUMO gap is observed, and, unlike the trans adducts (1 : 1 and 1 : 2) for which the HOMO was computed to be a pure d orbital, the one of the cis-trinuclear compounds mixes a d orbital with a π* one of each N2 ligands. This may translate into a more favourable electrophilic attack on the N2 ligands instead of the metal centre, while a stabilized N2-centered LUMO should ease electron transfer, suggesting Lewis acids could be co-activators for electro-catalysed N2 reduction. Experimental UV-vis spectra for the tungsten family of compounds were compared with TD-DFT calculations (CAM-B3LYP/def2-TZVP), allowing to assign the low extinction bands found in the visible spectrum to unusual low-lying MLCT involving N2-centered orbitals. As significant red-shifts are observed upon LA coordination, this could have important implications for the development of visible light-driven nitrogen fixation.

Cover Feature: Asymmetrical Platinum and Rhodium Dinuclear Complex Strongly Bound to Filled d Complexes by Unbridged Pt−Metal Bonds: Toward Heterometallic‐Extended Metal Atom Chains (Chem. Eur. J. 16/2023)

Heterometallic trinuclear or pentanuclear complexes aligned as Rh−Pt−M or Rh−Pt−M−Pt−Rh (M=Pt, Pd, Cu), respectively, were kinetically and thermodynamically synthesized and characterized. These polynuclear complexes, in which three kinds of metals form straight one-dimensional structures with metal–metal bonds, are presented as sweet, three-colored, Japanese dango dumplings. More information can be found in the Research Article by K. Uemura and co-workers (DOI: 10.1002/chem.202204057).

Trinuclear cyanido-bridged MII-WV complexes (M = Mn, Co): Crystal structures and magnetic properties

Two new trinuclear heterometallic complexes, [{MnII(L)(CH3OH)}2{WV(CN)8}](NO3)·CH3OH·H2O (2) and [{CoII(L)(CH3OH)}2{WV(CN)8}](NO3)·CH3OH·H2O (3), have been assembled from mononuclear cationic modules, [M(L)(H2O)2]2+, and [W(CN)8]3– as a metalloligand (M = MnII; CoII, L is the pentadentate ligand resulting from the condensation reaction between 2,6-diacetylpyridine and semicarbazide). The crystal structure of the manganese(II) precursor, [MnII(L)(H2O)2](NO3)2·H2O 1, and of the trinuclear compounds 2 and 3 have been solved. Within the three compounds the 3d metal ions show a pentagonal bipyramidal coordination geometry. The magnetic properties of compounds 2 and 3 have been investigated. Compound 2 shows an irregular spin state structure with S = 9/2 arising from MnII-WV antiferromagnetic interactions (J = −8.00 cm−1, H = -2JS1S2). In compound 3, the CoII-WV exchange interaction was found ferromagnetic (J = +12.3 cm−1).

A Bis-(carbone) Pincer Ligand and Its Coordinative Behavior toward Multi-Metallic Configurations.

Carbones are divalent carbon(0) species that contain two lone pairs of electrons. Herein, we have prepared the first known stable and isolable free bis-(carbone) pincer framework with a well-defined solid-state structure. This bis-(carbone) ligand is an effective scaffold for forming monometallic (Ni and Pd) and trinuclear heterometallic complexes with Au-Pd-Au, Au-Ni-Au, and Cu-Ni-Cu configurations. Sophisticated quantum-theoretical analyses found that the metal-metal interactions are too weak to play a significant role in upholding these multi-metallic configurations; rather, the four lone pairs of electrons within the bis-(carbone) framework are the main contributors to the stability of the complexes.

A Bis‐(carbone) Pincer Ligand and Its Coordinative Behavior toward Multi‐Metallic Configurations

AbstractCarbones are divalent carbon(0) species that contain two lone pairs of electrons. Herein, we have prepared the first known stable and isolable free bis‐(carbone) pincer framework with a well‐defined solid‐state structure. This bis‐(carbone) ligand is an effective scaffold for forming monometallic (Ni and Pd) and trinuclear heterometallic complexes with Au−Pd−Au, Au−Ni−Au, and Cu−Ni−Cu configurations. Sophisticated quantum‐theoretical analyses found that the metal–metal interactions are too weak to play a significant role in upholding these multi‐metallic configurations; rather, the four lone pairs of electrons within the bis‐(carbone) framework are the main contributors to the stability of the complexes.

