The trans Effect

Substitution Reaction Kinetics

2.5 Factors Influencing the Reactivity of Square Planar Platinum(II) Complexes Complexes

2.5.5 Non-Participating Ligand

2.5.5.1 The trans Effect

The trans effect is the influence exerted by a non-labile ligand in a square planar complex on the rate of replacement of a labile ligand trans to it.⁴ This concept of trans influence was first observed by Werner in the early 20^th century.^9b,55 By 1926 Chernyaev used the concept of trans effect to synthesize isomeric square planar Pt(II) complexes.^14a The trans ligand (L) labilize the ligand trans to it (X). In this case the

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spectator ligands (A2) facilitate the replacement by the incoming nucleophile Y.^14b (Equation 2.40).

M X

L Y L M Y X (2.40)

A A

A A

In an associative mechanism, a greater trans effect increases the rate of substitution for square planar complexes. Using a standard complex, studies have established the general order of labilization by trans-directing ligands to be:^2,9b,14a,b

CN , C 2H4, CO, NO > R3P ≈ H^≈ SC(NH2)2 > CH₃^ > C₆H₅^ > SCN^ > NO₂^ >

I> Br^ >Cl^ > NH3 > OH^> H2O

A ligand high up in the trans effect series would have a greater trans effect.

Kinetically, the trans effect can be large, hence, with a good trans labilizing ligand the effect may increase up to 10⁶ or more.^14a Since trans effect is a kinetic phenomenon, in order to understand it better, its effect at the ground state and the transition state should be considered. For example, a stabilization of the transition state and destabilization of the ground state leads to a reaction coordinates marked by smaller energy barrier leading to a higher reactivity.

At this point it is important for one to differentiate the two closely related but characteristically distinct concepts, trans effect and trans influence. The trans effect provides information about the trans ligand on ground state as well as the transition state, while trans influence is a ground state phenomenon which involves the ground state effects such as ground state bond length.^7c,9b,46 To understand the trans effect better, it is important to know both σ-and π-bonding effects. These effects involve the ground state orbitals shared by the metal M, the leaving group X and the trans ligand L^7c (Equation 2.40).

In a square planar Pt(II) complex, a good π-accepting trans ligand can stabilise the five coordinate transition state intermediate by accepting the electron density from metal centre.^14a The π-orbital in the metal centre is a filled dxy, dzy or dyz orbital and the electrons are transferred from these orbital to the empty p orbitals on the trans ligand, L.^14a This makes the metal centre highly electrophilic. Thus, the increase in the electron

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density due to the five coordinate transition state on the metal centre is reduced.

Hence, the activation energy is reduced and thus, the rate of substitution increases.

In the transition state, the leaving group and the trans ligand do not directly share the same p orbital. Thus, such a ligand has a greater σ-donacity in the ground state and is the dominant contributor for the ground state trans influence. Of the studies conducted to explain trans effect,⁵⁶ the π-bonding theory and the polarisation theory are the two very common theories.⁴ Trans effect due to an appended alkyldiammine pendent groups have been recently reported by Jaganyi et al.^28,36c for the substitution kinetics of mono and dinuclear Pt(II) complexes with thiourea nucleophiles.

The π-back bonding effect in a metal complexes was first introduced by Pauling⁵⁷ to account for the short Ni―C bond distances in Ni(CO)4. The π-bonding theory states that ligands with π-bonds, such as C2H4, PR3 and CO have stronger π-acceptor abilities thus, stabilise the transition state and therefore appear higher in the trans-effect series.^14a,46 A pair of electrons is donated from the ligand to platinum centre to form the σ-bond while the π-bond is formed by the overlap of electrons from the filled d orbital (either dxz or dyz) of platinum with the vacant orbital of the ligand.^14a,46

In general, removal of electron density from the platinum centre to the empty orbitals of the ligand weakens the M―X bond in the ground state.^14a Chatt et al.⁵⁸ and Orgel⁵⁹ independently proposed a π-bonding stabilization of the trigonal bipyramidal intermediate complex for the reaction of trans-PtA2LX with Y to form trans-PtA2LY where Y is the entering group, X is the leaving group and L is the trans ligand (Figure 2.13).

Figure 2.13 π-back donation of the electrons from the filled d orbital to the vacant orbitals of the trans ligand in PtA2LXY.^14a,46

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Chatt et al., stated that strong trans directing ligands enhance the substitution reactions by removing the electron density from the metal centre to the π-backbonding ligand.⁵⁸ One of the hybrid orbitals, either dxz or pz overlaps with the empty orbital on the trans ligand.⁴⁶ Thus, the other two orbitals readily accept the electron density from the incoming ligand thereby favouring a rapid reaction.^2,14a Orgel⁵⁹ also supported this theory indicating that the presence of the π-acceptor ligand in the five coordinate transition state lowers the energy of the intermediate because the electron density on the Pt(II) is reduced along the Pt―X and Pt―Y directions, thereby retaining the original configuration.^14a

Polarization theory, reported by Meerwein,⁶⁰ is based on weakening of the Pt―X bond due to the strong trans effect caused by the trans ligand in PtA2LX.^14a The charge on the Pt(II) induces a dipole in L, which in turn induces a dipole in the platinum metal which repels the negative charge in X^14a resulting in a weakening of the Pt―X bond. A diagrammatic representation of charge induced dipoles in L―Pt―X is shown in Figure 2.14. However, there are some contradictions to this theory. Thus, the effect of covalent bonding in such systems needs to be considered. Ligands of high polarisability are expected to form the strongest bonds with Pt(II).

