**4. Theoretical study of complexes of (olefin)Pt(PPh3)2**

**Figure 7.** Fragment of 1

256 New Advances in Hydrogenation Processes - Fundamentals and Applications

complexes.

H NMR spectra of diethyl maleate bis(triphenylphosphine)platinum(0) complex.

Wherever the equimolar mixture of both ligands was added to the solution of ethylene *bis*(triphenylphosphine)platinum(0), the formation of the equimolar quantities of both com‐ plexes of esters with platinum was prevented, but a complex with dimethyl fumarate was preferentially produced. It could be assumed that the resulting ratio reflected the different adsorption properties of both ligands. In order to verify that it was the case of a behavior resulting from an inter‐displacement of ligands, an experiment was conducted, in which first the complex with diethyl maleate was prepared and subsequently dimethyl fumarate added. Even under these circumstances, the ligand (diethyl maleate) displacement took place as well as the stabilizing of a ratio corresponding to the situation of a simultaneous addition of both ligands (**Figure 8**). This method has a certain limit related to a small dy‐ namic range of NMR measurements, and thus it is necessary to compare substrates as de‐ scribed above with near adsorptivities. On the other hand it implies that even minor changes in the absorptivity of substrate molecules can be registered using this procedure. In a series of synthesized complexes of (olefin)Pt(PPh3), the complexes with a near stability have always been selected, and NMR "*in situ"* competitive measurements performed. Based on them, a relative stability of the prepared complexes was determined. As shown below, this sequence very well correlated with calculated bonding energies of the listed The purpose of these studies was to find a calculable correlation relationship between the structure and the stability of the complexes of the type (olefin)Pt(PPh3)2. The stability compar‐ ison was carried out by numerous available computational methods, that is, using both structural parameters (changes in the binding length and angles after coordination), then the quantification of the bond orders in alkene subunits, and finally the estimation of the binding energy between an alkene and Pt(PPh3)2. It was soon discovered and experimentally confirmed that the chemical nature of the bond metal‐alkene is rather nearer the description of cyclopro‐ pane ring (see above) than the potential π‐bond. These models assume that within the formalism of valence bonds, the compound Pt(PPh3)2‐alkene can be considered as a derivative of platinum(II) rather than platinum(0), providing the change in hybridization of carbon atoms from *sp*<sup>2</sup> to *sp*<sup>3</sup> . Many computational approaches to compounds of the type [(ole‐ fin)Pt(PPh3)2] require to tackle one important parameter, that is, a structural simplification leading to a significant reduction in the computing time, which is carried out on the assumption that phenyl groups are simply regarded as hydrogens, and in particular triphenylphosphine groups are regarded as phosphines. This simplification reduces the number of atoms in the calculation by 60 and even more significantly reduces the number of basic functions, which leads to a major acceleration of the computation. It is known that this simplification does not affect the structural and energetic results, although the steric hindrance on the platinum atom is relatively substantially altered.

The results of molecular modeling in the given group of substances were generally confirmed by the experimentally observed geometry of molecules, while any potential discrepancies have been notably dependent on the parameterization. Moreover, some aspects based on the theoretical approach allowed evaluating also such quantities as the reduction of values in the order of C‐C bond, the binding energy of Pt‐olefin, and other parameters. The contributions of donation and reversed donation to the bond platinum‐olefin were also addressed on the theoretical basis of the orbital interaction method CDA (*charge decomposition analysis*). As a result, it was demonstrated that the π‐reversed donation (Pt to LUMO olefin) was the main binding contribution and thus affected the strength of Pt‐C bonds much stronger than the donation (**Figure 2**). In the presented study, a considerable attention has been devoted to quantum‐mechanical calculations on a group of (PPh3)2Pt(olefin) complexes, which were sufficiently reliably approximated by complexes with the structure (PH3)2Pt(olefin), **Figure 9**. This approximation led to a marked simplification of calculations; the number of atoms in resolved molecules was thus reduced by 60 and the number of base functions by 552 to 106 bases (while it is known that the computational requirements of chemical structures increase with the number of atoms in a molecule approximately exponentially). In the search of a suitable parameterization for calculating the structure, the convergence of binding energies in molecules was preferred from their geometrical properties (bond lengths of Pt‐C and Pt‐P and the angles of P‐Pt‐P and C‐Pt‐C). For the geometrical optimization, the method of B3LYP (functional density functional theory (DFT) hybrid) was selected with the base 6‐31G(d) for the lighter elements (Cl, P, S, O, N, F, C, and H) and the base LanL2DZ with the pseudopotential (ECP, *effective core potential*) LanL2 for Pt based on the study of *Frison* and *Grützmacher*[16], who utilized only a slightly larger "Basis Set II." To quantify the total energy of molecules, MP2 method was selected as a compromise solution for a better interpreting ability, and simulta‐ neously for only medium requirements on the computing time. For the calculations of Hessian (estimation of infrared (IR) spectra), the same parameterization was used as for the geometrical optimization, that is, B3LYP/6‐31G(d):LanL2DZ. The binding energy of olefin‐platinum bonds in the molecules of (olefin)Pt(PH3)2, where the olefin was R1 R2 C=CR3 R4 and bound by two σ‐ bonds with both carbons to platinum, has been interpreted as the dissociation energy of these bonds, that is, the enthalpy of decomposition reaction of an organometallic complex. Numer‐ ically, the binding energy is thus equal to the difference between the energies of a complex splitting (olefin and (PH3)2), and the complex (olefin)Pt(PH3)2, the final and the initial state, respectively:

**Figure 9.** Molecular structure of (PPh3)2Pt(CH2=CH2) (**a**) and (PH3)2Pt(CH2=CH2) (**b**).

Organometallics: Exploration Tool for Surface Phenomena in Heterogeneous Catalysis http://dx.doi.org/10.5772/65337 259

$$\mathrm{E}\_{\mathrm{B}} = \mathrm{E}\left[\mathrm{Pt}\left(\mathrm{PH}\_{3}\right)\_{2}\right] + \mathrm{E}\left[\mathrm{olefin}\right] - \mathrm{E}\left[\left(\mathrm{olefin}\right)\mathrm{Pt}\left(\mathrm{PH}\_{3}\right)\_{2}\right].\tag{1}$$

During the calculations, the binding energy of Pt-olefin bonds was enumerated. Therefore, it had primarily been necessary to carry out geometrical optimization on both free planar olefins R1 R2 C=CR3 R4 and then to evaluate their geometric properties. In olefins with a more complicated conformational behavior (e.g., unsaturated alcohols), a random inspection of the conformation space was performed related especially to those degrees of freedom where a free rotation around CC bonds occurred only with a low barrier (**Figure 10**). then the equilibrium geometry of olefins was introduced to the models of organometallic compounds (PH3)2Pt(R1 R2 C=CR3 R4 ), and these models were optimized without any restrictions on any degrees of freedom. The fragment (H3P)-Pt-(PH3) assumed the optimal geometry in the linear arrangement. The structure of the fragment (Ph3P)-Pt-(PPh3) was calculated as well, in which the linear structure of P-Pt-P has been analogically identified. These structures have been considered to be metastable. Although their observations have been reported, no credible spectroscopic evidence was provided. IR spectra have been estimated and optimized for different models of olefins and their complexes. The algorithm of molecular modeling of the complex (olefin)Pt(PPh3)2 is summarized in **Figure 11**.

**Figure 10.** Alkene CH2=CH-CR1 R2 R3 with allowed rotation around C-C bond.

#### **4.1. Structure-based effects**

theoretical approach allowed evaluating also such quantities as the reduction of values in the order of C‐C bond, the binding energy of Pt‐olefin, and other parameters. The contributions of donation and reversed donation to the bond platinum‐olefin were also addressed on the theoretical basis of the orbital interaction method CDA (*charge decomposition analysis*). As a result, it was demonstrated that the π‐reversed donation (Pt to LUMO olefin) was the main binding contribution and thus affected the strength of Pt‐C bonds much stronger than the donation (**Figure 2**). In the presented study, a considerable attention has been devoted to quantum‐mechanical calculations on a group of (PPh3)2Pt(olefin) complexes, which were sufficiently reliably approximated by complexes with the structure (PH3)2Pt(olefin), **Figure 9**. This approximation led to a marked simplification of calculations; the number of atoms in resolved molecules was thus reduced by 60 and the number of base functions by 552 to 106 bases (while it is known that the computational requirements of chemical structures increase with the number of atoms in a molecule approximately exponentially). In the search of a suitable parameterization for calculating the structure, the convergence of binding energies in molecules was preferred from their geometrical properties (bond lengths of Pt‐C and Pt‐P and the angles of P‐Pt‐P and C‐Pt‐C). For the geometrical optimization, the method of B3LYP (functional density functional theory (DFT) hybrid) was selected with the base 6‐31G(d) for the lighter elements (Cl, P, S, O, N, F, C, and H) and the base LanL2DZ with the pseudopotential (ECP, *effective core potential*) LanL2 for Pt based on the study of *Frison* and *Grützmacher*[16], who utilized only a slightly larger "Basis Set II." To quantify the total energy of molecules, MP2 method was selected as a compromise solution for a better interpreting ability, and simulta‐ neously for only medium requirements on the computing time. For the calculations of Hessian (estimation of infrared (IR) spectra), the same parameterization was used as for the geometrical optimization, that is, B3LYP/6‐31G(d):LanL2DZ. The binding energy of olefin‐platinum bonds