Synthesis and Characterisation of New {Fe2CrO} Heterotrinuclear Iron-chromium Clusters

Two new μ3-oxo trinuclear heterometallic Fe2IIICrIII complexes with furan-2-carboxylic and salicylic acids with the composition: [Fe2CrO(C4H3OCOO)6(CH3OH)3]NO3·0.5CH3OH and [Fe2CrO(C6H4(OH)COO)7(CH3OH)2]·2DMA were synthesized starting from iron(III) and chromium(III) salts mixture. The complexes structures were confirmed by elemental analysis, IR, Mössbauer spectroscopies, and X-ray analysis. The atomic absorption spectroscopy confirmed that the iron: chromium ratio is 2:1. The thermal properties of both heteronuclear complexes have been investigated in oxidizing and inert atmospheres revealing the stability of the trinuclear core up to 170 and 220°C, respectively.

Structural and Magnetic Characterization of Homo- and Heterometallic Trinuclear Ni(II) and Cu(II) Clusters with N2O6 Acyclic Polydentate Ligand

Homo- and heterometallic trinuclear complexes with an acyclic N2O6-type ligand, [Ni3(L)2(CH3COO)2]·H2O·MeOH (1) and [NiCu2(L)2(CH3COO)2]·3H2O (2) (H2L = N,N′-(hexane-1,6-diyl)bis(2-hydroxy-3-methoxybenzamide), were prepared, respectively, of which crystal structures as well as magnetic properties were characterized. The axial Cu-O bond lengths in complex 2 are 2.338–2.413 Å, unlike Ni-O, due to the axial bond elongation by the Jahn-Teller effect in Cu(II) complexes. Magnetic analyses revealed that both complexes showed intra-cluster ferromagnetic interactions with S = 3 for 1 and S = 2 for 2, respectively.

"Triple-Decker Sandwich" Containing Planar {B2E2Pd} Ring (E = S or Se).

In an effort to generate triple-decker complexes comprising a {PdCl2}moiety in the middle deck, we have explored the reactivity of [(Cp*M)2{μ-B2H2E2}], 1-4 (1: M = Co, E = S; 2: M = Co, E = Se; 3: M = Rh, E = Se; and 4: M = Ir, E = Se; Cp* = η5-C5Me5), with [PdCl2(COD)] (COD = 1,5-cyclooctadiene). The reactions led to the formation of a series of trinuclear heterometallic triple-decker complexes, [(Cp*M)2{μ-B2H2E2Pd(Cl)2}], 5-8 (5: M = Co, E = S; 6: M = Co, E = Se; 7: M = Rh, E = Se; and 8: M = Ir, E = Se). Formation of the complexes 5-8 occurred almost instinctively as a single product with the elimination of the COD ligand. These complexes are examples of novel triple-decker species having a planar bridging palladacycle ligand, in which the Pd metal exists as Pd(II) in an uncommon pseudo-octahedral environment with an elongated M-Pd bonding interaction. The new species, 5-8, have been characterized spectroscopically, and the structures of 5-7 were confirmed by single-crystal X-ray diffraction studies. The structure and bonding of these molecules were further analyzed with the help of density functional theory studies that found a strong electron donation from the B2E2 (E = S or Se) fragment of the middle ring to the axial metals, while a weak bonding interaction between group 9 metals and Pd.

Thermal analysis, synthesis and structural studies of heterometallic {Fe2MO} salicylate complexes

Four new trinuclear heterometallic molecular complexes with the {Fe2IIIMII μ3-O} core, where M = Mn(II), Ni(II), Cu(II) and Zn(II), have been synthesized by the reaction between iron nitrate and d-metal salts with ammonium salicylate in a mixture of solvents. The compounds were obtained as crystalline material suitable for single-crystal structure analysis with the following composition: [Fe2MnO(SalH)6(EtOH)(MeOH)2]·DMAA·EtOH·2MeOH·H2O (1); [Fe2NiO(SalH)6(EtOH)(MeOH)2]⋅DMAA⋅2MeOH⋅1.5H2O (2); [Fe2CuO(SalH)6(EtOH)(MeOH)2]⋅DMAA⋅4H2O (3); [Fe2ZnO(SalH)6(EtOH)(MeOH)2]⋅DMF⋅2MeOH⋅1.5H2O (4), where SalH = monodeprotonated salicylic ligand. The elemental analysis and IR spectra of the complexes 1–4 are in good agreement with the crystallographic data. The Mossbauer parameters are characteristic to iron(III) complexes with a spin value of S = 5/2. The thermal properties of all compounds have been studied in air and nitrogen atmosphere at the 20–1000 °C temperature range. The thermal analysis data revealed that after the elimination of external solvent molecules, the trinuclear core of the complexes is stable up to 250 °C.

