INTRODUCTION
The most common approach for determining reaction mechanisms of chemical reactions of coordination complex species is the interpretation of results from kinetics investigations. The experimental aspects of kinetics measurements, for the determination of rate laws and their interpretation and the use of the Eyring equation to derive the enthalpy and entropy of activation have been described thoroughly in by Atkins [1], The principal method of monitoring reactions of coordination compounds is UV/visible spectrophotometery as many reactions of transition metal complexes are accompanied by changes in the electronic spectra.
In coordination compounds metal atom or ions are surrounded by donor groups (anions on neutral molecules) that are called ligands. The type of groups that may surround a metal atom or ion are greatly varied, but they may be broadly considered to be of two types i.e. ligands that bind to metal atoms or ions through carbon atoms and ligands that do not. The former are involved in organometallic compounds. The branch of inorganic chemistry concerned with the remaining combined behaviour of central metal ions and their ligands is called coordination chemistry.
The main justification for classifying many substances as coordination compounds is that their chemistry can conveniently be described in terms of a central cation Mn+, about which a great variety of ligands L, and so on may be placed in an essentially unlimited number of combinations. The overall charge on the resulting complex [MLxLY Lz…] is determined by the charge on M, and the sum of the charges on the ligands.
The ability of a co-ordination compound to engage in reactions that result in displacing one or more ligands in its coordination sphere (by other ligands in solution, for instance) is called its lability. Those coordination compounds for which such substitution reaction are rapid, called labile, whereas those for which such substitution reactions proceed slowly (or not at all) are called inert. It is to be noted that these terms should not be confused with thermodynamic stability and unstability [2].Forexample [Co(NH3)6]3+ ion, which will persist for months in an acidic medium because of its kinetic inertness (slow reactivity) despite of the fact that it is thermodynamically unstable. As shown by the large equilibrium constant (K~1025)for reaction [Co(NH3)6]3+ +6H3O+[Co(H2O)6]3+ + 6NH4+; in contrast, the overall formation constant (β=1022) for the reaction Ni2+ + 4CN–[Ni(CN)4]2– indicates that the thermodynamic stability of [Ni(CN)4]2– is high.
A practical definition of the terms labile and inert can be given. Inert complexes are those whose substitution reaction have half life longer than a minute. Such reactions are slow enough to be studied by the classical techniques where the reagents are mixed and changes in absorbance, pH, gas evolution and so on.
Labile complexes are those that have half-lives for a reaction under a minute. Special techniques are required for monitoring such reactions, as they may appear to be finished within the time of mixing.
In the first transition series, virtually all octahedral complexes say those of Cr(III) and Co(III) and sometimes Fe(II), are normally labile, i.e., ordinary complexes came to equilibrium with additional ligands (including water) so rapidly that the reactions appear instantaneous by ordinary techniques of kinetic measurement. Complexes of Co(III) and Cr(III) ordinarily undergoes substitution reaction with half lives of hours, days, or even weeks at 25oC.
I. Ligand Substitution Reactions
In reference to coordination chemistry, the coordinated ligand to a central transition metal ion may be exchanged by a free ligand or the exchange of coordination central metal ion itself by a free metal ion in solution. The detailed knowledge obtained from the comprehensive kinetics of substitution reactions can be of immense importance in deducing most appropriate conditions under which new metal complexes can be synthesized. Therefore, such studies can definitely improve the older methods of preparation of such complexes in general and refining the available analytical procedures depending on coordination chemistry. Several excellent reviews [3-5] and books[6,7]have been published on the kinetics and mechanism of inorganic reactions in order to visualize the nature of a variety of intermediates and transition states produced during the course of such reaction.
I.1 General Mechanism of Ligand Substitution Reactions
In these reactions, a coordinated ligand is substituted by an uncoordinated ligand. The simplest and most common of these is the swapping of coordinated water with solvent water (known as water exchange) but any ligand can in principle exchange with another. There are three classes of ligand substitution reaction mechanism:
In the dissociative mechanism, the M-X bond is broken before the entering group Y attaches. The coordination number of M decreased by one in the transition state. This mechanism is denoted as D corresponding to the notation SN1 used for substitution at a carbon centre developed by Hughes and Ingold[8].
In the associative mechanism, the entering group attaches to M before any weakening of the M-X bond occurs. The coordination number of M is increased by one in the transition state. The mechanism is denoted as A.
