Tris-chelate Complexes Research Articles

Union is strength: π–π stacking interactions are capable of preventing solid-state racemization of tris-chelate complexes

The first reliable observation of the solid-phase racemization of the entire tris-chelate complex is reported. The decisive effect of the π–π stacking on this process is revealed.

Features of Cadmium(II) Complexation with N-Donor Heterocyclic Ligands in the Presence of the Octadecahydroeicosaborate Anion

The process of cadmium(II) complexation with 1-ethyl-2-(4-methoxyphenyl)azobenzimidazole (L1), 2,2′-bipyridyl (L2), and 1,10-phenanthroline (L3) in the presence of the [trans-B20H18]2– anion has been studied. It has been found that the nature of the organic ligand affects the composition and structures of the resulting compounds. Complexation with ligand L1, a benzimidazole derivative, has yielded binuclear complex [CdL12NO3(μ-NO3)CdL1(NO3)2] (1), which contains no boron cluster anion. In the case of azaheterocyclic ligands L2 and L3, depending on the Cd : L ratio, heteroleptic complexes [CdL2NO3]2[trans-B20H18] (L = L2 (2); L3 (3)) with coordinated nitrate ions (Cd : L = 1 : 2) or tris-chelate complexes [CdL3][trans-B20H18] (L = L2 (4); L3 (5)) with the boron cluster anion as a counterion (Cd : L = 1 : 3) have been isolated. The structures of complexes [CdL12NO3(μ-NO3)CdL1(NO3)2] (1) and [Cd(L2)2NO3]2[trans-B20H18] (2) have been determined by X-ray diffraction.

Synthesis of Nickel(II) Complexes Based on Dialkylphosphorylpyridines and Their Cytotoxic Activity

Bis-chelate nickel complex of composition [L2Ni(H2O)2](BF4)2 (4), where L = 1,3-dibenzhydryl-5-(pyridin-2-yl)-5-oxo-1,3,5-diazaphosphorinane, and tris-chelate complexes 5 and 6 of composition [L3Ni](BF4)2, where L = 2-diethylphosphorylpyridine and 2-diisopropylphosphorylpyridine, based on 1,4-N,O ligands L2–L4 with phosphoryl groups of acyclic and cyclic structure have been prepared. The structure and composition of complexes 4–6 were confirmed by mass-spectrometry, IR spectroscopy, thermogravimetric and elemental analysis. The structure of complexes 4-EtOH and 5 in crystals has been established by X-ray diffraction study. Cytotoxic properties toward M-HeLa and HuTu80 cancer cells and Chang Liver normal cell lines for the previously prepared nickel(II) complexes 1–3 based on 2-(oxophospholane)pyridines and the nickel(II) complexes obtained in this work have been studied. It has been found that the complexes with ligands containing cyclic phosphoryl groups show higher cytotoxicity toward cancer cells as compared with the complexes with their acyclic analogs.

Features of Copper(I) Complexation with Benzimidazole Derivatives in the Presence of the closo-Dodecaborate Anion

The process of copper(I) complexation with organic ligands as luminophores 1-(1-methylbenzimidazol-2-yl)-N-phenylmethanimine (L1), 1-ethyl-2-(4-methoxyphenyl)azobenzimidazole (L2), and 1-(1-benzylbenzimidazol-2-yl)-N-cyclohexylmethanimine (L3) in the presence of the closo-dodecaborate anion [B12H12]2– has been studied. It has been shown that in acetonitrile, a redox reaction proceeds to form copper(II) tris-chelate complexes [CuIIL3][B12H12]. Using diiodomethane as a solvent, we have succeeded in stabilizing copper in the +1 oxidation state; as a result, mixed-ligand binuclear complexes [CuI2L2(μ-I)2] containing no boron cluster anion have been isolated. The structures of complexes [CuII(L1)3][B12H12] and [CuI2(L3)2(μ‑I)2] have been determined by X-ray diffraction.

