DFT-BS study on the magnetic coupling interaction in a series of (R-Bpmp)Mn2(μ-OAc)2 complexes

The magnetic properties of a series of dinuclear Mn^II systems are investigated by the calculations based on density functional theory combined with broken-symmetry approach (DEF-BS). It is found that there are weak antiferromagnetic interactions in these systems with different bridging ligands. The changing trend of the magnetic coupling constants J indicates that with the electronegativity of the increasing bridging ligands, the antiferromagnetic coupling interaction is weakened. The analyses of the magnetic orbitals and the spin densities show that the weakly antiferromagnetic couplings in these systems are due to the vertical magnetic d orbitals and the weak spin delocalization. These results should be instructive for the design of new molecular magnetic materials.     

Improving the calculation of magnetic coupling constants in MRPT methods

The magnetic coupling in transition metal compounds with more than one unpaired electron per magnetic center has been studied with multiconfigurational perturbation theory. The usual shortcomings of these methodologies (severe underestimation of the magnetic coupling) have been overcome by describing the Slater determinants with a set of molecular orbitals that maximally resemble the natural orbitals of a high‐level multiconfigurational reference configuration interaction calculation. These orbitals have significant delocalization tails onto the bridging ligands and largely increase the coupling strengths in the perturbative calculation. © 2014 Wiley Periodicals, Inc.     

混合桥基双核配合物中的轨道互补与反互补效应

在混合桥基的双核体系中,金属中心的磁轨道的线形组合可与相同对称性的桥基最高占据轨道(HOMO'S)相互作用.如果两种桥联配体稳定同一磁轨道组合,则称它们以互补方式起作用;反铁磁偶合就会被加强.反之,如果桥联配体稳定不同的磁轨道组合,则称它们以反互补方式起作用;这将减小反铁磁偶合.本文就互补和反互补效应以及相关的磁学知识做一简要概述.     

Magnetic properties of Cu(II) bischelate complexes with 3-imidazoline nitroxides. 2. Anab initio analysis of exchange interaction mechanisms

The mechanism responsible for the emergence of ferromagnetic exchange interactions in bischelate complexes of Cu²⁺ with enaminoketone derivatives of 3-imidazoline nitroxide CuL₂ is studied by ab initio quantum chemical methods. The parameters J_cu-L and J_L-L’ of exchange interactions between the unpaired electrons of the paramagnetic centers (Cu²⁺ ion and N-O groups of nitroxyl ligands L and L’) of these complexes were calculated in terms of the full 3x3 configuration interaction between the singlet states constructed in a basis set of molecular orbitals of unpaired electrons. It is shown that for variations of the structure of the coordination polyhedron around the Cu²⁺ ion from square planar to tetrahedral the exchange interactions between the unpaired electrons of the paramagnetic centers is ferromagnetic J_Cu-L >J_L -L’>0, which agrees with the data of magnetic measurements. The principal mechanism of exchange interactions in CuL₂ complexes is the delocalization mechanism that is due to a minor transfer of spin density from the 3d-orbitals of Cu²⁺ to the Σ-orbitals of the N-O groups of L and L’ ligands. Translated fromZhurnal Strukturnoi Khimii, Vol. 38, No. 5, pp. 850–856, September-October, 1997.     

Interpretation of the indirectly induced field at nonmagnetic atoms in Fe-Y garnets

The indirectly induced magnetic fields at Sn⁴⁺ replacing Fe³⁺ in octahedral positions are derived in the MO LCAO [molecular orbitals by linear combinations of atomic orbitals] semiempirical approximation, with self-consistent charges on the atoms, for a part of the crystal containing Sn⁴⁺ and Fe³⁺ (in tetrahedral positions), the two being linked via oxygen ions. The absence of conduction is due to the very restricted overlap between the MO of the parts. The results explain the observed induced field and also predict certain possible effects, in particular unpaired spin density on other lattice ions.     

Determination of the number of unpaired electrons in metal-complexes. A comparison between the Evans’ method and susceptometer results

1,4,8,11-Tetraazacyclotetradecane (cyclam) is widely known as an ideal ligand for chelating heavy metal ions such as Ni²⁺ and Cu²⁺. In this work, the consequences of chelation on the preference for high spin or low spin configuration were investigated for Fe³⁺, Ni²⁺, Cu²⁺ and Cr³⁺. Two methods were used to determine the number of unpaired electrons in the complex. First the dependence of magnetic susceptibility change of the crystalline powder as a function of temperature increase was measured. In the second case, the Evans’ method was used to obtain information about the number of unpaired electrons in the dissolved complexes. In some cases, such as Fe³⁺ and Ni²⁺, a discrepancy between the results obtained with the two methods was noticed. The results are influenced by the geometry of the complex, the aggregation state and the dimensions of the ions.     