The interchange mechanism lies in between these extremes as it involves the synchronous weakening of the M-X bond and attachment of Y. It is denoted as I and corresponds to the notation SN2 used in organic chemistry[7]. If bond weakening makes a larger contribution to the energy of the transition state then the interchange mechanism is labelled Id. When bond attachment makes a larger contribution, the mechanism is labelled, Ia. When the contributions are equal, the label I is used. This reformulation was made by Langford and Gray[9].
I.2Mechanism of substitution in Octahedral Complexes
I.2.1 Dissociative Mechanisms
In dissociative mechanisms, the rate determining step involves bond breaking and the energy required for this determines the activation energy.
(a) D Mechanism
In the D mechanism, there are only two elementary steps. In step (I), the complex LnMX gains enough energy to break completely the M-X bond.
The 5-coordinate intermediate L5M may exist long enough to be detectable. In step (2), the intermediate L5M reacts with Y. It must be noted that Y is often solvent if this can coordinate as it is in large excess.
(b) Id Mechanism
In the Id mechanism, as the M−X bond starts to break, M begins to from a new bond with Y. The entering group Y must be present in the region around the complex L5MX when the M−X bond begins to break. Thus before the substitution occurs, Y must enter the outer sphere of L5MX,
Ligands X and Y then swap over in the rate determining step,
before X completely leaves the outer sphere of L5MY,
It should be noted that, unlike in the D mechanism, no 5-coordiante intermediate is predicted.
I.2.2Associative Mechanism
(a) A Mechanism
In the A mechanism, step (I) involves the formation of a 7-coordiante intermediate, which may exist long enough to be detectable. This is the rate determining step and the activation energy is determined by the bond making to the entering group and the ensuing steric crowding around M. In step (2), the products are then formed by breaking the M-X bond in the intermediate:
L5MXY L5MY + X
(b) IaMechanism
The Ia mechanism is very similar to that shown above for the Id mechanism. The difference between the two lies in the nature of the 7-coordiante transition state in the rate determining step:
- In the Id case, the M-X bond is very sensitive to the approach of Y and begins to weaken when Y is relatively far away. As bond breaking is more important than bond making, the activation energy is determined to a large degree by the strength of the M-X bond. In the limit, the
M-X, bond breaks when Y is absent and a genuinely 5-coordiante
intermediate is formed and the mechanism becomes D.
- In the Ia case, the M-X is unaffected until Y gets closer to M. As bond making is more important than bond breaking, this determines the activation energy. In the limit, the M-Y bond is formed before any weakening of the M-X bond. A genuinely coordinate intermediate is formed and the mechanism becomes A.
Where L represents a non labile ligand, X is the leaving ligand and Y the incoming ligand.
I.3Water Exchange in Aqua Ions
Since many reactions in which complexes are formed occur in aqueous solution. One of the most fundamental reactions in which the water ligands in the aqua ion [M(H2O)n]m+ are displaced from the first coordination shell by other ligands included. Here is the simple case in which the new ligand is another water molecule that is the water exchange reaction. It is convenient to divide the ions into four classes[10], depending on these rate constants for water exchange[2].
Class I. The exchange of water is extremely fast, First-order exchange rate constants are on the order of 108 s-1, which approaches the maximum possible rate constant (calculated to be 109 to 1011s-1 for a diffusion controlled reaction). The complexes are bound by essentially electrostatic forces and include the complexes of the alkali metals and larger alkaline earth metals. The metal ions are characterized by low charge and large size; Z2/r ratios range up to about 10×10-28 C2 m-1.
Class II. The exchange of water is fast. First-order rate constants range form 105 to 108 s-1, Metal ions belonging to this group are the dipositive transition metals, Mg2+, and tripositive lanthanides. These ions form complexes in which the bonding is somewhat stronger than in those of Class I ions, but LFSEs are relatively small. The Z2/r values for ions in this category range from about 10 to 30 ×10-28 C2 m-1.
Class III. The exchange of water is relatively slow compared with Classes I and II, although fast on an absolute scale, with first-order rate constants of 1 to 104 s-1, The metal ions of this group are most of the tripositive transition metal ions stabilized to some extent by LFSE, and two very small ions, Be2+ and Al3+. The Z2/rratio are greater than about 30×10-28 C2 m-1.
ClassIV. The exchange of water is slow, these are the only inert complexes. First-order rate constants range from 10-1 to 10-9 s-1.These ion are comparable in size to Class III ions and exhibit considerable LFSE: Cr3+ (d3), Ru3+ (low spin d5), Pt2+ (low spin d8). Best estimates for Co3+ , which oxidized water and is therefore unstable in aqueous solution, also place it in this class.