Magnetic Properties of Adducts of Trinuclear Heterometallic Complexes with Acetonitrile: Quantum Chemical Study

Magnetic Properties of Adducts of Trinuclear Heterometallic Complexes with Acetonitrile: Quantum Chemical Study

Synthesis, structure and magnetic properties of three CuII2LnIII complexes (Ln = Pr, Nd and Sm) with an unsymmetrical Schiff base ligand

Three heterometallic trinuclear complexes of general formula, [{(CH3CN)CuL}2(µ-NO3)2Ln(NO3)] [Ln = Pr (1), Nd (2) and Sm (3)] have been synthesized by reacting a neutral metalloligand [CuL], derived from a N2O2 donor unsymmetrically di-condensed di-Schiff base, H2L (where H2L = N-α-methylsalicylidene-N΄-salicylidene-1,3-propanediamine) with the respective lanthanide nitrates in 2:1 ratios. X-ray single-crystal structure reveals that these isomorphic complexes (1–3) possess C2-symmetric trinuclear units where two terminal metalloligands are coordinated to the central Ln(III) ion in transoid manner. In all three complexes, distorted tetradecahedron coordination geometry is formed around the Ln(III) ion by four μ2-phenoxido oxygen atoms from two (CuL) units and three chelating nitrato coligands. The two terminal Cu(II) ions possess distorted octahedral geometry. The dc magnetic susceptibilities and field dependent magnetization measurements of all three complexes reveal the occurrence of antiferromagnetic interactions between Cu(II) and Ln(III) ions.

Isomerism in heterometallic trinuclear complexes with salen type Schiff base ligands

The use of neutral Cu-II and Ni-II complexes of salen type Schiff bases as metalloligand in the synthesis of heterometallic poly nuclear complexes are known since long but the potential of isomerization phenomenon in the resulted complexes has not been realized. A systematic study in the authors' laboratory for the last ten years revealed that the trinuclear heterometallic complexes can exist in different isomeric forms e.g. linear-bent isomers, linkage isomers, coordination position isomers, supramolecular isomers, polymorphs, etc. The focus of the present review is mainly on the syntheses, structural characterization and magnetic properties of these isomeric products.

Rare trinuclear NiII2MII complexes (MII = Mn, Fe and Co) with a reduced Schiff base ligand: Synthesis, structures and magnetic properties

Abstract Three new trinuclear hetero-metallic NiII2MII complexes with MII = Mn, Fe and Co have been synthesized using a [NiLR] “metalloligand”, where H2LR = N,N′-bis(2-hydroxybenzyl)-1,3-propanediamine. All complexes have been characterized by elemental analysis, spectroscopic methods, single crystal XRD and magnetic and electrochemical studies. In the three complexes, in addition to the double phenoxido bridges, the two terminal NiII atoms are linked to the central MII [M = Mn(1), Fe(2) and Co(3)] ion by means of a bridging carboxylato co-anion, giving rise to a linear NiII-MII-NiII structure. Variable temperature magnetic susceptibility measurements show the presence of weak ferromagnetic and antiferromagnetic exchange interactions mediated through the double phenoxido bridges with J values of +8.5 and −3.0 cm−1 for complexes 1 and 2, respectively. Compound 3 shows the presence of antiferromagnetic interactions. Cyclic voltammetry shows a common quasi-reversible one electron oxidation corresponding to the Ni(II)/Ni(III) process in 1–3 and an irreversible M(II)/M(III) oxidation for Mn(1) and Fe(2).