> R2 C=CR3

bonds with both carbons to platinum, has been interpreted as the dissociation energy of these bonds, that is, the enthalpy of decomposition reaction of an organometallic complex. Numer‐ ically, the binding energy is thus equal to the difference between the energies of a complex splitting (olefin and (PH3)2), and the complex (olefin)Pt(PH3)2, the final and the initial state,

R4 and bound by two σ‐

in the molecules of (olefin)Pt(PH3)2, where the olefin was R1

258 New Advances in Hydrogenation Processes - Fundamentals and Applications

**Figure 9.** Molecular structure of (PPh3)2Pt(CH2=CH2) (**a**) and (PH3)2Pt(CH2=CH2) (**b**).

respectively:

As a general rule, the double bonds of carbons associated with platinum(0) manifested their extension compared to the other functional groups. This has also been supported by the determined significant alterations in the constitutional and electronic properties of the olefins. The angle between P-Pt-P and C-Pt-C markedly differs among various organometallic substances due to dissimilar properties of each olefin. Platinum(0) was bound with the olefin functional groups via two σ(Pt-C) bonds. Among the listed olefins, all were arranged in ηνfashion to platinum with no free rotation, to the contrary of rotations described in the case of ethylene in platinum(II) derivatives such as Zeise's salt K[Cl3Pt(CH2=CH2)].

**Figure 11.** Flowchart of utilization of methods of molecular modeling in the group of "species" (PH3)2Pt(R1 R2 C=CR3 R4 ), (H3P)‐Pt‐(PH3) and R1 R2 C=CR3 R4 .

The implicit solvation method has not been applied any further, as the continuum solvation had negligible effects, the parameters as the vibrational frequencies, optimal geometries, energies, and NMR magnetic‐shielding tensors, etc., as proven by a separated computation (pulse‐code modulation (PCM) method, dielectric constant of dichloromethane). **Table 1** shows the global assessment of energetic and structural outcomes resulted from the final computations. The values of binding energy *EB* were applied as to assess the thermodynamic properties of Pt‐olefin electron‐overlap interaction: *Δν* indicates the change in the vibrational wave number of C=C‐stretching/contracting mode following the complex production, whereas *ΔR* is the extension primarily generated by the strength of metal‐olefin complex. The pyra‐ midalization *δ* was determined as an average angle of R1‐C=C‐R3 and R2‐C=C‐R4 dihedrals; thus, it has been applicable also to asymmetrical olefins, to the contrary of the previous papers [4, 14], where the pyramidalization angle was determined merely for R2C=CR2, that is, olefins shaping complexes in C2V symmetry.

In order to make comparison and consistency with the previous studies, a reference compound was selected, that is, ethylene, whose results are further discussed in detail. Yates [19] carried out broad computations related to the consistency and attained the binding energy of 111.9 kJ/ mol in (PH3)2Pt(CH2=CH2) at MP2/6‐31G(d):LANL2DZ level, however, remarkably lower quantities for the arrangements containing PPh3 ligands. Nunzi et al. [16] determined the "bond dissociation energy," amended with BSSE (basis set superposition error), as 102 kJ/mol at local density approximation (LDA)‐DFT level of theory using mixed ζ‐quality Slater‐type orbital (STO) basis sets. Nevertheless, the published experimental values [17] for (PPh3)2Pt(CH2=CH2) of Mortimer (152 ± 18 kJ/mol, gas‐phase) and Kirkham et al. (11.6 ± 1.6 kJ/ mol) appeared to be relatively inaccurate.


fashion to platinum with no free rotation, to the contrary of rotations described in the case of

ethylene in platinum(II) derivatives such as Zeise's salt K[Cl3Pt(CH2=CH2)].