I.4 Formation Reaction
The formation of metal complexes takes place in media where usually water acts as a solvent. The rate of these reactions vary from very slow to very fast. The generally accepted mechanism for complex formation was originally proposed by Eigen and Tamm [11-13]. For complexes of unidentate ligands it involves the formation of an outer sphere complex between solvated metal ion and the incoming ligand followed by loss of a solvent molecule from this outer sphere complex to give the desired species. The mechanism of formation of complexes of multidentate ligands with the minor modification that the ring closure may constitute the rate determining step also comes in this category.
I.5Dissociative Reactions
The dissociation of metal complexes can be considered as the reverse of the complex formation. These reactions are, generally much slower than the formation reactions. The mechanism proposed for the complex formation also accounts for the dissociation rates of complexes bearing unidentate, bidentate or ambidentate ligands. In case of complexes of bidentate or ambidentate ligands, the rate constant depends upon opening of the chelate ring or sometimes rupture of penultimate metal-ligand bonds. Both these situations have been encountered frequently with outgoing bidentate or multidentate ligand groups. The presence of an acid generally enhances the dissociation rate because of protonation of the released ligand stablises the intermediate relative to the fully coordinated form [16-20].
I.6Types of Ligand Substition Reaction
Ligands substitution reactions of coordination compounds have been studied as intensively as many class of inorganic reactions. The kinetics of these processes have been investigated extensively for octahedral and to a lesser extent for square planer complexes. A very wide span of rate is found ranging from the extremely slow exchange of CN with [NiCYDT]2- (no evidence for the formation of [Ni(CN)4]2-, t1/2= 1440 hours) [21] to the almost diffusion controlled exchange of H2O between [Cu(H2O)6]2+ and water (t1/2= 10-8 sec) [22]. The following four types of substitution reaction have been reported in coordination chemistry.
I.6.1Unidentate Ligand Substitution Reactions (excess ligand).
This section is concerned with the kinetic and mechanisms of substitution in complexes where unidentate ligands substitution with unidentate or multidentate ligands taken place.
I.6.1.1 Unidentate by Unidentate Ligands
The substitution of one unidentate ligand by another is the simplest case to consider and has been extensively used for investigating the mechanisms of substitution in many octahedral complexes as well as some square-planer complexes. The general reactions involve the substitution of a monodentate ligand present in the inner-coordination sphere in the solvent media.
There are two basic mechanism for unidentate ligand substitution in aqueous solution.For example the nickel-hexamine system, the first mechanism would be a dissociative type.
[Ni (NH3)6]2+[Ni (NH3)5]2+ + NH3 (1)
[Ni (NH3)6]2+[Ni (NH3)6]2+(2)
The second would be a bimolecular mechanism involving water molecules:
[Ni (NH3)6]2+ + H2O [Ni (NH3)5]H2O]2+ + NH3 (3)
[Ni (NH3)5]2++ NH3[Ni (NH3)6]2++ H2O(4)
It is difficult to distinguish between such mechanism in aqueous solutions. However, the lack of NH3 attack on [Ni (NH3)6]2+ and the fact that a change of 30% in H2O concentration produced no observable effect support a dissociative mechanism [23].
The reactivity of low-spin Ru(II) complexes with respect to substitution by Unidentate ligand has received little attention in comparison with the large number of data available for other octahedral, low-spin complexes such as those of Co(III) [24] Rh(III) [24] and Ru(II) [25,26]. However, the kinetics and mechanism of ligand substitution in pentacyano (ligand)ruthenate(II) [Ru(CN)5L](3-n)- complexes have been the subject of considerable interest, in recent past, for several reasons[27-34].These low spin Ru(II) species represent models for active sites in biological system and the reactions with imidazole (ImidH) have been investigated in this regard[30]. Another feature of these complexes is their use in redox reactions with metalloproteins[25].
The kinetics of substitution reaction of a series of complexes of the type [Fe(CN)5L](3-n)-have been studies by Toma et al. [35,36] where L was an aromatic nitrogen heterocycle and the substituting ligand was nitroso-R-salt. The rate of substitution varied with the nature of L and a saturation kinetics typical of rate-determining loss of L from the complex followed by rapid addition of the incoming ligand was reported. This is the first study[6] about monodentate ligand substitution reactions of simple low-spin Fe(II) complexes which generally proceed by D or SN1 (lim) mechanism.