A theoretical analysis of the explanation of the significant differences in antiferromagnetic interactions between homologous μ-alkoxo and acetate bridged dicopper(II) complexes: ab initio and semi-empirical calculations

A magnetostructural classification of dimmers, containing the Cu (μ-alkoxo) Cu core, based on data obtained from X-ray diffraction analysis reported in the literature has been performed. In these complexes, the local geometry around the copper ions is generally a square planar and each copper ion is surrounded by one N atom and three O atoms. The influence of the overlap interactions between the bridging ligands and the metal (Cu) d orbitals on the super-exchange coupling constant has been studied by means of ab initio Restricted Hatree–Fock molecular orbital calculations. The interaction between the magnetic d orbitals and highest occupied molecular orbitals of the acetate oxygens has been investigated in homologous μ-acetato-bridged dicopper(II) complexes which have significantly different −2J values (the energy separation between the spin-triplet and spin-singlet states). In order to determine the nature of the fronter orbitals, Extended Hückel molecular Orbital calculations are also reported. Ab initio restricted Hartree–Fock calculations have shown that the acetato bridge and the alkoxide bridge contribute to the magnetic interaction countercomplementarily to reduce antiferromagnetic interaction.     

Linear Copper Complex Arrays as Versatile Molecular Strings: Syntheses,Structures, Luminescence,and Magnetism

The defined linear arrangement of metal atoms in discrete coordination complexes or polymers is still one of the most intriguing challenges in synthetic chemistry. These chain arrangements are of fundamental importance, because of their potential applications as molecular wires and single molecule magnets (SMM) in microelectronic devices on a molecular scale. Oligonuclear Group 11 metal complexes with suitable bridging ligands, specifically those that are based on copper as the first choice of a cheap precursor coinage metal, are of particular interest in this regard. This is due to the superior luminescence properties of these linear clusters that often show d¹⁰⋅⋅⋅d¹⁰ interactions in their molecular structures. The combination of Cu^I with heavier coinage metal ions results in tunable emissive arrays that are also stimuli-responsive. Thus, both linear multinuclear Cu^I and linear heteropolymetallic Cu^I/Ag^I as well as Cu^I/Au^I clusters are excellent candidates for applications in molecular/organic light-emitting devices (OLEDs). Alternatively, paramagnetic multinuclear cupric arrays are prominent as potential molecular wires with enhanced magnetic properties through multiple coupled d⁹ centers. This Review covers the whole range of linear multinuclear assemblies of cuprous and cupric ions in complexes and coordination polymers, their syntheses, photophysical behavior, and magnetic properties. Moreover, recent advances in the rapidly progressing field of hetero-Cu^I/Ag^I and Cu^I/Au^I molecular strings are also discussed.     

A Theoretical Account of the Coupling between Metal- and Ligand-centred Spins

This study addresses the magnetic interaction between paramagnetic metal ions and the radical ligands taking the [Cu^II(hfac)₂(imVDZ)] and [M^II(hfac)₂(pyDTDA)] (imVDZ=1,5-dimethyl-3-(1-methyl-2-imidazolyl)-6-oxoverdazyl; hfac=(1,1,1,5,5,5)hexafluroacetylacetonate; pyDTDA=4-(2′-pyridyl)-1,2,3,5-dithiadiazolyl), (M=Cu, Ni, Co, Fe, Mn) compounds as reference systems. The coupling between the metal and ligand spins is quantified in terms of the exchange coupling constant (J) in the platform of density functional theory (DFT) and the wave function-based complete active space self-consistent field (CASSCF) method. Application of DFT and broken symmetry (BS) formalism results ferromagnetic coupling for all the transition metal complexes except the Mn(II) complex. This DFT-BS prediction of magnetic nature matches with the experimental finding for all the complexes other than the Fe(II)-pyDTDA complex, for which an antiferromagnetic coupling between high spin iron and the thiazyl ligand has been reported. However, evaluation of spin state energetics through the multiconfigurational wave function-based method produces the S=3/2 ground spin state for the iron-thiazyl in parity with experiment. Electronic structure analyses find the overlap between the metal- and ligand-based singly occupied molecular orbitals (SOMOs) to be one of the major reasons attributing to different extent of exchange coupling in the systems under investigation.     

