3)(PH3)2] Compound: Ab Initio Gas Phase Reaction Mechanism and Solvent Effects Using Continuum Models

A isomerização cis → trans do composto quadrático plano d 8 [Pt(Cl)(SnCl 3)(PH3)2] foi investigada utilizando-se o nível ab initio de cálculo MP4(SDQ)//MP2. As estruturas otimizadas, localizadas na superfície de energia potencial em fase gasosa, indicam que esta reação se processa através de um estado de transição quase-tetraédrico. A influência dos efeitos eletrônicos dos ligantes no mecanismo da reação foi investigada utilizando-se o método de Análise de Decomposição de Carga (CDA), o qual forneceu suporte para a compreensão do forte efeito trans do ligante SnCl3. O efeito devido ao solvente na energia de reação em fase gasosa foi avaliado utilizando-se os modelos contínuos SCRF e IPCM. Em ambos os casos um aumento na barreira de energia para o processo foi observado, sendo que a estabilidade termodinâmica dos isômeros cis e trans foi alterada pela solvatação.


Introduction
The square planar geometry is quite common amongst compounds of transition elements having d 8 configuration, for example Rh(I), Ir(I), Pd(II), Pt(II) and Au(III).These 16electron complexes can act as precursors, intermediates or products in several catalytic processes, where they can participate in associative elementary steps (in which they are readily converted into a 18-electrons compound) or in associative reactions, in which they can act as a 14-electrons species [1][2][3] .The existence of cis → trans isomerism in square planar d 8 complexes is well known.The isomerization mechanism of compounds involving platinum group metals is of particular interest due to their catalytic properties and to the fact that some isomers of this group show antitumor activity 4 .For example the cis isomer of cisplatin, [Pt(Cl) 2 (NH 3 ) 2 ], is a very active antitumor agent but, the trans isomer is inactive 4a .On the other hand, the Pd analog of cisplatin, [Pd(Cl) 2 (NH 3 ) 2 ], is inactive 4b .Some questions become apparent such as (i) how does the geometric arrangements affect the reactivity?(ii) what is the influence of the ligands and solvent on the relative stability of these isomers and on the isomerization mechanism?
Since the early 50's several studies have appeared, trying to rationalize the factors which affect the relative stability of the isomers, as well as the reaction mechanism, for compounds of the MX 2 L 2 type (where M=Pd(II), Pt(II); X=halide ion and L is a phosphine ligand).Indeed several interesting general rules about the solvent effects, electronic effects of the phosphine and the electronegativity of the halides were obtained [5][6][7] .Most of these studies were carried out for compounds where the Pt-X bond is of the same nature.The situation becomes more challenging in compounds of the M(PR 3 ) 2 XY type where the Pt-X and Pt-Y bonds are dissimilar, as in the process exemplified in Scheme 1. Compounds with the general formulation [Pt(Cl)(SnCl 3 )(PR 3 ) 2 ], possessing the Pt-Sn bond, have been shown to be highly active and selective in hydroformylation of olefins 8 .These compounds are formed in situ when SnCl 2 reacts with cis-[Pt(Cl) 2 (PR 3 ) 2 ] through an insertion mechanism.
of the stationary points located on the Potential Energy Surface (PES) for the gas phase cis→trans isomerization reaction of the [Pt(Cl)(SnCl 3 )(PH 3 ) 2 ] heterobimetallic compound, as well as to study the nature of the Pt-ligand bonds.The second goal of this work is to evaluate the solvent effects on the gas phase PES for this reaction.
Recently much theoretical effort has been directed to the development of new methodologies to better understand reactive processes in solution.These methods can be classified according to the form they treat explicitly the solutesolvent interactions, such as Molecular Mechanics force fields (MM) 15 , Empirical Valence Bond method (EVB) 16 and the whole variation of hybrid Quantum Mechanics/ Molecular Mechanics potentials (QM/MM) 17 .The other models are based on an implicit approach, which treats the solvent as a dielectric continuum medium in which the solute molecule is surrounded by a cavity, such as the Self Consistent Reaction Field method (SCRF) 18 , the Polarizable Continuum Model (PCM) 19 , the Isodensity Polarizable Continuum Model (IPCM) 20 and the Generalized Conductor-like Screening Model (GCOSMO) 21 .The implicit and explicit approaches to treat the solute-solvent interactions have been successfully applied to understand the role of the solvent in some relevant organic reactions 22 .Despite some progress, the study of solvent effects on reactions involving transition metal compounds remains a challenge.Since many of these compounds are active catalytic species in homogeneous catalysis, which occurs in solution, these methodologies have to be extended to this kind of reactions.The explicit treatment of the solute-solvent interaction when the solute molecule is an organometallic compound is a hard task because of the intermolecular potential parametrization.To the best of our knowledge, the only study reported so far for such kind of reactions is due to Morokuma and co-workers 23 , who used the Honda-Kitaura potential 24 to investigate the ligand substitution reaction Based on the arguments presented in this section, this work can be viewed as an attempt to extend the continuum solvent approaches to study organometallic reactions in solution, as well as to test the applicability of these models to such reactions.