In contrast to the work on the pentacyanoferrate(II) complexes described above, very little work has been reported on the analogous Ru(II) system until recently. A detailed kinetic study of ligand substitution reactions of substituted [Ru(CN5)L](3-n)(where Ln+ is dimethyl sulphoxide or nitrogen heterocycle and entering ligand Ym+ is primarily dimethyl sulphoxide or N-methylpyrazinium cation) has been made previously [30].The substitution kinetics in [Ru(CN)5en]3- complexes by pyrazine have also been studied in the recent past by Olabe etal.[31]. The reactions of [Ru(CN)5L)(3-n)-in presence of an excess of Ym+resulted in a first order formation of [Ru(CN)5Y](3-m)–or loss or [Ru(CN)5L)(3-n)-. The deviation of the first order behaviour after 4 or 5 half lives (-24 hrs) were observed in some reactions and are attributed to possible side processes such as cyanide substitution and dimer formation [37]. The dissociative [D or SN1 (lim)] mechanism of ligand substitution is proposed for exchange of the L and Y ligands in [Ru(CN)5L)(3-n)- complexes according to eq. (5).
Where n and m are the charges on the ligands L and Y respectively. The limiting reaction rates, at sufficiently large concentrations of entering ligand Ym have been observed with all leaving ligands as described above. The exchange of L and Y has been found to obey a second order rate law as.
[Ru(CN)5L)(3-n)- = kY [Ru(CN)5H2O3-][Ln] (6)
The rate constants and activation parameters of the dissociation reactions of [Ru(CN)5L)(3-n)- complexes have been compiled in Table 1.1.
Table 1.1 Rate and activation parameters for ligand sunstitution of Ln+ by Ym+ in [Ru(CN)5L](3–n)– complexes (pH = 7.00, I = 0.10MNaCl)
Ln+ / Ym+ / 105k–Ls–1 / H (kJ mol–1) / S (JK–1 mol–1) / Ref.N-Mepyz+ / Me2SO
Py
Im
pyz / 6.31
5.97
6.74
5.91 / 102.402.1 / 20.98.3 / 27
27
27
27
27
27
27
27
pyrpyr+ / Me2SO
Py
Im / 4.17
3.79
4.32 / 100.31.7 / 8.34.2
bpy / Me2SO / 6.79 / 92.80.8 / -12.54.2
Pyz / Me2SO
N–
Mepyz+ / 1.77
1.57 / 93.67.1 / -20.920.9
Py / Me2SO
N-mepyz+ / 3.34
3.39 / 107.01.3 / 29.216.7
Im / N-mepyz+ py / 10.70
11.70 / 92.82.9 / –8.38.3
Isonic– / Me2SO
N-mepyz+ / 1.67
1.33 / 104.91.2 / 16.74.2
Me2SO / M-mepyz+ Py
Pyz / 0.85
0.81
0.77 / 1021.7 / –4.24.2
en / Pyz / 11.40 / 832 / 335 / 34
34
34
34
34
34
34
bpe / tri / 51.20 / 781 / 181
PCI / tri / 10.10 / 922 / 161
PyCN / tri / 18.10 / 902 / 151
isox / tri / 20.60 / 861 / 192
mtr / tri / 52.20 / 801 / 212
tt / tri / 68.40 / 881 / 222
The rate constants and activations parameters reported for formation and dissociation reactions of the analogous [Fe(CN)5L](3–n)– complexes are also compiled in Table 1.2.
Table 1.2 : Kinetic and activation parameters at 298 K for the formation and dissociation of various pentacyano(ligand)ferrate(II) complexes.
Ligand / kfM–1s–1 / kd104,
s–1 / H
(kJ mol–1) / S
(JK–1 mol–1) / Ref.