260 New Advances in Hydrogenation Processes - Fundamentals and Applications

**Figure 11.** Flowchart of utilization of methods of molecular modeling in the group of "species" (PH3)2Pt(R1

The implicit solvation method has not been applied any further, as the continuum solvation had negligible effects, the parameters as the vibrational frequencies, optimal geometries, energies, and NMR magnetic‐shielding tensors, etc., as proven by a separated computation (pulse‐code modulation (PCM) method, dielectric constant of dichloromethane). **Table 1** shows the global assessment of energetic and structural outcomes resulted from the final computations. The values of binding energy *EB* were applied as to assess the thermodynamic properties of Pt‐olefin electron‐overlap interaction: *Δν* indicates the change in the vibrational wave number of C=C‐stretching/contracting mode following the complex production, whereas *ΔR* is the extension primarily generated by the strength of metal‐olefin complex. The pyra‐ midalization *δ* was determined as an average angle of R1‐C=C‐R3 and R2‐C=C‐R4 dihedrals; thus, it has been applicable also to asymmetrical olefins, to the contrary of the previous papers [4, 14], where the pyramidalization angle was determined merely for R2C=CR2, that is, olefins

In order to make comparison and consistency with the previous studies, a reference compound was selected, that is, ethylene, whose results are further discussed in detail. Yates [19] carried out broad computations related to the consistency and attained the binding energy of 111.9 kJ/ mol in (PH3)2Pt(CH2=CH2) at MP2/6‐31G(d):LANL2DZ level, however, remarkably lower quantities for the arrangements containing PPh3 ligands. Nunzi et al. [16] determined the "bond dissociation energy," amended with BSSE (basis set superposition error), as 102 kJ/mol at local density approximation (LDA)‐DFT level of theory using mixed ζ‐quality Slater‐type orbital (STO) basis sets. Nevertheless, the published experimental values [17] for (PPh3)2Pt(CH2=CH2) of Mortimer (152 ± 18 kJ/mol, gas‐phase) and Kirkham et al. (11.6 ± 1.6 kJ/

(H3P)‐Pt‐(PH3) and R1

R2 C=CR3 R4 .

shaping complexes in C2V symmetry.

mol) appeared to be relatively inaccurate.

R2 C=CR3 R4 ),


**a** Coordination via C=C double bond; **<sup>b</sup>**Coordination via C≡C triple bond, vibrational shift for C≡C stretch; **c–f**Syntheses of complexes, see Refs. [9, 12, 13, 16], respectively; **<sup>g</sup>**Vibrational shift of the C≡C stretch and elongation of the C≡C bond; **<sup>h</sup>**Vibrational shift of the C=S stretch and elongation of the C=S bond; **<sup>i</sup>** Vibrational shift of the O=O stretch normal mode and elongation of the O=O bond.

**Table 1.** The calculated parameters of coordination compounds of type (PH3)2Pt (olefin).

In our study, for a completely optimized complex (PH3)2Pt(CH2=CH2) (**Figure 12**), the binding energy of Pt‐ethylene bonds was calculated as 116.8 kJ/mol at MP2/6‐31G(d):LANL2DZ level of theory. The relatively minor discrepancy from the outcomes of Yates [15] could have been produced by our experimental setup, which has underwent a minor optimization. The binding energy for our entirely optimized model of (PPh3)2Pt(CH2=CH2) was determined at B3LYP/ 6‐31G(d):LANL2DZ level, yielding a value of only 9.9 kJ/mol (in comparison to 51.9 kJ/mol for the PH3 analog). Regarding the geometrical parameters, the C=C bond in (PH3)2Pt(CH2=CH2) stretched from 1.331 (free) to 1.425 (coordinated) by 0.094 Å. For the PPh3 analog, the coordi‐ nated value was 1.430 Å, which was nearer to 1.434 Å as measured in the experiment. In the complex (PH3)2Pt(CH2=CH2), having C2V symmetry, both Pt‐P bonds were of the same length 2.157 Å, while in (PPh3)2Pt(CH2=CH2) [CS symmetry], two different Pt‐P bond lengths were calculated (2.356 and 2.366 Å). The mean experimental value was 2.268 Å. The σ‐Pt‐C bonds in (PH3)2Pt(CH2=CH2) were 2.157 Å long. Regarding the PPh3 analog, the bond length was calculated 2.153 Å (mean value of 2.168 and 2.138 Å). Both of these achieved results were overestimated in comparison to 2.112 Å (average) as determined by the experiment [18]. The next aim was to determine the similarity of Pt(0)‐olefin organometallics from olefins coordi‐ nated on metal surfaces. In our study, the vertical distance of platinum and C=C bond in (PH3)2Pt(CH2=CH2) was 2.036 Å, while for ethylene adsorbed in di‐*σ*‐bridge position on the Pt(111) surface, comparable values were obtained. Moreover, regardless of the calculations performed at different theory levels, the bond dissociation energies were 116.8 kJ/mol in the case of organometallic compounds (MP2 value, this study) and 106.1 kJ/mol in the case of C2H4 on p(2x2)‐Pt(111) (generalized gradient approximation (GGA)‐DFT calculation, [15]). Thus, the values were comparable within these arrangements. Additionally, the thermody‐ namics of the complete reaction of molecular oxygen with platinum(0) complex has been examined. It has been recognized that ethylene in the complex of (C2H4)Pt(PR3)2 is substituted by dioxygen producing (O2)Pt(PR3)2. This reaction proceeded well, providing the exposure of (C2H4)Pt(PR3)2 to the ambient air. Having substituted **H** for **R**, the reaction enthalpy was determined according to the following schema:

$$\mathrm{P}\left(\mathrm{C}\_{2}\mathrm{H}\_{4}\right)\mathrm{Pt}\left(\mathrm{PR}\_{3}\right)\_{2}\left(\mathrm{g}\right) + \mathrm{O}\_{2}\left(\mathrm{g}\right) = \left(\mathrm{O}\_{2}\right)\mathrm{Pt}\left(\mathrm{PR}\_{3}\right)\_{2} + \mathrm{C}\_{2}\mathrm{H}\_{4}\left(\mathrm{g}\right),\tag{2}$$

finally obtaining the value of *ΔHr <sup>298</sup>* = −77.4 kJ/mol, a relatively average figure for an exothermic reaction.

**Figure 12.** Molecular structure of ethylene (a, left) and (ethylene)Pt(PH3)2 (b, right).

#### *4.1.1. Propadiene, butadiene, and vinylacetylene*

**Group/compound** *EB* **[kJ/mol] Δ***v* **[cm-1] Δ***R* **[Å]**  *δ* **[°]**

Coordination via C=C double bond; **<sup>b</sup>**Coordination via C≡C triple bond, vibrational shift for C≡C stretch; **c–f**Syntheses of complexes, see Refs. [9, 12, 13, 16], respectively; **<sup>g</sup>**Vibrational shift of the C≡C stretch and elongation of the C≡C bond;

In our study, for a completely optimized complex (PH3)2Pt(CH2=CH2) (**Figure 12**), the binding energy of Pt‐ethylene bonds was calculated as 116.8 kJ/mol at MP2/6‐31G(d):LANL2DZ level of theory. The relatively minor discrepancy from the outcomes of Yates [15] could have been produced by our experimental setup, which has underwent a minor optimization. The binding energy for our entirely optimized model of (PPh3)2Pt(CH2=CH2) was determined at B3LYP/ 6‐31G(d):LANL2DZ level, yielding a value of only 9.9 kJ/mol (in comparison to 51.9 kJ/mol for the PH3 analog). Regarding the geometrical parameters, the C=C bond in (PH3)2Pt(CH2=CH2) stretched from 1.331 (free) to 1.425 (coordinated) by 0.094 Å. For the PPh3 analog, the coordi‐ nated value was 1.430 Å, which was nearer to 1.434 Å as measured in the experiment. In the complex (PH3)2Pt(CH2=CH2), having C2V symmetry, both Pt‐P bonds were of the same length 2.157 Å, while in (PPh3)2Pt(CH2=CH2) [CS symmetry], two different Pt‐P bond lengths were calculated (2.356 and 2.366 Å). The mean experimental value was 2.268 Å. The σ‐Pt‐C bonds in (PH3)2Pt(CH2=CH2) were 2.157 Å long. Regarding the PPh3 analog, the bond length was calculated 2.153 Å (mean value of 2.168 and 2.138 Å). Both of these achieved results were overestimated in comparison to 2.112 Å (average) as determined by the experiment [18]. The next aim was to determine the similarity of Pt(0)‐olefin organometallics from olefins coordi‐ nated on metal surfaces. In our study, the vertical distance of platinum and C=C bond in (PH3)2Pt(CH2=CH2) was 2.036 Å, while for ethylene adsorbed in di‐*σ*‐bridge position on the Pt(111) surface, comparable values were obtained. Moreover, regardless of the calculations performed at different theory levels, the bond dissociation energies were 116.8 kJ/mol in the case of organometallic compounds (MP2 value, this study) and 106.1 kJ/mol in the case of C2H4 on p(2x2)‐Pt(111) (generalized gradient approximation (GGA)‐DFT calculation, [15]). Thus, the values were comparable within these arrangements. Additionally, the thermody‐ namics of the complete reaction of molecular oxygen with platinum(0) complex has been examined. It has been recognized that ethylene in the complex of (C2H4)Pt(PR3)2 is substituted by dioxygen producing (O2)Pt(PR3)2. This reaction proceeded well, providing the exposure of (C2H4)Pt(PR3)2 to the ambient air. Having substituted **H** for **R**, the reaction enthalpy was