Features of the formation of d10 metal complexes with benzimidazoles derivatives in the presence of the closo-decaborate anion

Copper(I) complexation with luminophore ligands 1–(1–methylbenzimidazol-2-yl)-N-phenylmetanimine (L1), 1-ethyl-2-(4-methoxyphenyl)azobenzimidazole (L2) and 1-(1-benzylbenzimidazol-2-yl)-N-cyclohexylmethanimine (L3) with different types of the linker group and different functional substituents has been studied in the presence of the closo-decaborate anion. For comparison, a number of zinc(II) and cadmium(II) complexes with the [B10H10]2− anion and ligand L3 has been isolated and characterized. The effect of the nature of the complexing metal on the composition and structures of final complexes has been determined. For Zn(II), a tris-chelate complex was obtained with the boron cluster acting as a counterion. In the case of Cd(II) and Cu(I), mononuclear and binuclear complexes have been obtained, respectively, with the boron cluster anion [B10H10]2− coordinated by the metal atom via 3c2e (CdHB and CuHB) bonds. The structures of complexes [Zn(L3)3][B10H10]∙4CH3CN, [Cd(L3)2[B10H10]]∙2CH3CN, [Cu2(L2)2[μ–B10H10]] and [Cu2(L3)2[μ-B10H10]] have been determined by single-crystal X-ray diffraction. An analysis of the electronic absorption spectra of the obtained compounds indicates that the luminophore properties of ligands L are retained in the complexes under consideration.

Coordination Chemistry of the Parent Dithiocarbamate H2NCS2-: Organometallic Chemistry and Tris-Chelates of Group 9 Metals.

Two tris-chelate complexes of cobalt and rhodium and two complexes of Ru(II) of dithiocarbamate, [S2CNH2]-, were synthesized. The complexes were spectroscopically characterized by IR, NMR, UV-vis, and MS and structurally characterized by X-ray diffraction. The structural features of the rhodium complex were compared to those of other tris-chelate Rh(III) dithiocarbamate complexes and are characterized by a change in the ground-state geometry in comparison to expected octahedral tris-chelate complexes. This was confirmed both experimentally by X-ray diffraction and theoretically using DFT calculations. The inversion barriers of Rh(Bz2dtc)3, Ir(Bz2dtc)3, and Rh(Et2dtc)3 were determined using VT-NMR in DMSO. These barriers were found to be surprisingly low for heavy group 9 elements of d6 tris-chelate complexes: values of 16.7, 17.1, and 16.4 kcal/mol were calculated, respectively. By comparing structural features, we are able to determine that the activation barrier for the inversion of stereochemistry of Rh(H2dtc)3 must have a similarly low value. A modified version of the Bailar twist involving an intermediate with C3h geometry was proposed as the mechanism of inversion.

Electrochemical mechanism of CO2 reduction mediated by NiII(tpa) (tpa = tris(2-pyridylmethyl)amine) complexes: An integral view

In this paper, we report the synthesis, characterization, as well as electrochemical, spectroelectrochemical, and density functional theory (DFT) calculations of a series of novel NiII complexes with a flexible tetradentate ligand, tris(2-pyridylmethyl)amine) (tpa). The complexes [NiII(tpa)(H2O)(MeCN)](BF4)2, [NiII(tpa)(NO3)2], and [NiII(tpa)Cl2] were studied for their ability to electrochemically reduce CO2. These complexes were compared with the tris-chelate complex [Ni(bpy)3](BF4)2 to understand the role of flexible tetradentate ligands containing a pyridinic ring in the electrochemical reduction of CO2. Spectroelectrochemical experiments and theoretical studies were performed to rationalize the electrochemical reductions occurring and to predict the redox potential and pathway of CO2 reduction.

Cobalt complexes with biguanide derivatives – Synthesis, structure and antiviral activity

The reactions of biguanide derivatives NRR′(NH)CNHC(NH)NH2 ((RR′bigH: R, R′ = Bui, H; Pr, H; -CH2CH2OCH2CH2-) with cobalt salts have been studied. For the Co3+ ion, tris-chelate complexes with neutral bidentate coordinated derivatives of biguanides ([Co(RR′bigH)3]Cl3 (R, R′= Bui, H; Pr, H)) were obtained, as well as a complex with bidentate coordinated deprotonated morpholine derivative of biguanide [Co((RR′bigH)3] (R, R′= -CH2CH2OCH2CH2-). In the case of Co2+ ion, the [Co(MorphbigH2)Cl3] complex was obtained, in which the ligand is protonated and monodentate coordinated. The structure of the two complexes, [Co(iBuHbigH)3]Cl3 and [Co(MorphbigH2)Cl3], was determined using single crystal X-ray diffraction analysis. The newly obtained compounds, in contrast to the complexes obtained by us earlier, did not show activity against the influenza virus but turned out to be active against the second type of herpes virus. A preliminary analysis of the influence of the metal complex structures and functional groups of its ligands on the antiviral activity indicates that the number of free functional groups in ligands plays a more important role than of the metal complex structures.