Pyridine / 365 / 11.0 / 103.7(67.3)* / 46.0(29.3)* / 36,38
4-Methylpyridine / - / 11.5 / 100.3) / 37.6 / 36
Isonicotinamide / 296 / 7.3 / 108.7(66.0) / 58.5 (25.1) / 36,38
4-Picoline / 354 / - / –(63.1) / –(16.7) / 38
Pyrazine / 380 / 4.2 / 110.3 (64.4) / 58.5 (20.9) / 36
N-Methylpyrazinium / 550 / 2.8 / 114.9 (70.2) / 74.6 (41.8) / 36
Dimethylsulphoxide / 240 / 0.75 / 110.8 (64.4) / 46.0 (16.7) / 36
4, 4- Bipyridine / - / 6.2 / 110.8) / 66.9 / 36
Thiourea / 286 / 390.0 / 69.4 (65.6) / -37.6 (20.9) / 39
Allylthiourea / 196 / 451.0 / 68.1 (69.8) / -41.8 (33.4) / 39
Dimethylthiourea / 238 / 813.0 / 75.2 (64.8) / -12.5 (16.7) / 39
Glycinate / 28.0 / 26.7 / 97.0 (61.5) / 29.2 (-12.5) / 40
Imidazole / 240 / 13.3 / 101.6 (63.5) / 41.8 (12.5) / 40
N–Histidine / 320 / 5.3 / 105.3 (64.4) / 46.0 (20.9) / 40
N3–Histidine / 320 / 1090 / 91.1 (64.4) / 41.8 (20.9) / 40
Cyanide / 38 / - / - / - / 41
Thiocyanate / 64 / - / - / - / 41
3-Cyanopyridine / 370 / - / - / - / 41
Nitrile / 42 / - / - / - / 41
1 / 2 / 3 / 4 / 5 / 6
DMNA / - / 12.0 / - / - / 41
Aniline / - / Ca 0.20 / - / - / 42
Cyclohexylamine / - / 13.56 / 96.5 / 46.0 / 40
Ethanolamine / - / 7.72 / 97.8 / 38.0 / 42
Morpholine / - / 7.16 / 103.2 / 53.9 / 42
NH3 / 190 / 12.0 / 102.4 / 63.1 / 42
MeNH3 / 130 / 4.46 / 103.2 / 53.9 / 42
Me2NH / 80 / 7.79 / 100.3 / 49.7 / 42
Me3N / 60 / 12.2 / 91.4 / 28.8 / 42
BunNH2 / 250 / 7.47 / 105.7 / 66.9 / 42
EtNH2 / 180 / 7.54 / 104.1 / 38.0 / 42
PrnNH2 / 200 / 7.38 / 107.0 / 71.0 / 42
Pipyridine / - / 8.43 / 94.9 / 29.0 / 43
en / 330 / 51.5 / 97.0 / 37.7 / 43
enH+ / 620 / 104.0 / 99.9 / 50.1 / 43
Sulphite / - / 0.57 / Ca119.5 / Ca75.2 / 43
Nitrosobenzene / - / 0.016 / Ca117.0 / Ca41.8 / 43
N-Methylimidazole / 418 / 32.4 / 81.5 / - / 44
Isonicotinohydrazide / 325 / 7.3 / 107.8 / 59.8 / 45
pd / - / 54.0 / 100.5 / 58.7 / 46
pdH+ / - / 83.0 / 100.5 / 53.7 / 46
bd / - / 46.0 / 102.4 / 58.7 / 46
bdH+ / - / 69.0 / 104.5 / 62.7 / 46
ptd / - / 45.0 / 103.7 / 58.7 / 46
ptdH+ / - / 64.0 / 101.6 / 53.7 / 46
hxd / - / 41.0 / 100.5 / 45.8 / 46
hxdH+ / - / 53.0 / 96.5 / 37.8 / 46
1, 4–Tx / - / 5.71 / 112.0 / 71.0 / 47
1, 4–DT / - / 5.58 / 105.0 / 44.0 / 47
1, 3–DT / - / 3.39 / 108.0 / 50.0 / 47
* Numbers in parenthesis give the values of H and S for the formation reactions.
Shephered et al.[48,49]and Toma et al. [36,38]have recently employed these complexes and binuclear derivatives in spectroscopic studies for comparing the behaviour of low spin d6 moieties [35-36]viz. [Fe(CN)5L)](3-n) ,[Ru(CN)5L]3-and [Ru(NH3)5]2+. Malin and co-workers [35-36]have observed that the intermediate [Fe(CN)5]3 is quite insensitive to the nature and charge on the attacking reagent. Coelho and co-workers have also made the similar observations [47] on the kinetic studies of [Fe(CN)5L)(3-n) (where L= sulphur heterocylic ligands). There has been several reports in literature on the complex formation reaction involving [Fe(CN)5H2O]3- complex ion beside the discussion on the tendency of this ion to dimerize at higher concentration and the nature of the dimeric species [50].
However, the replacement of coordinated water ligand in [Fe(CN)5H2O]3- ion by other ligands, such as aromatic nitrogen heterocycles which gives rise to an equally extensive series of substituted pentacyanoruthenate(II) complexes [27]. Hoddenbagh and Macartney have recently reported that results of kinetic study of the substitution reactions of [Ru(CN)5H2O]3- ion [51], which implicates an ion-pair dissociative mechanism with a water exchange rate of 10±5 s-1 one order of magnitude lower than found for the [Fe(CN)5H2O]3-ion.