(*Z*‐1,2‐bis(phenylsulfonyl)ethylene)Pt(PH3)2

(carbon disulfide)Pt(PH3)2

**Table 1.** The calculated parameters of coordination compounds of type (PH3)2Pt (olefin).

(dioxygen)Pt(PH3)2

262 New Advances in Hydrogenation Processes - Fundamentals and Applications

**<sup>h</sup>**Vibrational shift of the C=S stretch and elongation of the C=S bond; **<sup>i</sup>**

determined according to the following schema:

**Various compounds** (vinyltrimethylsilane)Pt(PH3)2

**a**

and elongation of the O=O bond.

(bicyclo[2.2.0]hex‐3(5)‐ene)Pt(PH3)2 **165.3** 461.09 0.1248 – (bicyclo[4.2.1]non‐1(8)‐ene)Pt(PH3)2 **145.8** 261.94 0.0992 –

**<sup>c</sup> 128.3** 463.25 0.0938 29.35

**<sup>e</sup> 74.8** 341.57**<sup>h</sup>** 0.1291**<sup>h</sup>** –

**<sup>f</sup> 194.3** 637.49**<sup>i</sup>** 0.1849**<sup>i</sup>** –

**<sup>d</sup> 118.5** 332.58 0.1140 39.20

Vibrational shift of the O=O stretch normal mode

Even though these hydrocarbon groups contain only alkenic or alkynic functions on the subject multiple bond, the computation disclosed that they retain enough strength upon coordination. It can be deduced that the functions CH=CH2 (vinyl) and C≡CH (ethinyl) apparently favored accepting electrons. Moreover, interrupting the conjugation of multiple bonds is an energy‐ requiring procedure which has an inversely proportional effect on the dissociation energy. Different bond strengths were recorded for vinylacetylene (but‐1‐ene‐3‐yne). Compared to buta‐1,3‐diene or propadiene, either double or triple bond coordinated to the platinum(0) core atom with different bond strengths. As described in preceding computations and assessments, the triple bond arrangement was the ultimately favored constitution. This observation has been also substantiated during the examining of a relatively well‐proceeding substitution reaction of olefinic complexes with substances bearing acetylenic group. It could have been initiated by the triple bond, a stronger electron‐acceptor versus the C=C double bond acting as the substituent. Allene (propadiene) formed robust bonds to Pt(0), accompanied by a high‐bond dissociation energy, and only a subtle wave number drop of C=C stretch (~275 cm‐1) of the bound double bond.

#### *4.1.2. Unsaturated alcohols*

Unsaturated alcohols with −OH group in α‐ or β‐position to the double bond manifested a lower stability in the complex than ethylene. Apparently, the electron donor‐acceptor characteristics associated with the hydroxy group in the complexes are rather shifted toward the electron donor. The decline in C=C stretch wave numbers was comparable to the one determined for the ethylene complex. The hydroxyl group bound in α‐ position to the C=C bond yielded complexes with a lower stability. Other alcoholic complexes, such as hex‐1‐en‐3‐

ol and allyl alcohol (prop‐2‐en‐1‐ol) gave slightly lower values of dissociation energy than for ethylene, suggesting their relative instability. Furthermore, allyl alcohol complexes were described to have been synthesized [19], yet verified only preliminarily by NMR. Neverthe‐ less, this discovery indicates the potential for the production of alike derivatives. Having mentioned the minimal stability of the complex of hept‐1‐en‐4‐ol, a substantial decrease of the C=C stretch wave number was determined as well. The methyl groups in 2‐methylbut‐3‐ en‐2‐ol did not appear to have a noticeable impact on the complexes mainly owing to their pronounced sterical hindering (bulkiness) taking rather an aloof effect. On the other hand, they displayed a slightly higher stability compared to allyl alcohol complexes.