Method of Calculation
Full geometry optimizations were performed at the second-order Møller-Plesset perturbation (MP2) level of theory, without any symmetry constraint, using the LANL2DZ effective core potential (ECP) and valence double-x basis set of Hay and Wadt 25 for the Pt and Sn atoms.For the P and Cl atoms, the split-valence basis set 6-31G(d) 26 , which The 31 P and 195 Pt Nuclear Magnetic Resonance (NMR) characterization of the complexes that are formed in solution when SnCl 2 reacts with cis-[Pt(Cl) 2 (PR 3 ) 2 ] was studied by Pregosin and Sze 9 .The reaction scheme proposed by these authors is shown in Scheme 2. These authors found that the insertion of SnCl 2 into the Pt-Cl bond in cis-[Pt(Cl) 2 (PR 3 ) 2 ] is followed by rapid isomerization.The electronic effects of the phosphine and SnCl 3 -ligands on the thermodynamic stability of the isomers in the gas phase, as well as the insertion mechanism, were recently investigated theoretically by our group 10 .The application of this Pt-Sn catalytic system on the first step of the hydroformylation reaction was also investigated theoretically 11 .
There is a consensus that three mechanisms can explain the cis→trans isomerism: the consecutive displacement mechanism 3 , the Berry pseudorotation mechanism 12,13 and the dissociative pathway 14 .The operating mechanism will depend on the nature of the solvent, electronic effects of the ligands and temperature.The aim of this work is to investigate the reaction mechanism for the cis→trans isomerization showed in Scheme 1, using PH 3 as a model phosphine.The work can be divided in two parts: the first one will focus on the electronic and structural effects of the ligands on the thermodynamic stability includes a set of five d polarization functions, was used.For the spectator hydrogen atoms, we used a smaller splitvalence 3-21G 26 basis set.The valence basis set for the tin atom was augmented with a set of five d polarization functions, with an exponential coefficient of 0.180 27 .All the stationary points located on the gas phase potential energy surface for the cis-[Pt(Cl)(SnCl 3 )(PH 3 ) 2 ] → trans-[Pt(Cl)(SnCl 3 )(PH 3 ) 2 ] isomerization reaction (see Figure 1) were characterized as minima or transition state through harmonic vibrational frequency calculations.The calculated harmonic frequencies were also used to compute the entropy contribution to the energy variation.To obtain better energetic results, we carried out single-point calculations at the fourth-order Møller-Plesset perturbation level of theory (MP4) with single, double and quadruple excitations MP4(SDQ) on the MP2 optimized geometries, denominated MP4(SDQ)//MP2, using the same basis set.
The nature of the Metal-Ligand (M-L) bonds was investigated through the Charge Decomposition Analysis (CDA) of Frenking and co-workers 28 , in which a Linear Combination of Fragment Orbitals (LCFO) is performed and the charge donation, charge backdonation and repulsive polarization between the molecular fragments are obtained.The CDA method has been shown to be a very suitable method to investigate the nature of M-L interactions 29 .
The solvent effects on the reaction energetics were evaluated using the thermodynamic cycle shown in Scheme 3, where the indices g, sol and solv stand for gas phase, solution and solvation, respectively.

