Summary:  Hexachloroplatinic acid reacts with n-butanol to give the but-2-ene complex H[PtCl3(η2-C4H8)] (3) as intermediate, which reacts with trimethylsilyl-substituted acetylenes R‘C⋮CSiMe3 (R‘ = H, Me, SiMe3) with cleavage of the silyl groups to form platina-β-diketones [Pt2(μ-Cl)2{(COR)2H}2] [R = Me (2a), Et (2b)]. The constitution of 2 was determined by microanalysis, NMR (1H, 13C) spectroscopy, IR, Raman, and mass spectroscopy. The structure of 2a was determined by X-ray diffraction.

The entire catalytic cycle of the 1,4-polymerization of butadiene has been theoretically studied according to the π-allyl-insertion mechanism. This has been performed using density functional theory (DFT) with cationic butenylbis(ligand) and neutral dimeric butenyl complexes as the catalyst. The calculations give a clear insight into the kinetic and thermodynamic control of the catalytic activity and cis-trans selectivity as well as into the elucidation of the stereoregulation mechanism. The supposed π-allyl-insertion mechanism was supported in all essential features by this research. The stability and reactivity of different isomers of η 4 -butadiene π-complexes was calculated to be very similar, regardless of the donor-acceptor ability of the neutral or anionic ligand. The thermodynamically more stable syn-butenyl forms are also more reactive than the anti counterparts. The intrinsic reactivity diminishes while the ligand's donating ability increases. The favored pathway proceeds in an exothermic process as follows: starting from stable syn-butenyl η 4 -cisbutadiene complexes, followed by a required ligand conversion via prone butadiene transition states, and subsequently anti-syn isomerization of the actual anti insertion product to a new transoid C 4 unit in the polymer chain. The polymer chain should not have any stereoselectivity in the methylene groups. Alternative pathways (e.g., via anti-butenyl prone butadiene transition states, thus forming a cis-1,4 polymer, or the direct generation of trans-1,4-products by inserting trans-butadiene) are strongly unfavored by higher kinetic barriers. The rate-determining step is the cis-butadiene insertion for the neutral complexes and the anti-syn isomerization for the cationic complexes. To achieve a well-balanced description of both thermodynamic and kinetic control of trans-1,4-polymerization of butadiene, a careful modeling of the organophosphorus ligand's basicity was necessary. † Martin-Luther-Universitä t Halle-Wittenberg. ‡ a 4s-SS was chosen as the reference point in each case; numbers in parentheses include the zero-point correction while those in italics are the Gibbs free energies. b The activation energy relative to the corresponding isomer of 4. c The activation energy relative to 4s-SS. Figure 7. Different configurations and modes of coordination of the butenyl anion (R, organyl, f.i., the growing polybutadienyl chain).

The 1,4-cis polymerization of butadiene according to the π-allyl insertion mechanism has been studied theoretically by density functional theory (DFT) for the ligand free cationic butenylnickel(II) complexes [Ni(C 3 H 5 )(C 4 H 6 )] + , I and [Ni(C 3 H 5 )(C 4 H 6 )(C 2 H 4 )] + , II. DFT energy profiles have been determined for the insertion of s-cis-butadiene into the (η 3 -butenyl)nickel-(II) bond in the supine and prone orientations of the reacting ligands. The primary goal of this study aims to show that the insertion of cis-butadiene into the nickel(II)-allyl bond can occur within the π-coordination of the reacting parts which is characterized by an insertion barrier that should make the process feasible. Due to the lack of coordinative saturation of nickel(II) in the simpler model I, the insertion was calculated to be endothermic, and no clear difference between the supine/prone arrangements was apparent. The influence of the next double bond of the growing polymer chain for an adequate description of the geometrical aspects, as well as reliable energetics of the insertion, was demonstrated by II. The insertion was calculated to be exothermic by 11.6 kcal/mol for supine and 17.3 kcal/mol for prone, while the activation barrier was estimated to be 26.4 kcal/mol for supine and 3.9 kcal/mol for prone. Preference is given to the prone orientation in kinetic and in thermodynamic control.

ABSTRACT:The structure of ethylene, allyl, and butadiene -complexes with Ni 0 2q Ž . and Ni were optimized with density functional theory DFT . The different kinds of coordinative interaction between the olefins and the metal had been discussed for the frontier orbitals of the given symmetry. Both components of the donation and back-Ž . donation bond were characterized by charge density analysis CDA and natural bond Ž . orbital NBO populations. In case of the butadiene ligand the s-cis and s-trans conformer and the transition state of their rearrangement were localized and characterized by their Ž . vibrational spectra. The calculated ionization of bis -allyl nickel can be well represented by comparing the DFT energies of the neutral molecule and its cationic configurations Ž .

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