Terdentate ligands

By far the most important metal containing dyes are derived from OjO-dUiydroxyazo stmctures in which one of the two azo nitrogen atoms and the two hydroxyl oxygen atoms are involved in bonding with the metal ion. Thus these dyes serve as terdentate ligands. In the case of metal ions with a coordination number of four, eg, Cu(H), the fourth position is usuaUy occupied by a solvent molecule (47). 

The dissociation rate constant measures directly the value of k 2 in (4.52). The strain resident in multi-ring complexes is clearly demonstrated by some hydrolysis rate studies of nickel(II) complexes. The AFT values for the first bond rupture for Ni(II)-polyamine complexes fall neatly into groups. They are highest for en, containing the most strain-free ring (84 kJ mol ), about 75 kJ mol for complexes with terdentate ligands and only —63 kJ mol for complexes of quadridentate and quinquedentate amines and with NH3 itself. See also Ref. 109. 

In this group, Lj and L2 represent symmetrical terdentate ligands, which may be identical. The main product from the reaction of [Co(NH3)5Cl]Cl2 with dien is the s-fac Co(dien) + isomer 5 a. On heating solutions of this isomer for long times, small quantities of the other isomeric forms, u-fac and mer, result, 5b, and c (R, =R2=H). Separation of the three forms is carried out by using Sephadex cation exchange columns. Characterization of the isomers is possible by nmr which together with absorption spectroscopy, is used to study the interconversions. 

S and Se-Donor Ligands. The potentially terdentate ligand thiodiethanol, S(CH2CH20H)2, forms navy-blue [Cr S(CH2CH20H)2 Cl2] on refluxing... 

The Schiff base a-A -methyl-S-methyl-P-A -(2-pyridyl)methylenedithiocarba-zate (L) (66) behaves as a neutral terdentate ligand and [Cr(L)Cl3] has been isolated from reaction of the ligand and CrCl3,6H20 in ethanol. Pyruvic acid thio- and seleno-semicarbazones (H2L) complex with chromium(iii), to form species such as [Cr(L2)] and [Cr(L)(HL)]. ... 

The structure of bis-(l-methyl-3-o-chlorophenyltriazene-l-oxide)cobalt(ii) (84) is unusual in that it contains two terdentate ligands giving rise to the rare [C0N2O2CI2] core with no co-ordinated halide ions however, the Cl atoms are only weakly co-ordinated (Co—Cl = 2.98 This and related complexes... 

The potentially terdentate ligand, thiodiethanol (88), forms 1 1 complexes and 1 2 complexes with C0CI2. The former are five-co-ordinate and the latter octahedral. 1 1 Complexes of Co and diethylenetrithioacetic acid, ethylene-dithiodiacetic acid, and ethylidenetetrathiotetra-acetic acid have all been reported." " Potassium monothiobenzoate reacts with an equimolar quantity of CoSO in aqueous solution to yield green crystals of the pseudo-tetrahedral dinuclear complex [Co2(SOCPh)4 ], which contains both bidentate and bridging thiobenzoate ligands. ... 

A new route to the complexes (177) has been reported by the reaction of isonitroso-acacH and NHOAc) in the presence of NH3 and an alkylamine in 50% aqueous ethanol. Both [Ni(acetaldioxime) Cl2] and [Ni(acetamide)4 -(11 0)2] have been shown to be six-co-ordinate in the solid state. The first report of neutral monohalogeno-terdentate ligand nickel(ii) complexes has... 

Complexes of nickel(ii) with ethylenedithiodiacetate, diethylenetri-thioacetate, and ethylidenetetrathiotetra-acetate have been reported. The structure of ethylenedithiodiacetatonickel(ii) dihydrate has been reported. The potentially terdentate ligand, thiodiethanol, forms both 1 1 and 1 2 complexes with nickel. 

Infinite chain of cations with bridging CIO groups. Tetragonal co-ordination Trigonal-bipyramidal with bipy-N atoms in equatorial plane Terdentate ligands give distorted octahedral co-ordination Square-planar... 

The Ns-terdentate ligand bis(2-pyridylcarbonyl)amine, Hbpca = (54), is also a / -diketone, and ean therefore act as a bridging ligand. In the complexes [Fe(bpca)2][M (hfac)2]2, where = Fe, Mn, or Ni, the central iron, in its high ligand field Ng environment, is low-spin, while the iron eoordinated to hfac in the first complex is, as expected for its tris-/3-diketonate environment, high-spin." ... 

AF values for cyanide attack at [Fe(phen)3] +, [Fe(bipy)3] + and [Fe(4,4 -Me2bipy)3] " in water suggest a similar mechanism to base hydrolysis, with solvation effects dominant in both cases. Cyanide attack at [Fe(ttpz)2] , where ttpz is the terdentate ligand 2,3,5,6-tetrakis(2-pyridyl)pyr-azine, follows a simple second-order rate law activation parameters are comparable with those for other iron(II)-diimine plus cyanide reactions. Interferences by cyanide or edta in spectro-photometric determination of iron(II) by tptz may be due to formation of stable ternary complexes such as [Fe(2,4,6-tptz)(CN)3] (2,4,6-tptz= (66)). ... 

The iron(II) compounds [FeL2](C104)2 MeCN, [FeL2](BPh4)2 2MeCN and [FeL2](PF6)2, where L is the terdentate ligand 2,6-bis(3,5-dimethylpyrazol-l-ylmethyl)pyridine, all show temperature-... 

There appears to be a relatively fine balance between mer and fac geometries for octahedral trichloroiron(III) complexes. Thus it was established some time ago that [Fe(phen)(solvent)Cl3] adopted the fac geometry for solvent = H20 or MeOH/ but mer for solvent = DMF. The geometric and steric demands of terdentate ligands such as 1,3,5-trimethyltriazacyclohexane, ... 

A similar mechanism may occur for systems involving Cu(II) coordinated to ter-dentate or bidentate ligands in which the oxidized complex exists primarily as a 1 2 complex while, upon reduction, the 1 1 complex predominates. Kandegedara et al. [48] reported that the system involving copper and the terdentate ligand [9]aneS3 (LBE) appears to involve such a mechanism as illustrated in Scheme 4 ... 

The Scheme 4 mechanism is likely to apply to other Cu(II/I) systems involving terdentate ligands that are not sterically hindered. For example, it is suspected that the apparent irreversibility of the CV s observed by Neves and coworkers [123] for the Cu(II/I) system involving (2-hydroxybenzyl-2-pyridylmethyl) amine... 

A. Kandegedara, Ph.D. Dissertation, Electron-Transfer Kinetics of Copper Complexes with Macrocyclic Terdentate Ligands, Wayne State University, 2001. 

The importance of the carboxylate donors is underlined by a study of the lanthanide coordination chemistry of the similar terdentate ligand 2,6 -bis( 1 -pyrazol-3 -yl)pyridine, L24 (63). The complex structure of [Tb(L24)3][PF6]3, shown in Fig. 11, appears to be fairly robust in methanolic solution, with Horrocks analysis (q = 0.6) suggesting the 9-coordinate structure is retained the small quenching effect of outer sphere coordination explains the q-value. However, in aqueous solution, the lability of the ligands dramatically changes the luminescence. Whilst the emission decays are not exactly single exponential, approximate lifetimes in H20 and DoO suggest a solvation value of 4-5. 

Terdentate ligands. By analogy with ethylenediamine and glycine, we expect diethylenetriamine (dien) and iminodiacetate ion (IDA2-) to be... 

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