#### *4.1.3. Cyanoethylenes*

**Table 1** depicts the tendency that can be read as the higher the amount of cyano groups on ethylene, the higher the stability of the complexes. Nitrile groups, strongly attracting electrons, supersede the double bond effect of sterical hindrance, and cause the energy level of HOMO orbitals in cyanoethylenes to be decreased in direction to the vacant orbitals of platinum. Previous studies of charge decomposition analysis predominantly indicated that π‐back donation largely contributed to the final production of the platinum‐olefin complex. The methyl group in methacrylonitrile, abundant with electrons, apparently caused the final complex to be less stable than the complex coordinated with acrylonitrile. Regarding 1,2‐ dicyanoethylenes, *E*‐isomer (fumaric acid dinitrile) compared to *Z*‐isomer (maleic acid dinitrile) displayed a relatively higher stability (Δ*E* = 1.3 kJ/mol).

#### *4.1.4. Compounds containing α-carbonyl group(s)*

The length of an alkyl group in ester products of acrylic acid had a significant impact on the electron‐acceptor role of the carbonyl group. Hence, the adjoining Pt‐C bond was relatively strong and unaffected. Fumaric acid esters displayed an analogous drift, that is, diethyl esters were observed to have their stability only marginally lower compared to dimethyl esters. Moreover, it is prudent to take into account any disruption of conjugated bonds in α,β‐ unsaturated compounds. Plain ketones, for example, but‐1‐en‐3‐one, produce complexes with a reasonably high stability. The simplest unsaturated aldehydes are a palpable example, for example, acrylic aldehyde (prop‐2‐en‐1‐al), which retains even a higher stability (i.e., higher dissociation energy). Other products of acrylic acid (amide, methylester) were demonstrated to have an average stability (125.4 and 122.1 kJ/mol, respectively). The highest dissociation energy (154.1 kJ/mol) was found in maleic acid anhydride owing to an intense π‐electrons attraction by the C=C bond and only a minor sterical constrains due to the absence of any massive substituents.

#### *4.1.5. Halogen containing compounds*

Halogenated hydrocarbons present a useful model for the review of donor/acceptor electronic impacts on the C=C double bond in olefins. For example, 3‐chlorobut‐1‐ene produced a reasonably rigid complex, which was noteworthy given its high pyramidalization (33.7°). Perfluorinated ethylenes, when bond to bis(triphenylphosphine)platinum, yielded a consid‐ erably elevated stability compared to perchlorinated products, probably owing to a geometric hindrance of chlorine atoms. Furthermore, carbon‐halogen bonds are evidently more stretched after the coordination in C2F4 than in C2Cl4, feasible due to a reduced conjugation of fluorine atoms with C=C bond. Moreover, trifluoromethyl groups in *E*‐CF3CF=CFCF3 incline to coordinate to a greater stability compared to the products of direct fluoro‐substitution in C2F4, which led to obtaining a dissociation energy among the highest observed for the Pt‐C2 bonding (165.2 kJ/mol). Halogen atoms located in a position influencing the C=C double bond plausibly contribute to producing relatively stable organometallic complexes.

#### *4.1.6. Strained olefins*

ol and allyl alcohol (prop‐2‐en‐1‐ol) gave slightly lower values of dissociation energy than for ethylene, suggesting their relative instability. Furthermore, allyl alcohol complexes were described to have been synthesized [19], yet verified only preliminarily by NMR. Neverthe‐ less, this discovery indicates the potential for the production of alike derivatives. Having mentioned the minimal stability of the complex of hept‐1‐en‐4‐ol, a substantial decrease of the C=C stretch wave number was determined as well. The methyl groups in 2‐methylbut‐3‐ en‐2‐ol did not appear to have a noticeable impact on the complexes mainly owing to their pronounced sterical hindering (bulkiness) taking rather an aloof effect. On the other hand,

**Table 1** depicts the tendency that can be read as the higher the amount of cyano groups on ethylene, the higher the stability of the complexes. Nitrile groups, strongly attracting electrons, supersede the double bond effect of sterical hindrance, and cause the energy level of HOMO orbitals in cyanoethylenes to be decreased in direction to the vacant orbitals of platinum. Previous studies of charge decomposition analysis predominantly indicated that π‐back donation largely contributed to the final production of the platinum‐olefin complex. The methyl group in methacrylonitrile, abundant with electrons, apparently caused the final complex to be less stable than the complex coordinated with acrylonitrile. Regarding 1,2‐ dicyanoethylenes, *E*‐isomer (fumaric acid dinitrile) compared to *Z*‐isomer (maleic acid

