**2. Model of adsorbed complex: (olefin)Pt(PPh3)2 complex**

The compounds of the type (olefin)Pt(PPh3)2 belong to a group of intensively studied substan‐ ces. One of the reasons for their study are the binding ratios in these molecules. These compounds, are characterized by their structure, which is noticeably similar to the adsorbed state of olefins on the catalyst surface based on transition metals (di‐*σ*‐adsorbed complex). The adsorbed complex, as the key structure of a heterogeneously catalyzed reaction, and its problematic structural characterization in comparison to the characterization of (ole‐ fin)Pt(PPh3)2 complexes, is the reason why the described group of substances has been gaining much interest of "catalytic chemist" despite the fact that the mentioned complexes do not exhibit the catalytic activity in of a homogeneous hydrogenation catalyst. The concept of the similarity of these compounds with the adsorbed di‐*σ*‐complexes was initiated from the following experimental observations:


**Figure 1.** Three‐membered ring (cyclopropane type) formed by coordination of alkene to Pt(0).

fin)Pt(PPh3)2 seems to offer a suitable model of adsorbed surface complexes as the key struc‐ tures in the transformation of a reactant to products. These compounds are platinum organometallic compounds containing coordinated olefins, which in a sense resemble the morphology of the metal surface and chemisorptions occurring on it. Although these cova‐ lent compounds cannot catalyze homogeneous hydrogenation, such as the rhodium analog Rh(PPh3)3Cl, their structure very well approximates the adsorbed state on heterogeneous platinum catalysts. The model of organometallic compounds (olefin)Pt(PPh3)2 represents an attractive alternative for the application to molecular modeling in heterogeneous catalysis. By comparing values with a similar significance (adsorption *vs*. dissociation energy) or the differences in geometric data, very good qualitative (corresponding order of stabilities) and

The compounds of the type (olefin)Pt(PPh3)2 belong to a group of intensively studied substan‐ ces. One of the reasons for their study are the binding ratios in these molecules. These compounds, are characterized by their structure, which is noticeably similar to the adsorbed state of olefins on the catalyst surface based on transition metals (di‐*σ*‐adsorbed complex). The adsorbed complex, as the key structure of a heterogeneously catalyzed reaction, and its problematic structural characterization in comparison to the characterization of (ole‐ fin)Pt(PPh3)2 complexes, is the reason why the described group of substances has been gaining much interest of "catalytic chemist" despite the fact that the mentioned complexes do not exhibit the catalytic activity in of a homogeneous hydrogenation catalyst. The concept of the similarity of these compounds with the adsorbed di‐*σ*‐complexes was initiated from the

**•** The organization of alkenes has significantly altered when coordinated with platinum(0). Unsaturated carbonic atoms, diverging from the plane of the double bond near the platinum atom, cause a deviation identical to the decrease in strength and extending the carbonic

**•** The olefin double bond in platinum(0) complexes diverges in a minor angle from the triangular plane arranged by platinum, midpoints of the olefinic double bond and the

H NMR) spectrum of platinum(0)‐olefin complexes

hybridization, which could be interpreted as a result of the production of

hybridization of the carbon atom

**•** The platinum(0)‐olefin bond appeared firm in the complexes.

revealed that during coordination of olefin, the starting *sp*<sup>2</sup>

three‐membered ring (cyclopropane type) (**Figure 1**).

even quantitative (correlating *E*ads and *E*diss) correlations can be achieved.

250 New Advances in Hydrogenation Processes - Fundamentals and Applications

**2. Model of adsorbed complex: (olefin)Pt(PPh3)2 complex**

following experimental observations:

double bond.

remaining two ligands.

was near the *sp3*

**•** Proton nuclear magnetic resonance (1

The bonding mode in Pt(0)‐olefin complexes has usually been explained by means of a modified Dewar‐Chatt‐Duncanson approach, which assumed two simultaneous and partly mutually dependent interactions of the orbitals of the metal and alkenes, namely donation and back donation (**Figure 2**). The significance of donation can be sufficiently described us‐ ing the classical donation of πC=C alkene electrons to vacant *d*‐orbitals of platinum(0). The back donation has been perceived as the interaction of occupied orbitals of platinum (in the band theory of solids, these were the bands laying below the Fermi energy level) with πC=C\* olefin orbital. The reverse donation has a significant impact on the stability of the complex.

The lower is the electron density on C=C double bond (causing a stronger withdrawal of electrons), the lower is the level of πC=C\* orbital and thus a lower energy difference between πC=C\* alkene orbital and the occupied *d*‐orbitals of Pt. As a result, a stronger interaction occurs between πC=C\* and occupied Pt (*d*) orbitals, which constructs a stronger bond of Pt‐alkene. If a common alkene is described as ethylene‐bearing electron‐acceptor substituents, then the substituents have the tendency to stabilize the complex by shifting highest occupied molecular orbital/lowest unoccupied molecular orbital ((HOMO)/LUMO) energy levels in alkene toward the planes of Pt(PPh3)2. To the contrary, the groups increasing their electron density destabilize the complex to such an extent that the process of preparation could be rendered impossible. **Figure 3** shows the general order of stability in the group of ethylene derivatives bearing typical smaller substituents.

**Figure 2.** Orbital description of the bonds in the olefin-metal models: (1) Dewar-Chatt-Duncanson model derived for organometallic compounds; (2) Newns-Anderson model derived for surface complex substrate-catalyst.

**Figure 3.** General order of coordinated olefins stability.