Electronic structure of linear thiophenolate-bridged heteronuclear complexes [LFeMFeL](n)(+) (M = Cr, Co, Fe; n = 1-3): a combination of kinetic exchange interaction and electron delocalization

The electronic properties of the isostructural series of heterotrinuclear thiophenolate-bridged complexes of the general formula [LFeMFeL](n)(+) with M = Cr, Co and Fe where L represents the trianionic form of the ligand 1,4,7-tris(4-tertbutyl-2-mercaptobenzyl)-1,4,7-triazacyclononane, synthesized and investigated by a number of experimental techniques in the previous work(1), are subjected now to a theoretical analysis. The low-lying electronic excitations in these compounds are described within a minimal model supported by experiment and quantum chemistry calculations. It was found indeed that various experimental data concerning the magnetism and electron delocalization in the lowest states of all seven compounds are completely reproduced within a model which includes the electron transfer between magnetic orbitals at different metal centers and the electron repulsion in these orbitals (the Hubbard model). Moreover, due to the trigonal symmetry of the complexes, only the electron transfer between nondegenerate orbital, a(1), originating from the t(2g) shell of each metal ion in a pseudo-octahedral coordination, is relevant for the lowest states. An essential feature resulting from quantum chemistry calculations, allowing to explain the unusual magnetic properties of these compounds, is the surprisingly large value and, especially, the negative sign of the electron transfer between terminal iron ions, beta'. According to their electronic properties the series of complexes can be divided as follows: (1). The complexes [LFeFeFeL](3+) and [LFeCrFeL](3+) show localized valences in the ground electronic configuration. The strong antiferromagnetic exchange interaction and the resulting spin 1/2 of the ground-state arise from large values of the transfer parameters. (2). In the complex [LFeCrFeL](+), due to a higher energy of the magnetic orbital on the central Cr ion than on the terminal Fe ones, the spin 3/2 and the single unpaired a(1) electron are almost localized at the chromium center in the ground state. (3). The complex [LFeCoFeL](3+) has one ground electronic configuration in which two unpaired electrons are localized at terminal iron ions. The ground-state spin S = 1 arises from a kinetic mechanism involving the electron transfer between terminal iron ions as one of the steps. Such a mechanism, leading to a strong ferromagnetic interaction between distant spins, apparently has not been discussed before. (4). The complex [LFeFeFeL](2+) is characterized by both spin and charge degrees of freedom in the ground manifold. The stabilization of the total spin zero or one of the itinerant electrons depends on beta', i.e., corresponds to the observed S = 1 for its negative sign. This behavior does not fit into the double exchange model. (5). In [LFeCrFeL](2+) the delocalization of two itinerant holes in a(1) orbitals takes place over the magnetic core of chromium ion. Although the origin of the ground-state spin S = 2 is the spin dependent delocalization, the spectrum of the low-lying electronic states is again not of a double exchange type. (6). Finally, the complex [LFeCoFeL](2+) has the ground configuration corresponding to the electron delocalization between terminal iron atoms. The estimated magnitude of the corresponding electron transfer is smaller than the relaxation energy of the nuclear distortions induced by the electron localization at one of the centers, leading to vibronic valence trapping observed in this compound.     

Oligo-m-phenyleneoxalamide copper(II) mesocates as electro-switchable ferromagnetic metal-organic wires