Rare examples of diphenoxido-bridged trinuclear NiII2FeIII complexes with a reduced salen type Schiff base ligand: Structures and magnetic properties

Three new trinuclear hetero-metallic complexes, [(NiLR)2Fe(N3)3] (1), [(NiLR(H2O))2Fe(C6H5CH2CO2)2]·(HSO4) (2) and [(NiLR(H2O))2Fe(C6H5CO2)2]·(HSO4)·(H2O)·(CH2Cl2) (3) have been synthesized using [NiLR] as a “metalloligand” (where H2LR=N,N′-bis(2-hydroxybenzyl)-1,3-propanediamine). All complexes have been characterized by elemental analysis, spectroscopic methods, single crystal XRD and magnetic study. In the angular trinuclear units of 1, the two terminals [NiLR] coordinate through double phenoxido bridges to the central FeIII ion which is penta-coordinated having terminally coordinated azide ion. The two terminal NiII centers are connected to each other and also to neighbouring units through μ1,3-azido bridges to form an alternating chain. On the other hand, complexes, 2 and 3 are linear discrete trinuclear species [NiII–FeIII–NiII] in which two terminal octahedral [NiLR] units coordinate to the central octahedral FeIII ion, located on a crystallographic centre of symmetry, through a μ2-phenoxido oxygen atom and a bridging carboxylato ion. Variable temperature magnetic susceptibility measurements show the presence of antiferromagnetic exchange interactions in 1 mediated through the phenoxido bridges (J1=−33.2cm−1,) and μ1,3-N3 single bridges (J2=−19.9cm−1 and J3=−16.7cm−1). On the other hand, compounds 2 and 3 show ferromagnetic coupling interactions mediated through the double phenoxido bridges with J values of +4.9 and +3.0cm−1 for 2 and 3, respectively.

Synergistic treatment of triple-negative breast cancer by doxorubicin and thioridazine and its nano-formulation

Three new trinuclear hetero-metallic complexes, [(NiLR)2Fe(N3)3] (1), [(NiLR(H2O))2Fe(C6H5CH2CO2)2]·(HSO4) (2) and [(NiLR(H2O))2Fe(C6H5CO2)2]·(HSO4)·(H2O)·(CH2Cl2) (3) have been synthesized using [NiLR] as a “metalloligand” (where H2LR = N,N′-bis(2-hydroxybenzyl)-1,3-propanediamine). All complexes have been characterized by elemental analysis, spectroscopic methods, single crystal XRD and magnetic study. In the angular trinuclear units of 1, the two terminals [NiLR] coordinate through double phenoxido bridges to the central FeIII ion which is penta-coordinated having terminally coordinated azide ion. The two terminal NiII centers are connected to each other and also to neighbouring units through μ1,3-azido bridges to form an alternating chain. On the other hand, complexes, 2 and 3 are linear discrete trinuclear species [NiII–FeIII–NiII] in which two terminal octahedral [NiLR] units coordinate to the central octahedral FeIII ion, located on a crystallographic centre of symmetry, through a μ2-phenoxido oxygen atom and a bridging carboxylato ion. Variable temperature magnetic susceptibility measurements show the presence of antiferromagnetic exchange interactions in 1 mediated through the phenoxido bridges (J1 = −33.2 cm−1,) and μ1,3-N3 single bridges (J2 = −19.9 cm−1 and J3 = −16.7 cm−1). On the other hand, compounds 2 and 3 show ferromagnetic coupling interactions mediated through the double phenoxido bridges with J values of +4.9 and +3.0 cm−1 for 2 and 3, respectively.

Electrochemically induced transformations of bi- and trinuclear heterometallic vinylidene complexes containing Re, Pd and Fe

Redox-induced transformations of bi-Cp(CO)2RePd(μ-CCHPh)(PPh3)2 (2), Cp(CO)2RePd(μ-CCHPh)(P-P) [P-P=η2-Ph2P(CH2)2PPh2 (dppe) (3), η2-Ph2P(CH2)3PPh2 (dppp) (4)], Cp(CO)2ReFe(μ-CCHPh)(CO)4 (5) and trinuclear Cp(CO)2ReFe2(μ3-CCHPh)(CO)6 (6), CpReFePd(µ3-CCHPh)(CO)5(P-P) [P-P=dppe (7), dppp (8)] heterometallic vinylidene complexes derived from Cp(CO)2ReCCHPh (1) were investigated by electrochemical methods. The oxidation processes of clusters 7 and 8 were studied by EPR spectroscopy. The electrochemical properties of clusters 7 and 8 were shown to depend on the nature of the chelate diphosphine ligand at the palladium center in contrast to complexes 3 and 4. The oxidation of complexes 2–4 was found to result in the Re-Pd, Pd-C bond cleavage and formation of rhenium complex 1 and Pd-containing fragments. The oxidation of 7 and 8 resulted in the formation of the radical cations 7+ and 8+, which sequentially transformed into a number of intermediate products: Cp(CO)2RePd(μ-CCHPh)(P-P), [Fe(CO)3]+, [CpReFe2(µ3-CCHPh)(CO)6]+ and others. The final products of their oxidation in both cases were triangular clusters (CO)8Fe2Pd(P-P) [P-P=dppe (9), dppp (10)] and rhenium complex 1. The molecular structure of cluster 7 was established by X-ray diffraction analysis.