Separation of Isomers and Mechanisms of Inversion of Stereochemistry of Group 9 d6 Tris-Chelate Complexes of Hinokitiol.

Tris-chelate complexes of Co(III), Rh(III), and Ir(III) with 4-isopropyltropolone (hinokitiol or β-thujaplicin) form by the substitution of carbonate and chloride ligands from group 9 trivalent metal salts. The new complexes are neutral, are readily soluble in most organic solvents, and are brightly colored with strong charge transfer bands. The fac isomers of Co(hino)3 and Rh(hino)3 were isolated from the mixture by fractional recrystallization from ethanol. The remaining mixtures were respectively enriched by 5:3 and 4.4:3 for the mer isomer. The 1H NMR data show that the complexes exhibit remarkable stereochemical lability, which is unusual for diamagnetic d6 group 9 metals, with rotational barriers of 14.2 and 18.2 kcal/mol found for the inversion of stereochemistry of Co(hino)3 and Rh(hino)3. The low activation barriers, as well as the analysis of some key structural parameters, suggest that the inversion of stereochemistry occurs via a trigonal-twist (Bailar) mechanism. Facile substitution of a single hinokitiol ligand in the cobalt complex with ethylenediamine to form [Co(en)(hino)2]Cl also indicates that the tris-chelates are substitutionally and configurationally labile.

Features of the formation of zinc(II) and cadmium(II) complexes with the inner-sphere and outer-sphere position of the decahydro-closo-decaborate anion in the presence of azaheterocyclic ligands

In the present work, we studied reactions between Cd(II) or Zn(II) salts and the closo-decaborate anion [B10H10]2− in the presence of azaheterocyclic ligands L (L = 2,2′-bipyridyl (bipy), 2,2′-bipyridylamine (bpa), and 1,10-phenanthroline (phen)). It was found that, depending on the nature of organic ligands L, solvents, and synthetic conditions, complexes with the inner-sphere and outer-sphere positions of the boron cluster were formed with different composition and structures. The structure of tris-chelate complex [Zn(bipy)3][B10H10] and mixed-ligand complexes [Zn(bpa)2(NO3)]2[B10H10], [Cd(bipy)2[B10H10]], and [Zn(bpa)2[B10H10]] were studied by X-ray single-crystal diffraction. Compounds [Cd(bipy)2[B10H10]] and [Zn(bpa)2[B10H10]] contain the coordinated boron cluster anion, the latter complex being the first example of a zinc(II) complex with the coordinated boron cluster anion.

Compounds of the types Pn(pyS)3 (Pn = P, As, Bi; pyS: pyridine-2-thiolate) and Sb(pyS) x Ph3–x (x = 3–1); molecular structures and electronic situations of the Pn atoms