Double‐stranded copper(II) string complexes of varying nuclearity, from di‐ to tetranuclear species, have been prepared by the Cu^II‐mediated self‐assembly of a novel family of linear homo‐ and heteropolytopic ligands that contain two outer oxamato and either zero ( 1 b ), one ( 2 b ), or two ( 3 b ) inner oxamidato donor groups separated by rigid 2‐methyl‐1,3‐phenylene spacers. The X‐ray crystal structures of these Cu^II_n complexes (n=2 ( 1 d ), 3 ( 2 d ), and 4 ( 3 d )) show a linear array of metal atoms with an overall twisted coordination geometry for both the outer CuN₂O₂ and inner CuN₄ chromophores. Two such nonplanar all‐syn bridging ligands 1 b – 3 b in an anti arrangement clamp around the metal centers with alternating M and P helical chiralities to afford an overall double meso‐helicate‐type architecture for 1 d – 3 d . Variable‐temperature (2.0–300 K) magnetic susceptibility and variable‐field (0–5.0 T) magnetization measurements for 1 d – 3 d show the occurrence of S=nS_Cu (n=2–4) high‐spin ground states that arise from the moderate ferromagnetic coupling between the unpaired electrons of the linearly disposed Cu^II ions (S_Cu=1/2) through the two anti m‐phenylenediamidate‐type bridges (J values in the range of +15.0 to 16.8 cm^?1). Density functional theory (DFT) calculations for 1 d – 3 d evidence a sign alternation of the spin density in the meta‐substituted phenylene spacers in agreement with a spin polarization exchange mechanism along the linear metal array with overall intermetallic distances between terminal metal centers in the range of 0.7–2.2 nm. Cyclic voltammetry (CV) and rotating‐disk electrode (RDE) electrochemical measurements for 1 d – 3 d show several reversible or quasireversible one‐ or two‐electron steps that involve the consecutive metal‐centered oxidation of the inner and outer Cu^II ions (S_Cu=1/2) to diamagnetic Cu^III ones (S_Cu=0) at relatively low formal potentials (E values in the range of +0.14 to 0.25 V and of +0.43 to 0.67 V vs. SCE, respectively). Further developments may be envisaged for this family of oligo‐m‐phenyleneoxalamide copper(II) double mesocates as electroswitchable ferromagnetic ‘metal–organic wires’ (MOWs) on the basis of their unique ferromagnetic and multicenter redox behaviors.     

同结构GdIIIMII(M=Cu，Ni，Co，Fe，Mn)配合物磁性理论研究

基于DFT-BS方法,在不同泛函方法和基组下计算[CuIIGdIII{pyCO(OEt)py C(OH)(OEt)py}3]2+及3d-Gd异金属配合物的磁耦合常数,结果表明,PBE0/TZVP(Gd为SARC-DKH-TZVP)水平可用于描述其磁学性质。顺磁中心CuII、GdIII与桥联配位氧原子间存在较强的轨道相互作用,其磁轨道主要由GdIII的4fz3、4fz(x2-y2)轨道、CuII的3dx2-y2轨道和桥联配位原子O的p轨道组成。顺磁中心CuII离子以自旋离域作用为主,GdIII离子以自旋极化作用为主,顺磁中心CuII自旋离域作用对桥联氧原子的影响大于顺磁中心GdIII的自旋极化作用。在同结构3d-Gd配合物中,随着MII离子未成对电子的增加,顺磁中心间自旋密度平方差越大,顺磁中心MII和GdIII之间的反铁磁性贡献越大,其磁耦合常数越小。     

Structural,Electronic, and Optical Properties of Metallo Base Pairs in Duplex DNA: A Theoretical Insight

Using density functional theory calculations, we investigated the structural, energetic, electronic, and optical properties of recently synthesized duplex DNA containing metal‐mediated base pairs. The studied duplex DNA consists of three imidazole (Im) units linked through metal (Im‐M‐Im, M=metal) and four flanking A:T base pairs (two on each side). We examined the role of artificial base pairing in the presence of two distinctive metal ions, diamagnetic Ag⁺ and magnetic Cu²⁺ ions, on the stability of duplex DNA. We found that metal‐mediated base pairs form stable duplex DNA by direct metal ion coordination to the Im bases. Our results suggest a higher binding stability of base pairing mediated by Cu²⁺ ions than by Ag⁺ ions, which is attributed to a larger extent of orbital hybridization. We furthermore found that DNA modified with Im‐Ag⁺‐Im shows the low‐energy optical absorption characteristic of π–π*orbital transition of WC A:T base pairs. On the other hand, we found that the low‐energy optical absorption peaks for DNA modified with Im‐Cu²⁺‐Im originate from spin–spin interactions. Additionally, this complex exhibits weak ferromagnetic coupling between Cu²⁺ ions and strong spin polarization, which could be used for memory devices. Moreover, analyzing the role of counter ions (Na⁺) and the presence of explicit water molecules on the structural stability and electronic properties of the DNA duplex modified with Im‐Ag⁺‐Im, we found that the impact of these two factors is negligible. Our results are fruitful for understanding the experimental data and suggest a potential route for constructing effective metal‐mediated base pairs in duplex DNA for optoelectronic applications.     