TS Trans TS Trans
Trans TS

Cis Trans Cis Trans
Trans Cis where DE 0 solv (N) is the solvation energy of the species N, obtained by the difference between the energy of the What Scheme 3 tells us is that, in order to follow the reaction path in solution, we need to perform vibrational frequency calculations at every point on the gas phase PES necessary for the calculation of the free energy of solvation of each species, that is, of course, a difficult task.What we did was to assume that the gas phase PES has the same profile of the PES in solution so, Scheme 3 can be written as follows:

Cis Trans Cis Trans
TS Cis species N in gas phase and in solution.As formulated, we only add the difference in solvation energy of the involved species to the gas phase free energy.We can use even classical expressions to estimate the solvation energy of the species involved 30 .
Two continuum methods were used to evaluate the solvation energy of the Cis, TS and Trans species [∆E 0 solv (Cis), ∆E 0 solv (TS) and ∆E 0 solv (Trans)].These methods have in common the fact that the solute molecule is treated as a charge distribution embedded in a polarizable and continuum dielectric (solvent), characterized by a dielectric constant e.The solvation energy is described as a function of the interaction between the solute charge distribution (ρ) and the induced charges on the dielectric (solvent) (σ) (eq.4).The solute-solvent interaction is reduced to the determination of the electric potential (Φ σ ) generated by the charge distribution s, which is a consequence of the solvent polarization by the solute (eq.4).
The two continuum models used in this work differ in the way that the electrostatic potential Fs is obtained.
The first method used was the SCRF (Self Consistent Reaction Field) 18 , in which a homogeneous solute charge distribution is assumed and so, the cavity defined for the solute is symmetric (spherical in this case).In this situation, the electric potential inside this cavity can be expressed as a multipolar expansion (eq.5) and the solvation energy is given by E solv =V int /2.
The first term on this multipolar expansion(l = 0) leads to the Born equation 18e : in which q is the solute net charge.If neutral molecules are considered, as in the present study, the next term which contributes to the solute-solvent interaction energy is l = 1, which gives the Onsager equation 18f : ( ) where ε is the solvent dielectric constant, µ is the solute dipole moment and a is the cavity radius which is occupied by the solute molecule.So, the solute charge distribution is represented by a single-center multipolar expansion, truncated at the dipolar term.As can be seen from equations 5-7, the SCRF model is strongly dependent on the cavity radius assumed for the solute molecule.In this study, the spherical cavity radius for the solute was obtained taking half the maximum distance between non-bonded atoms and adding half the van der Waals radii of the atoms which form this maximum distance.Using this approach, we obtained a cavity radius a 0 of 4.472 Å for the Cis isomer, 4.099 Å for the Trans isomer and 4.196 Å for TS.
The other method used to study the solvent effects was the IPCM (Isodensity Polarizable Continuum Model) 20 , which uses a more realistic molecular-shape cavity, derived from the solute electron density.This method overcomes two main deficiencies of the SCRF approach: the assumption of a uniform charge distribution and the symmetric shape of the cavity.In the IPCM method, s is given by eq. 8.
( ) where Φ ρ -designates the contribution from the solute charge distribution to the potential and Φ σ -is the contribution due to the induced charge distribution on the dielectric.The determination of the electrostatic potential Φ σ (eq.9) is carried out self-consistently.
In all IPCM calculations an isodensity value of 0.001 electrons Å -3 was used.The SCRF and IPCM calculations were carried out in CH 2 Cl 2 (e = 9.080) at the MP2 level of theory.The reason to choose this solvent is that the cis-[Pt(Cl)(SnCl 3 ) (PH 3 ) 2 ], trans-[Pt(Cl)(SnCl 3 )(PH 3 ) 2 ] isomerization was experimentally observed in dichloromethane 9 , so some comparisons can be made.As CH 2 Cl 2 is a non-coordinating solvent, consequently no specific solute-solvent interaction can be expected.The system investigated here seems to be good to test the adequacy of continuum solvent approaches to organometallic reactions.All the calculations reported here were performed using the GAUSSIAN-94 package 31 .