Abstract The compounds Pn(pyS)3 (Pn = P, As, Sb, Bi) were synthesized from the respective chloride (Pn = P, As, Sb) or nitrate (Bi), pyridine-2-thiol (pySH) and triethylamine (NEt3) as a supporting base in THF (P, Sb), CHCl3 (As) or methanol (Bi). Sb(pyS)3 was also obtained from the reaction of SbCl3 with LipyS (prepared in situ) in methanol. The compounds Sb(pyS)2Ph and Sb(pyS)Ph2 were prepared in a one-pot reaction starting from SbCl3 and SbPh3 (1:1 ratio). Upon Cl/pyS substitution, the resulting reaction mixture allows for a facile separation of the products in hot hexane. P(pyS)3 and As(pyS)3 crystallize isostructurally to the reported structure of Sb(pyS)3 with κ-S-bound pyS ligands. These crystal structures feature close Pn···Pn contacts which are most pronounced for the arsenic derivative. Bi(pyS)3 adopts a different molecular structure in the solid state, which features two chelating (κ 2-S,N-pyS) ligands and a κ-S-bound ligand. The presence of N→Bi interactions between the nitrogen atom of the κ-S-pyS ligand and the Bi atom of another molecule renders this structure a polymer chain along the crystallographic b axis with Bi⋅⋅⋅Bi van-der-Waals contacts. The structures of this set of Pn(pyS)3 compounds were also studied in solution using 1H NMR spectroscopy, revealing equivalent pyS ligands in discrete Pn(pyS)3 molecules. The molecular structure of Sb(pyS)Ph2 was optimized by quantum chemical methods, and a comparison with the structures reported for the other Sb/pyS/Ph combinations reveals Sb(pyS)2Ph to feature the strongest Sb···N interactions with the κ-S-pyS ligand. The results of 1H NMR spectroscopic investigations of the compounds Sb(pyS) x Ph3–x (x = 3–0) suggest the Ph protons in ortho position to be incorporated into intramolecular C–H···S contacts for x = 2 and 1. Natural localized molecular orbital (NLMO) calculations were employed in order to gain insights into the electronic situations of the Pn atoms and Pn–R bonds (R = S, C), especially for the effects caused by formal substitution of Pn in the compounds Pn(pyS)3 and the ligand patterns in the compounds Sb(pyS) x Ph3–x (x = 3–0). For the latter series of compounds, the electronic situation of the Sb atom was further studied by 121Sb Mössbauer spectroscopy, providing a correlation between the calculated electron density at Sb [ρ(0)] and the experimentally observed isomer shift δ. The missing link between group 15 and group 13 metal compounds of the type M(pyS)3, compound Al(pyS)3, was synthesized in this work. In the solid state (confirmed crystallographically), the mer isomer of this tris-chelate complex with distorted octahedral Al coordination sphere was found. This coordination mode was confirmed for the solution state (CDCl3) by 1H and 13C NMR spectroscopy at T = −40 °C.

“Turn-on” fluorescent sensor for Th4+ in aqueous media based on a combination of PET-AIE effect

Previously reported fluorescent sensors for Th4+ experienced emission quenching or generated false positive signal upon aggregate formation in aqueous media. Herein, a simple and novel thorium sensor (CDB-BA) based on cyanodistyrene structure was designed and synthesized, which integrated the highly emitting characteristic of AIE effect and off-on response of PET modulation for the first time to construct the “turn-on” fluorescent probe for Th4+. Besides excellent selectivity, CDB-BA exhibited remarkable fluorescent enhancement which was linearly related to the concentration of Th4+ in the range of 0.25–8 μM. The detection limit was attained 0.074 μM, which was lower than that of most previously reported sensors. The mechanism of tris-chelate complex of CDB-BA with Th4+ was confirmed by mass spectra, IR spectra and DFT calculation. The excellent Th4+ sensing ability of CDB-BA was successfully applied to detecting Th4+ on TLC plates, in real water samples and living-cell imaging. This work suggested that the combination of AIE and PET photophysical mechanism could offer the merits of minimized background and enhanced signal fidelity to develop novel “turn-on” fluorescent probe in complicated aqueous environment and biological research.

Synthesis and Characterization of 3-Hydroxy-2-(5'-Hydroxypentyl)-4H-Pyran-4-one and Study of Its Complexation with Iron(III)

With the special focus to obtain a new tridentate ligand able to coordinate Fe(III), 3-hydroxy-2-(5'-hydroxypentyl)-4H-pyran-4-one (5-H) has been synthesized. Iron/ligand solution chemistry has been investigated in aqueous solution by ESI-MS and cyclic voltammetric measurements. The physiologic predominant specie is the tris-chelate complex as Job test attested. The electrode potential of the [FeIII(5)3]/[FeII(5)3]– couple was evaluated to be –0.54 V vs. SCE, pH 7.4.

Tris-chelated complexes of nickel(II) with bipyridine derivatives: DNA binding and cleavage, BSA binding, molecular docking, and cytotoxicity