The length of an alkyl group in ester products of acrylic acid had a significant impact on the electron‐acceptor role of the carbonyl group. Hence, the adjoining Pt‐C bond was relatively strong and unaffected. Fumaric acid esters displayed an analogous drift, that is, diethyl esters were observed to have their stability only marginally lower compared to dimethyl esters. Moreover, it is prudent to take into account any disruption of conjugated bonds in α,β‐ unsaturated compounds. Plain ketones, for example, but‐1‐en‐3‐one, produce complexes with a reasonably high stability. The simplest unsaturated aldehydes are a palpable example, for example, acrylic aldehyde (prop‐2‐en‐1‐al), which retains even a higher stability (i.e., higher dissociation energy). Other products of acrylic acid (amide, methylester) were demonstrated to have an average stability (125.4 and 122.1 kJ/mol, respectively). The highest dissociation energy (154.1 kJ/mol) was found in maleic acid anhydride owing to an intense π‐electrons attraction by the C=C bond and only a minor sterical constrains due to the absence of any

Halogenated hydrocarbons present a useful model for the review of donor/acceptor electronic impacts on the C=C double bond in olefins. For example, 3‐chlorobut‐1‐ene produced a reasonably rigid complex, which was noteworthy given its high pyramidalization (33.7°). Perfluorinated ethylenes, when bond to bis(triphenylphosphine)platinum, yielded a consid‐

they displayed a slightly higher stability compared to allyl alcohol complexes.

264 New Advances in Hydrogenation Processes - Fundamentals and Applications

dinitrile) displayed a relatively higher stability (Δ*E* = 1.3 kJ/mol).

*4.1.4. Compounds containing α-carbonyl group(s)*

*4.1.3. Cyanoethylenes*

massive substituents.

*4.1.5. Halogen containing compounds*

The C=C bond in strained olefins coordinated to extraordinarily highly stable complexes, as the sp2 ‐carbon atoms apparently well‐sp3 ‐hybridized. The modified compounds were nomi‐ nated based on available information found in the literature illustrating their feasible prepa‐ ration and analytical detection. The complexes of each selected strained olefins revealed high‐ binding energies. Pyramidalization of substituents was assessed; however, unsatisfactorily high figures were obtained. Subsequently, this extent was determined for acyclic substitution on the C=C double bond. Nevertheless, once the complex settled down, the deformation of substituents from the previous position was exceptionally significant within this group of complexes. **Figure 13** illustrates one of the instances: bicyclo[2.2.0]hex‐3(5)‐ene complex.

**Figure 13.** Bicyclo[2.2.0]hex‐3(5)‐ene free form (a, left) and complex (b, right).

#### *4.1.7. Various compounds*

Selected compounds (further elaborated in various publications) contained different types of double bonds in their structure and their stability in the coordinated state was calculated. For example, in the case of organosilicon compounds, particularly vinyltrimethylsilane, CH2=CH‐ Si(CH3)3 delivered average quantities for *EB* and further features assessed, inferring that the electronic properties of silicon atoms induced comparable effects as carbon. On the other hand, *rac*‐*Z*‐1,2‐bis(phenylsulfonyl)ethane [15], yielded surprising data as the obtained binding energy for the coordination of the computed products of (*R,R*‐isomer) was minor compared to ethylene. Nonetheless, the complex was prepared from (PPh3)2Pt(CH2=CH2) by substituting the subject olefins. In order to assess the impact of the racemates, it is advisable to figure out the average of binding energies of all enantiomers. The previous tests revealed that carbon disulfide (CS2) adduct was in a solution of the olefin complex produced with a swift reaction rate, while the calculated binding energy was unexpectedly minimal. The cause could have been the solvent‐stabilization effect in a carbon disulfide solution, since adduct supported the induction of dipoles in a near vicinity of the molecule. The preparation of dimeric polynuclear complexes could be dismissed by means of X‐ray analyses [20]. Following the reaction of olefin complexes with molecular oxygen, dioxygen adduct (O2)Pt(PH3)2 was noticeably produced without a reversible option, yielding by far a complex with the maximum computed stability value as the core metallic atom and its oxidation state assumed the form of Pt(II) [21].