Geometries
The MP2 optimized structural parameters obtained for the reactants cis-[Pt(Cl)(SnCl 3 )(PH 3 ) 2 ], (Cis), Transition State (TS), and the product trans-[Pt(Cl)(SnCl 3 )(PH 3 ) 2 ], (Trans) are shown in Figure 1.The main structural parameters obtained are in good agreement with experimental findings.For example, the optimized angles around the platinum metal, for the Cis and Trans isomers are a little distorted from the expected optimal value of 90° for a d 8  In organometallic chemistry the structural parameters are a very rich source of information, from where some electronic effects of the ligands can be qualitatively analyzed, such as the ability of some ligands to labilyze the bond trans to it, the so called trans influence.For example, the Pt-P bond distance trans to the SnCl 3 group in the Cis isomer (2.413 Å) is greater than the Pt-P bond in the Trans isomer (2.320 Å), indicating that the SnCl 3 group weakens the Pt-P bond trans to it.The Pt-Cl bond in Cis (2.394 Å) trans to the PH 3 ligand is shorter than is the Pt-Cl bond length in Trans (2.439 Å) , also indicating the trans influence of the SnCl 3 group.The Pt-Sn bond length of 2.495 Å in TS is shorter than the values found for the Cis (2.561 Å) and Trans (2.523 Å) isomers.This can be qualitatively explained by the fact that in TS the SnCl 3 group does not have any ligand directly bonded trans to it and so, it can make a more effective interaction with the platinum atom.What these structural parameters tell us is that the SnCl 3 -ligand is a stronger trans director than the PH 3 ligand and this explains qualitatively why the isomerization takes place.These facts will be discussed quantitatively later.

Vibrational spectra
The calculated vibrational frequencies for the Pt-Cl, Pt-Sn, Pt-P and Sn-Cl bonds are shown in Table 1.The values quoted in Table 1 are in good agreement with experimental values obtained for similar compounds 35 .As can be seen from Table 1, the trans effect of the SnCl 3 group is reflected in the vibrational spectra for the Cis and Trans isomers.The stretching frequency ν(Pt-P) for the PH 3 trans to SnCl 3 (259 cm -1 ) in the Cis isomer is shifted 114 cm -1 to the low frequency region, compared with the ν(Pt-P) for the Pt-P bond cis to SnCl 3 .It is interesting to note that the ν(Sn-Cl) and ν(Pt-Cl) appear in the same region of the spectra which makes difficult its experimental attribution 35 .The calculated frequencies show that the most important vibrational frequencies are present in the low frequency region of the infrared (I.R) spectrum, which makes difficult the experimental identification of the Cis and Trans isomers by infrared spectroscopy.The transition state TS has one negative eigenvalue of 80 cm - 1 and the normal mode associated with this frequency is shown in Figure 2. We have re-optimized the geometry of the TS structure as a minimum energy point on the PES in an attempt to verify if it optimizes to one of the isomers or to a probable intermediate.We found that the initial structure is already a stationary point on the PES and when harmonic frequency analysis is performed we found one small imaginary frequency characterizing a first-order TS

2.
Normal mode exhibiting one negative eigenvalue in the Hessian matrix of TS.The figure shows a pseudorotation of the Cl-Pt-P1 angle (see Figure 1).structure, in spite of being on a flat region of the PES.

Electronic effects of the ligands
The nature of the M-L interactions was analyzed through the Charge Decomposition Analysis, CDA 28 , using the MP2 wavefunction.The results are shown in Table 2.There are several interesting points to be addressed here.First, as can be seen from Table 2, in all combinations analyzed, the magnitude of the charge donation from the PH 3 fragment to the platinum fragment, [Pt(Cl)(SnCl 3 )(PH 3 )], is greater than is the backdonation term, which indicates that the PH 3 fragment is a poor π-acceptor ligand.The comparisons of the extent of backdonated charges of the two PH 3 ligands in the cis-[Pt(Cl)(SnCl 3 )(PH 3 ) 2 ], (Cis isomer) can give some ideas, indirectly, about the amount of backdonation of the SnCl 3 ligand.For example, the extent of backdonation from the PH 3 fragment trans to the SnCl 3 group in the Cis isomer (0.075 e) is lower than backdonation from the PH 3 fragment cis to SnCl 3 (0.202 e).This indicates that the SnCl 3 fragment is withdrawing electron density from the platinum atom, which in turn, reduces the electron density available for donating to the PH 3 group.So, The PH 3 is competing unevenly with the SnCl 3 group to withdraw electron density from the platinum and this explains quantitatively why the isomerization does occur.In fact, the backdonation from the platinum fragment, [Pt(Cl)(SnCl 3 )(PH 3 )], to the PH 3 fragment trans to another PH 3 ligand in the Trans isomer (0.139 e), gives support to this assertion.
A quantitative measurement of the trans effect of the SnCl 3 group can be seen through the M-L interaction energy quoted in Table 2.As can be seen, the Pt-P bond energy evaluated for the PH 3 fragment cis to SnCl 3 in the Cis isomer (50.9 kcal mol -1 ) is 26.3 kcal mol -1 higher than the Pt-P bond energy calculated for the PH 3 fragment trans to SnCl 3 (24.6 kcal mol -1 ) in this same isomer.That is, the SnCl 3 ligand can weaken the Pt-P bond trans to it by ca. 26 kcal mol -1 .