3d-4f heterometallic trinuclear complexes derived from amine-phenol tripodal ligands exhibiting magnetic and luminescent properties.

A new family of 3d-4f heterometallic trinuclear complexes, namely [M2LnL2]·2ClO4·H2O (H3L = tris(((2-hydroxy-3-methoxybenzy1)amino)ethyl)amine, where M = Ni, Ln = Gd (1), Dy (2), and M = Zn, Ln = Gd (3), Dy (4)) were synthesized via the reaction of H3L and Ln(NO3)3·6H2O and Ni(ClO4)2·6H2O or Zn(ClO4)2·6H2O in a 2 : 1 : 2 ratio in the solution. Complexes 1-4 consisted of three metal ions arranged in an isosceles triangle manner. Magnetic properties investigations showed that complexes 1 and 2 exhibited weak ferromagnetic coupling between the Ni(ii) and Ln(iii) ions, whereas complex 4 displayed lanthanide single-ion magnetic properties. The alternating current (ac) magnetic susceptibilities of 4 revealed that both the in-phase (χ') and out-of-phase (χ'') signals are frequency- and temperature-dependent, which are typical features of the field-induced slow relaxation of the magnetization with Ueff = 124.5 K. Complex 4 also exhibited an obvious butterfly-shaped hysteresis loop at 2 K, indicating that it is a single-ion magnet. Moreover, complex 4 showed stronger fluorescent emissions, which were typical narrow emission bands of lanthanide ions. Therefore, complex 4 can be considered as a molecular magnetic and luminescent material. Comparably, complex 2 showed very weak DyIII-based emissions because the paramagnetic NiII ions quench the fluorescence and thereby lower the population of the triplet state.

Exploring the coordinative adaptation and molecular shapes of trinuclear CuM(II) (M = Zn/Cd) complexes derived from salen type Schiff bases: structural and theoretical studies.

Three new trinuclear hetero-metallic complexes [(CuL)2Zn(NCS)2] (1), [(CuL(R))2Zn(NCS)(μ1,1-NCS)] (2) and [(CuL(R))2Cd(μ1,3-NCS)2] (4) have been synthesized using [CuL] and [CuL(R)] as "metalloligands" (where H2L = N,N'-bis(salicylidene)-1,3-propanediamine and H2L(R) = N,N'-bis(2-hydroxybenzyl)-1,3-propanediamine). All three complexes are characterized by elemental analysis, spectroscopic methods and single crystal XRD. Complex 1 is an angular trinuclear species, in which two terminal four-coordinate square planar "metalloligands" [CuL] are coordinated to a central Zn(ii) through double phenoxido bridges along with two mutually cis nitrogen atoms of terminal isothiocyanate ions as is usually found in such complexes. In contrast, in complex 2, the two terminal "metalloligands" [CuL(R)] are square pyramidal, as one of the SCN(-) ions makes an unusual μ1,1-NCS bridge between copper centers while the other one coordinates to Zn(ii) through a N atom in a usual fashion making its geometry also square pyramidal. For 4 which possesses an angular trinuclear structure, in addition to double phenoxido bridges from two terminal [CuL(R)], both the SCN(-) ions are S-bonded to Cd(ii) and form a bridge (cis-μ1,3-SCN) between Cd(ii) and each of the terminal Cu(ii) ions. This structure is different from its unreduced analogue in which NCS(-) was N-terminal coordinated to Cd(ii) (3/3'). All the structures have been optimized using density functional theory (DFT) calculations. It has been found that for H2L, optimized structures like 1 and 2 differ only by 0.4 kcal mol(-1) but the H2L(R) structure 2 is more stable by 5.5 kcal mol(-1) than the structure resembling 1. For Cd(ii) complexes also, H2L optimized structures such as 3 and 4 do not differ significantly in energy (1.0 kcal mol(-1)) but the H2L(R) structure 4 is more stable than that of 3 by 4.6 kcal mol(-1). In fact, structure 4 has been found to be the most stable one among the other possible isomers of H2L(R).