Is [Cu3(L2)Cl2]2+ a molecule with geometrically spin frustration? DFT study on magnetic competing interaction in hetero-bridged tricopper(II) complex

The theoretical calculations on magnetic exchange interaction of the hetero-bridged tricopper(II) complex [Cu₃(L²)Cl₂]²⁺ and a related binuclear copper(II) model are carried out by using the density functional theory combined with the broken-symmetry approach. Meanwhile, one strategy computationally, so called isolated magnetic pair approach, is suggested to explore the spin frustration from geometry topology in poly-nuclear magnetic systems. It is found that the ferromagnetic coupling (J₂>0) of Cu1–Cu3 pair bridged by double μ–Cl ligands, in nature, is intrinsic, not resulted from geometrically spin frustration in the hetero-bridged tricopper(II) studied. However, in the whole molecule exist two competing contributions of antiferromagnetic and ferromagnetic coupling, and the antiferromagnetic coupling (J₁) from Cu1–(μ–OR)–Cu2 and Cu2–(μ–OR)–Cu3 pairs dominates the magnetic behavior of the whole molecular system. On the other hand, the variation of J₁/J₂ ratio affects significantly on magnetic properties of the system. The calculated effective magnetic moment μ_eff of 2.26μ_B at the OPerdew functional level is compared to experimentally observation of 2.70μ_B in the solution. The briefly analysis of molecular magnetic orbitals demonstrated that the two local magnetic orbitals on Cu1 and Cu3 ions are orthogonal each other, and primarily responsible for the intrinsic ferromagnetic coupling between Cu1–(μ–Cl)₂–Cu3. By comparison of spin population distributions for the Cu-triad and Cu-dimer the validity of the isolated magnetic pair approach is confirmed.     

Linear Chains of Magnetic Ions Stacked with Variable Distance: Ferromagnetic Ordering with a Curie Temperature above 20 K

We have studied the magnetic properties of the SURMOF‐2 series of metal–organic frameworks (MOFs). Contrary to bulk MOF‐2 crystals, where Cu²⁺ ions form paddlewheels and are antiferromagnetically coupled, in this case the Cu²⁺ ions are connected via carboxylate groups in a zipper‐like fashion. This unusual coupling of the spin ions within the resulting one‐dimensional chains is found to stabilize a low‐temperature, ferromagnetic (FM) phase. In contrast to other ordered 1D systems, no strong magnetic fields are needed to induce the ferromagnetism. The magnetic coupling constants describing the interaction between the individual metal ions have been determined in SQUID experiments. They are fully consistent with the results of ab initio DFT electronic structure calculations. The theoretical results allow the unusual magnetic behavior of this exotic, yet easy‐to‐fabricate, material to be described in a detailed fashion.     

Synthesis and magnetic properties of poly(p‐phthaloyl‐disacetylaceton‐ethylenediimine) metal complexes

Macrocyclic Schiff‐base ligand, bisacetylaceton‐ethylenediimine (BAE) and its transition metal complexes M(BAE) (M = Cu²⁺, Ni²⁺) were synthesized. The complexes having characteristics of aromatic systems and well‐defined one‐dimensional structures, reacted with p‐phthaloyl chloride, to obtain polymer complexes. The complexes were characterized by elemental analysis, inductively coupled plasma (ICP), FT‐IR, and thermal analysis and show good thermal stability. ESR spectra analysis discovered that there are free radicals in the chain of polymers, indicating that a weak magnetic spin‐exchange interaction operates between the metal ions and free radicals. It is found that, as the bridging p‐phthaloyl group is able to propagate the magnetic exchange interaction, the polymer complexes show paramagnetic properties by measurement of temperature dependence of the magnetic property, and obey Curie–Weiss law. Copyright © 2001 John Wiley & Sons, Ltd.     

Artificial metallo-DNA towards discrete metal arrays

DNA shows promise as a provider of a structural basis for the “bottom-up” fabrication of functionalized molecular building blocks. In particular, the replacement of hydrogen-bonded DNA base pairing for alternative one could possibly provide a novel tool for re-engineering DNA as well as for biological applications. This review describes our recent approaches to metal-based strategy directed towards self-assembled metal arrays within DNAs. Recently, we reported the synthesis of a series of artificial oligonucleotides, d(5′-G H _nC-3′) (n = 1-5), using hydroxypyridone nucleobases ( H ) as flat bidentate ligands. Right-handed double helices of the oligonucleotides, nCu²⁺·d(5′-G H _nC-3′)₂ (n = 1-5), are quantitatively formed through Cu²⁺-mediated alternative base pairing ( H -Cu²⁺- H ), where the Cu²⁺ ions are aligned along the helix axes inside the duplexes with the Cu²⁺-Cu²⁺ distance of 3.7 ± 0.1 Å. The Cu²⁺ ions are coupled in a ferromagnetic manner with one another through unpaired d electrons to form magnetic chains. This strategy represents a new method for self-assembled metal arrays in a predesigned fashion, leading to the possibility of metal-based molecular devices such as molecular magnets and wires.     