Relative energies
The relative energies obtained for the gas phase PES are quoted in Table 3 and the relative thermodynamic properties are shown in Table 4.As can be seen from Table 3, the activation energy, DE # , for the Cis→Trans isomerization is 26.9 kcal mol -1 and, the Trans isomer is ca.7.0 kcal mol -1 more stable than the Cis one in gas phase.Table 3 also shows that there is no significant change in energy when the electron correlation level is augmented up to fourth order of perturbation theory.The thermodynamic properties, evaluated at room temperature, shown in Table 4 give the following activation parameters for the gas phase Cis→Trans isomerization reaction: ∆S # = 2.0 cal K -1 mol -1 , ∆H # = 26.7 kcal mol -1 and ∆G # = 26.1 kcal/mol.The reaction proceeds with ∆G Cis→Trans of -7.7 kcal mol -1 , ∆H Cis→Trans of -7.0 kcal mol -1 and ∆S Cis→Trans of 2.8 cal K -1 mol -1 .From these data, it seems that the enthalpy changes will favor the Trans isomer much more than the entropy changes.This energetic favoring of the Trans isomer in gas phase may be due to internal bond energy changes in the Trans isomer.

Solution results
The solvation energy of Cis, TS and Trans are shown in Table 5.As can be seen, the SCRF results lead to higher stabilization energies in solution compared with the IPCM results.This can be explained by the fact that in the SCRF approach the solute-solvent interaction is modeled as a dipole-dipole interaction and so, this method has a marked dependence on the solute dipole moment (a quadratic dependence, µ 2 ), as can be seen in eq. 7. The SCRF results agree with the expected trend in solvation energy, where the species having higher dipole moment will be more stabilized in solution.That is, the Cis isomer (µ = 11.684debye) is more stabilized than Trans (µ = 3.015 debye) and TS (µ = 6.875 debye).The IPCM gave different stabilization energies in solution, where the Trans isomer is more stabilized than TS.This is because in the IPCM model we do not assume a homogeneous solute charge distribution as in the SCRF method.The more proper atomic charge distribution in IPCM may give atomic charge polarization effects, which give rise to deviations from what would be expected if only the solute total dipole moment is considered.Thus, a species with the smallest overall dipole moment may still have the largest solvent stabilization energy, as in this case.Of course, this different solvent stabilization energies obtained by these two methods, will be reflected on the relative stability of the Cis and Trans isomer in solution, ∆G 0 sol (Cis→Trans), as can be seen in the relative free energy in solution, shown in Table 6.In the SCRF model the Trans isomer is 4.8 kcal mol - 1 more stable than the Cis isomer.The IPCM model gave a reverse stability order, where the Cis isomer is 2.1 kcal mol -1 more stable than Trans.These IPCM results are in accordance with the experimental studies of Redfield and Nelson 6 .These authors evaluated the cis→trans relative stability of [Pd(Cl) 2 {PPh(CH 3 ) 2 } 2 ] in eleven different solvents and found that the cis isomer was more stable in all solvents analyzed.The assumption of a homogeneous charge distribution, the symmetric form to the cavity and the marked dependence on the solute dipole moment appear to make the SCRF approach more inconsistent than the IPCM.The sensitivity of the SCRF to the cavity radius (with a dependence of a 0 -3 ) is another problem.Different procedures to estimate a 0 can lead to different results.In fact, we have some results showing that in order to reproduce the IPCM results, we have to use explicitly some solvent molecules in the SCRF treatment, an approach called Super-Molecule + SCRF (SM +SCRF) 36 .The two methods agree only on the free energy of activation in solution, where we have an increase, compared with the gas phase results, in both methods (30.1 kcal mol -1 in the SCRF approach and 38.9 kcal mol -1 in IPCM).
Assuming that the IPCM is our best solution result, we have two factors competing with each other: the strong trans effect of the SnCl 3 ligand, which avoids placing the As was said before, several mechanisms can explain the cis→trans isomerization reaction in square planar compounds 12 .Most of these mechanisms are solvent assisted.The activation free energy found in this work for the Cis→Trans isomerization of [Pt(Cl)(SnCl 3 )(PH 3 ) 2 ], without the assistance of specific interactions of the solvent, seems to be high in gas phase (∆G # = 26.1 kcal mol -1 ) and in solution (∆G # SCRF = 30.1 kcal mol -1 , ∆G # IPCM = 38.9kcal mol -1 ) to explain this isomerization process.Combining the strong trans influence of the SnCl 3 ligand, as was shown in this work, and the ability of the Pt-Sn bond to stabilize penta-coordinated intermediates 11 , another route for this isomerization reaction could be the displacement of the PH 3 ligand trans to SnCl 3 by a solvent molecule, which catalyze the isomerization of another molecule (see Scheme 4)

