Details of Reaction Mechanism
The reactions of complexes are governed by the same kinetics as ordinary reactions. Transition State Theory says that the rate of a reaction is proportional to the equilibrium constant for formation of the activated complex, and leads to the result that fast reactions are favoured by positive activation entropies and small activation enthalpies (ΔG* = Eact).
|Reaction pathway for the complex reaction|
|The activated complex here is an associated complex, with both entering and leaving groups attached. This is common in the reactions in square planar complexes.|
When discussing reaction mechanisms, the following definitions are useful:
Lability and inert are kinetic terms.
Stable and unstable are thermodynamic statements.
The intimate mechanism refers to the details of the mechanism on the molecular scale.
Ligand Substitution of Square Planar Metal Complexes
Most square planar complexes are d8 species, such as RhI, IrI, NiII, PdII, PtII, and AuIII. The reaction is as shown, and is governed by the rate law shown.
|X is the leaving group; Y is the entering group; S is a solvent molecule.|
|ky is the rate constant for reaction with the entering group; ks with the solvent.|
The rate of substitution is affected by several factors: the role of the entering group, the role of the leaving group, the nature of the other ligands in the complex, and the effect of the metal center. The ligands not directly invloved in the reaction are known as spectator ligands; this is a slight misnomer, as they can have an influence on the process of the reaction, and need not merely spectate, as is discussed below.
|The square planar complex has the trans-ligand, T, the cis-ligands C, and the leaving group X.|
The Role of the Entering Group
The rate of reaction is proportional to the nucleophilicity of the entering group. This can be defined as the log of the ratio of ky and ks in the standard complex trans-[Pt(py)2Cl2]. The nucleophilicity, nPt, is defined as nPt = log(ky/ks).
There is no correlation between this scale and other nucleophilic properties such as basicity or redox potential, eg. strongly basic ligands are not necessarily reactive.
The relative rate dependent on entering group lies in the order,
Good nucleophiles are good entering groups.
The Role of the Leaving Group
This can be investigated by considering the reaction below.
The order of lability of the leaving group is
This order of reactivity reflects intrinsic reactivity of the complex, and parallels the strength of the metal-leaving group, M-Y, bond.
The Nature of the other Ligands: the Spectator Ligands
The other ligands effect the rate of substitution. However, the ligands trans– to and cis– to the leaving group have different effects.
The Trans Effect: this is best defined as the effect of a coordinated ligand upon the rate of substitution of ligands opposite.
In the substitution reactions of Pt(II) square planar complexes, the labilizing effect is in the order;
H2O = OH– = NH3 = amines = Cl– < SCN– = I– < CH3 < phosphines = H– < alkanes < CO < CN–
The trans effect is a kinetic phenomenon, but the labilization may arise from a stabilization of the transition state, or a destabilization of the reactant ground state, and hence a reduction in the activation energy and an increase in the rate of reaction.
In contrast, the trans influence is a purely thermodynamic phenomenon. Ligands may influence the ground state properties of groups to which they are located trans. These properties include the metal-ligand bond lengths, the vibration frequency or force constants. The trans influence series is;
R– = H– > CO = alkene = Cl– = NH3
The cis ligands may have an effect when the entering group is a relatively poor nucleophile. The order of influence for the cis effect is the same as that for the trans influence, but the magnitude of the effect is much smaller. This is because there is more direct electron interaction (in a molecular orbitaldescription) between ligands trans to each other than cis.
The Effect of the Metal center
The order of reactivity for a series of isovalent ions is NiII > PdII >> PtII.
The order of reactivity is the same order as the tendency to form 5-coordinate complexes. The more ready the formation of a 5-coordinate intermediate complex, the greater the stabilization of the transition state and the so the greater the rate enhancement.
This can give a huge effect, covering orders of magnitude, on the rate.
|M = Ni||ky = 33 mol-1s-1|
|M = Pd||ky = 0.58 mol-1s-1|
|M = Pt||ky = 6.7×10-6 mol-1s-1|
The size of the ligands can have an effect on the mechanism by which the reaction proceeds. Large ligands, or steric crowding, facilitate dissociative reactions and inhibit associative reactions.
The bulky groups can prevent the approach of attacking nucleophiles, but the reduction of coordination number in a dissociative reaction can relieve steric stress in the compound.
|Hydrolysis of cis-[PtClL(PEt3)2]|
|pyridene||8 x 10-2|
|2-methylpyridene||2 x 10-4|
|2,6-dimethylpyridene||1 x 10-6|
In the hydrolysis of cis-[PtClL(PEt3)2], the bulkier the ligand gets, the slower the reaction becomes, as it prevents the approach of the water molecule in the associative process characteristic of square planar complexes.
Stereochemistry of Substitution
On substitution of a square planar complex, the original geometry is usually preserved, ie. a cis-complex gives a cis-product and a trans-complex gives a trans-product.
This means that, in general, the intermediate is not long enough lived to allow any rearrangement to occur.
|The standard reaction pathway is that along the bottom, and only an intermediate long-lived enough to allow pseudorotation can give the alternative product.|
In the step labeled with a * in the above reaction, the intermediate complex must be long enough lived to allow a pseudorotation to occur, and so must be an Associative (A) intermediate.