Synthesis, coordination properties, and analytical applications of mixed donor macrocycles containing the 1,10-phenanthroline sub-unit

The coordination properties towards different metal ions of a new class of mixed N/S-, and N/S/O-donor macrocycles containing the 1,10-phenanthroline sub-unit in the cyclic framework are reviewed. The conformational constraints imposed by the heteroaromatic fragment onto the aliphatic portion of the ring determine the coordination mode of these ligands which can stabilise low-valent Ni⁺, Pd⁺, Pt⁺, and Rh⁺ metal complexes. Structural and thermodynamic aspects of the coordination chemistry of these ligands are considered together with possible applications as building blocks in the synthesis of multi-centred systems, and as template in the construction of extended polyiodide networks. However, solution studies demonstrate the inability of these ligands to work as selective and specific fluorescent chemosensors for heavy transition and post-transition metal ions and the formation constants evaluated for the formation of 1:1 complexes with Pb²⁺, Cd²⁺, Hg²⁺, Cu²⁺, and Ag⁺ in acetonitrile are of the same order of magnitude. Nevertheless, some of these macrocyles are extremely effective to recognise Cu²⁺ or Ag⁺ over the other metal ions in transport processes, and have been successfully used as neutral ionophore in the construction of PVC-based ionselective electrodes and supported liquid membranes for analytical detection and separation, respectively, of these metal ions.     

A Two‐in‐one Pincer Ligand and its Diiron(II) Complex Showing Spin State Switching in Solution through Reversible Ligand Exchange

A novel pyrazolate‐bridged ligand providing two {PNN} pincer‐type compartments has been synthesized. Its diiron(II) complex LFe₂(OTf)₃(CH₃CN) ( 1 ; Tf=triflate) features, in solid state, two bridging triflate ligands, with a terminal triflate and a MeCN ligand completing the octahedral coordination spheres of the two high‐spin metal ions. In MeCN solution, 1 is shown to undergo a sequential, reversible, and complete spin transition to the low‐spin state upon cooling. Detailed UV/Vis and ¹⁹F NMR spectroscopic studies as well as magnetic measurements have unraveled that spin state switching correlates with a rapid multistep triflate/MeCN ligand exchange equilibrium. The spin transition temperature can be continuously tuned by varying the triflate concentration in solution.     

Structure and magnetism of [M3](6/7+) metal chain complexes from density functional theory: analysis for copper and predictions for silver

The ground-state electronic structure of the trinuclear complex Cu3(dpa)4Cl2 (1), where dpa is the anion of di(2-pyridyl)amine, has been investigated within the framework of density functional theory (DFT) and compared with that obtained for other known M3(dpa)4Cl2 complexes (M = Cr, Co, Ni) and for the still hypothetical Ag3(dpa)4Cl2 compound. Both coinage metal compounds display three singly occupied x2-y2-like (delta) orbitals oriented toward the nitrogen environment of each metal atom, generating antibonding M-(N4) interactions. All other metal orbital combinations are doubly occupied, resulting in no delocalized metal-metal bonding. This is at variance with the other known symmetric M3(dpa)4Cl2 complexes of the first transition series, which all display some delocalized bonding through the metal backbone, with formal bond multiplicity decreasing in the order Cr > Co > Ni. An antiferromagnetic coupling develops between the singly occupied MOs via a superexchange mechanism involving the bridging dpa ligands. This magnetic interaction can be considered as an extension to the three aligned Cu(II) atoms of the well-documented exchange coupling observed in carboxylato-bridged dinuclear copper compounds. Broken-symmetry calculations with approximate spin projection adequately reproduce the coupling constant observed for 1. Oxidation of 1 removes an electron from the magnetic orbital located on the central Cu atom and its ligand environment; 1+ displays a much weaker antiferromagnetic interaction coupling the terminal Cu-N4 moieties via four ligand pathways converging through the x2-y2 orbital of the central metal. The silver homologues of 1 and 1+ display similar electronic ground states, but the calculated magnetic couplings are stronger by factors of about 3 and 4, respectively, resulting from a better overlap between the metal centers and their equatorial ligand environment within the magnetic orbitals.