Two nickel(II) complexes with substituted bipyridine ligand of the type [Ni(NN)3](ClO4)2, where NN is 4,4′-dimethyl-2,2′-bipyridine (dimethylbpy) (1) and 4,4′-dimethoxy-2,2′-bipyridine (dimethoxybpy) (2), have been synthesized, characterized, and their interaction with DNA and bovine serum albumin (BSA) studied by different physical methods. X-ray crystal structure of 1 shows a six-coordinate complex in a distorted octahedral geometry. DNA-binding studies of 1 and 2 reveal that both complexes sit in DNA groove and then interact with neighboring nucleotides differently; 2 undergoes a partial intercalation. This is supported by molecular-docking studies, where hydrophobic interactions are apparent between 1 and DNA as compared to hydrogen bonding, hydrophobic, and π–π interactions between 2 and DNA minor groove. Moreover, the two complexes exhibit oxidative cleavage of supercoiled plasmid DNA in the presence of hydrogen peroxide as an activator in the order of 1 > 2. In terms of interaction with BSA, the results of spectroscopic methods and molecular docking show that 1 binds with BSA only via hydrophobic contacts while 2 interacts through hydrophobic and hydrogen bonding. It has been extensively demonstrated that the nature of the methyl- and methoxy-groups in ligands is a strong determinant of the bioactivity of nickel(II) complexes. This may justify the above differences in biomolecular interactions. In addition, the in vitro cytotoxicity of the complexes on human carcinoma cells lines (MCF-7, HT-29, and U-87) has been examined by MTT assay. According to our observations, 1 and 2 display cytotoxicity activity against selected cell lines.Communicated by Ramaswamy H. Sarma

Synthesis and Characterization of Tris-chelate Complexes for Understanding f-Orbital Bonding in Later Actinides.

An isostructural family of f-element compounds (Ce, Nd, Sm, Gd; Am, Bk, Cf) of the redox-active dioxophenoxazine ligand (DOPOq; DOPO = 2,4,6,8-tetra- tert-butyl-1-oxo-1 H-phenoxazin-9-olate) was prepared. This family, of the form M(DOPOq)3, represents the first nonaqueous isostructural series, including the later actinides berkelium and californium. The lanthanide derivatives were fully characterized using 1H NMR spectroscopy and SQUID magnetometry, while all species were structurally characterized by X-ray crystallography and electronic absorption spectroscopy. In order to probe the electronic structure of this new family, CASSCF calculations were performed and revealed these systems to be largely ionic in contrast to previous studies, where berkelium and californium typically have a small degree of covalent character. To validate the zeroth order regular approximation (ZORA) method, the same CASSCF analysis using experimental structures versus UDFT-ZORA optimized structures does not exhibit sizable changes in bonding patterns. This shows that UDFT-ZORA combined with CASSCF could be a useful first approximation to predict and investigate the structure and electronic properties of actinides and lanthanides that are difficult to synthesize or characterize.

Synthesis and Structure of [М(DMF)6][B10H10] (M = Zn(II), Cd(II)) as Precursors for Solid-Phase Synthesis of Trischelate Complexes [М(L)3][B10H10

The reactivity of the decahydro-closo-decaborate anion in the complexation of metals being among intermediates acids in the concept of the Pearson’s theory of hard and soft acids and bases is studied systematically in the presence of competitive ligands L. In continuing these studies, we have synthesized Zn(II) and Cd(II) complexes of the composition [М(DMF)6][B10H10] (M = Zn(II), Cd(II)) and studied them by physicochemical analyses (IR and UV spectroscopy, X-ray powder diffraction). The possibility of their use in solid-phase synthesis of the corresponding trischelate complexes [M(Bipy)3][B10H10] has been found. Complexes [М(DMF)6][B10H10] are formed by the reaction between triethylammonium closo-decaborate with metal nitrates in DMF. The compounds are isostructural with the corresponding Co(II) and Ni(II) complexes [М(DMF)6][B10H10]. Specific interactions have been found between the BH groups of the boron clusters and CH groups of the DMF molecules. According to the data of IR and UV spectroscopy, the complexes contain specific B–H···C–H interactions. The corresponding tris-chelate complexes of the metals [M(Bipy)3][B10H10] are also isostructural.

Highly Luminescent Cyclometalated Iridium Complexes Generated by Nucleophilic Addition to Coordinated Isocyanides.

In this work, we report a new class of blue-emitting cyclometalated iridium complexes supported by acyclic diaminocarbene (ADC) ancillary ligands. These neutral, tris-chelated complexes are not obtainable via traditional synthesis routes and instead are generated through metal-mediated nucleophilic addition to a metal-bound isocyanide, which is followed by orthometalation of the ADC under mild conditions. Importantly, four of the variants exhibit efficient phosphorescence when immobilized in PMMA matrix, achieving quantum yields of 79% for blue emitters with a 2-(2,4-difluorophenyl)pyridine (F2ppy) C^N ligand and 30-37% for orange emitters with a 2-phenylbenzothiazole (bt) C^N ligand. Electrochemical studies demonstrate significantly higher-lying HOMO levels in the ADC complexes relative to the NHC analogues, a phenomenon that results in enhanced charge-transfer character in the excited states of the ADC complexes. This study demonstrates that ADC ancillary ligands not only give rise to new structures for Ir(III)-based phosphorescent emitters but also are promising targets for use in light-emitting devices and other thin-film optical applications.