Scheme 4
Despite the fact that CH 2 Cl 2 is a weakly coordinating solvent, the ability of such kind of solvents to displace strongly coordinated ligands from the transition metal can not be ruled out.For example, the loss of a PPh 3 from Wilkinson's catalyst, [RhCl(PPh 3 ) 3 ] 37 , is a crucial step in the catalytic cycle.Another example where this autocatalytic pathway takes place is in the isomerization of [PtCl 2 (CO)L] 38 , which proceeds spontaneously in several solvents of the weakly coordinating type.
As can be seen, this is the challenge of treating solvent effects in organonometallic reactions, that is, even weakly coordinating solvents can affect drastically the reaction pathway and in fact, all sort of specific interactions (coordination of the solvent, interactions with coordinated ligands through hydrogen bond etc.) must be taken into account and so, the continuum approaches can not explain these phenomena.

Conclusions
In this work we investigated the energetic and reaction mechanism for the gas-phase cis→trans isomerization reaction of [Pt(Cl)(SnCl 3 )(PH 3 ) 2 ], at the MP4(SDQ)//MP2 level of theory.The solvent effects on the reaction energetic were evaluated using the Self Consistent Reaction Field (SCRF) approach and the Isodensity Polarizable Continuum Model (IPCM).All the stationary points located on the gas-phase potential energy surface were fully optimized.The transition state (TS) structure obtained indicates that this reaction may proceed through a pseudorotation mechanism, leading to a quasi-tetrahedral structure for TS.The activation free energy obtained for the gas-phase reaction was 26.1 kcal mol -1 .No significant change in energy was observed when the correlation level is increased up to fourth order.The electronic effects of the ligands on the isomerization reaction were evaluated through the analysis of the MP2 wavefunction with the aid of the Charge Decomposition Analysis (CDA) method.This analysis indicates that SnCl 3 is a strong trans director capable of weakening the Pt-P bond trans to it by 26.3 kcal mol -1 .The CDA results gave explanation about why this isomerization does occur.
The solvent effects on the energetic of the gas phase reaction were evaluated using the SCRF and IPCM continuum approaches.In both cases an increase in the free energy of activation was observed (DG # = 30.1 kcal mol -1 using the SCRF approach and DG # = 38.9kcal mol -1 with the IPCM method).The IPCM results gave an inverse thermodynamic stability order, compared to the gas phase stability, which can be attributed to charge polarization effects on the stabilization of the Trans isomer.Combining the strong trans influence of the SnCl 3 ligand and the ability of the Pt-Sn to stabilize the penta-coordinated geometry, we proposed an autocatalytic mechanism for this reaction, which is solvent assisted.Based on the results presented here and on the facts that even weakly coordinating solvents can significantly change the reaction pathway through specific interactions, we believe that other treatments, taking into account all these interactions, are necessary to study organometallic reactions in solution.Our group is currently engaged in developing new tools for studying these reactions in solution.Despite the fact that the solvent dynamics was not explicitly included in the present study, work is in progress aiming to model the dynamic effects.
We are interested in understanding the substitution and isomerization reactions of square planar compounds, and other results will be available in future publications.

Table 3 .
Relative energies (in kcal mol -1 ) * for the three stationary points located on the gas phase PES for the isomerization reaction.

Table 5 .
Solvation energy of Cis, Ts and Trans obtained by the SCRF and IPCM methods*.

Table 6 .
Relative free energies in solution* PH 3 ligand trans to it and determines more stabilization of the trans-[Pt(Cl)(SnCl 3 )(PH 3 ) 2 ] isomer in gas phase, and the high dipole moment of the Cis isomer which makes it more stable in solution.