Figure 2.14 Distribution of Charge induced dipoles in the L―Pt―X coordinate of trans- PtA2LX.^14a

The σ- and π-trans effect is best described in the molecular orbital theory (MO theory).^14a

i σ-trans effect

A simplified MO diagram forPtCl₄^2is given in Figure 2.15. The most stable σ-orbitals tailed by the π-bonding orbitals are located mostly on the chloride groups followed by the anti-bonding orbitals of the σ and π-orbitals.⁴⁶ These orbitals are derived from the

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5d orbitals of Pt(II) and have four probable MO of which the



xy* is the most stable and

the _*

2 2 y x 

 is relatively the least stable MO.^14a,46 The higher energy pz valence orbitals are not involved in the σ-bond formation and the higher energy anti-bonding σ orbitals;

σs*, σx* and σy* are relatively the least stable from all.^14a

Figure 2.15 Molecular orbital representation showing the relative orbital energies in

 2

PtCl4 .^14a,46

In a square planar complexes, _{2 2}

y

dx _ , s,p_x, p_yand5 2

dz metal valence orbitals are used for the σ-bond formation.⁴⁶ However, only the two p orbitals have the right geometry for the trans directing properties. Therefore, in trans-PtA2LX, the leaving group, X and the trans ligand, L must share the same dsp² orbital in the ground state.^14a It is known that trans effect causes destabilization of the σ-trans ligand by elongation of the Pt―L bond via σ-donation. This in turn weakens the Pt―X bond due to the smaller share of

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the electrons available for the bonding,^14a thus, increases the rate of substitution of X (Figure 2.16). If the trans ligand is a stronger σ-donor, then the Pt―L shortens as a result of less electron density available for this bond and the Pt―X bond lengthens as reported by Jaganyi et al.^36c

L Pt X

L Pt X

(a) (b)

Figure 2.16 Representation of L― Pt―X bonding using σx MO (a) The σ-bond strength of L and X are almost equal. (b) Strong σ-donor ligand L, the σ-bond strength of L is much greater than that of X.^9b,14a

Langford and Gray⁶ used the σ-donation to explain the increase in the stabilization of the trigonal bipyramidal intermediate. In a ground state square planar complex, the samep_xorbital is used to form the L―Pt―X bonds.^14a Addition of the incoming ligand, Y from above the xy plane shifts X out of plane resulting in two suitable p orbitals (p_xandp_z) increasing the number of orbitals available for formation of bonds (Figure 2.17).^14a As a result, good σ-bonding ligands such asH^and CH3 which can donate to the extra p orbital can stabilize the σ structure in the trigonal bipyramidal intermediate through trans effect.^14a Jaganyi et al.^36c recently reported the trans effect due to the σ- donation of the alkyl pendent chains on the substitution kinetics of Pt(II) complexes by thiourea nucleophiles. It was reported that the alkyl chain on the trans N-donor atom of the chelate head group increases the rate of substitution of the aqua leaving groups due to the weaker trans influence of the alkyl amine donor group.

Pt x L

z

y

Y

Pt X L

Y

(a) (b)

X

A A A

A

Figure 2.17 The σ-trans effect due to the stabilization of the trigonal bipyramidal intermediate. (a) Only one p orbital is available for σ-bond formation of L and X.

(b) Two p orbitals are available for the σ-bonding of L, X and Y.^14a

36 ii π-trans effect

Good trans directing groups such as CO abd C2H4 stabilize the complex by strong π- back donation from the filled Pt(II) 5dxz or 5dyz to the empty π* (px or py) orbital of the ligand.^14a In the trigonal plane, the four orbitals are shared in π-bonding with the three ligands, L, X and Y. This stabilises the trigonal bipyramidal transition state if trans ligand, L, can form bonds with the π* orbitals which results in the transfer of electron density from the platinum centre to the π-acceptor ligands thereby lowering the energy of the system.^14a Thus, a good trans ligand lowers the activation energy of the reaction.^9b,14a This theory rates the trans ligands according to the following orders:^14a,17 C2H4 , CO > CNˉ > SCNˉ > Iˉ > Brˉ > Clˉ > NH3 > OHˉ

Recent studies reported^36a,b that trans π-accepting ligands can cause an increase in the substitution reaction of Pt(II) complexes by increasing the electrophilicity of the Pt(II) centre.