Direct Crystallization of Layered Silicates on the Surface of Amorphous Silica.

We overview the entire surface coverage of amorphous silica with layered silicates (smectite-like) using the direct crystallization technique under hydrothermal conditions. Various shapes (e. g., microspheres, fibers, microcapsules) have been available as the silica substrates, which involve partial dissolution triggered by the hydrolysis of urea for supplying a source of the layered silicates. Other sources (e. g., Li+ , Mg2+ , Al3+ ) have been supplied from aqueous solutions, participating the heterogeneous nucleation of the layered silicates. Because of partial dissolution of the silica substrates, the original shape has been preserved after the hydrothermal reactions. Cation exchange reactions, usually observed for conventional smectite systems, occurred by replacing the interlayer cations in the silicates with metal (a rare-earth ion) and organic (a quaternary long-chain alkylammonium, methylene blue, and a tris-chelated complex) cations. We have discussed the mechanism of the crystallization and displayed some applications of the hybrids (e. g., chiral discrimination, filtering, and magnetic recovery).

Crystal structure and EPR spectra of (Bu4N)2[V(dmit)3

The tris-chelate complex of vanadium(IV) (Bu4N)2[V(dmit)3] is prepared from VCl3 and (Bu4N)2[Zn(dmit)2] (dmit = isotrithionedithiolate C3S5 2–) and characterized by single crystal XRD and mass spectrometry. The complex crystallizes in the space group Pna21 and has a distorted octahedral environment of vanadium. The complex is paramagnetic and gives a characteristic EPR spectrum in both solution and solid phase. The g factors and hyperfine interaction constants are determined.

Disassembly of diruthenium(II,III) tetraacetate with P–N donor ligands

Valent-averaged diruthenium(II,III) tetraacetate, [Ru2(μ-O2CCH3)4(H2O)2](PF6), was reacted with various bidentate P–N donor ligands. These reactions led to the partial displacement of bridging carboxylate groups and concomitant cleavage of the Ru–Ru bond, producing novel complexes. Disassembly reactions with the aminophosphines; diphenyl-2-pyridylphosphine (dpppy) and 2-(diphenylphosphino)ethylamine (dppea), produced homoleptic tris-chelated species, fac-[Ru(dpppy-P,N)3](PF6)2 (III) and mer-[Ru(dppea-P,N)3](PF6)2 (VI), respectively. Disassembly reactions with aromatic aminophosphines; 2-(diphenylphosphino)methylpyridine (dppmpy) and 2-(diphenylphosphino)ethylpyridine (dppepy), led to heteroleptic tris-chelated complexes cis,cis,trans-[Ru(dppmpy-P,N)2(η2-O2CCH3)](PF6)·MeOH (IV) and cis,cis,trans-[Ru(dppepy-P,N)2(η2-O2CCH3)](PF6) (V), respectively, whereas reaction with the non-cyclic aminophosphine, 3-(diphenylphosphino)-1-propylamine (dpppa) and the non-cyclic iminophosphine, 2-(diphenylphosphino)benzylidinebenzylamine (dppbba), led to the heteroleptic tris-chelated complexes cis,cis,trans-[Ru(dpppa-P,N)2(η2-O2CCH3)](PF6·EtOH (VII) and cis,cis,trans-[Ru(dppbba-P,N)2(η2-O2CCH3)](PF6)·2MeOH·H2O (VIII), respectively. All complexes were fully characterized. Electrochemical studies (CV, OSWV) showed that the homoleptic tris-chelated complexes had higher oxidation potentials (Epa) than the corresponding heteroleptic tris-chelated complexes, due to the weak π-acceptor ability of the acetate group. Complex (III) has the highest Epa as it contains the strongest combination of π-accepting groups. The Epa values of the heteroleptic complexes were found to be in the order: dppbba (CHN)>dppmpy, dppepy (py)>dpppa (NH2), since amines are much poorer π-acceptors than pyridyl and imino groups. Complexes (III), (VI) and (VII) showed irreversible oxidation waves, while complexes (IV), (V) and (VIII) displayed quasi-reversible one electron processes.