was monitored by an in-stream thermocouple which had been previously calibrated against a second thermocouple in the sample position. Automatic peak search and indexing procedures yielded the monoclinic reduced primitive cell The final cell parameters and specific collection parameters are given in Table 7 .The 4143 raw intensity data were converted to structure factor amplitudes and their esd's by correction for scan speed, background, and Lorentz and polarization effects. No correction for crystal decomposition was necessary. Inspection of the azimuthal scan data showed a variation Imln/Imar = 0.75 for the average curve. An empirical correction for absorption, based on the azimuthal scan data, was applied to the intensities since it was not possible to accurately measure the sample crystal. Removal of systematically absent and redundant data left 3775 unique data.The structure was solved by Patterson methods and refined via standard least-squares and Fourier techniques. In a difference Fourier map calculated following refinement of all non-hydrogen atoms with anisotropic thermal parameters, peaks corresponding to the expected positions of most of the hydrogen atoms were found. A difference Fourier map calculated after inclusion of all other hydrogen atoms clearly showed the position of the hydride ligand attached to the metal. All hydrogen atoms were then allowed to refine with isotropic thermal parameters. A secondary extinction parameter was refined in the final cycles of leastsquares. The final residuals for 267 variables refined against the 3100 data for which F2 > 3 1 P 1 were R = 2.04%, wR = 2.53%, and GOF = 1.356. The R value for all 3775 data was 3.49%.The quantity minimized by the least-squares program was w(lFollFC1)* where w is the weight of a given observation. The p factor, used to reduce the weight of intense reflections, was set to 0.025 in the last cycles of the refinement. The analytical forms of the scattering factor tables of the neutral atoms were used, and all non-hydrogen scattering factors were corrected for both the real and imaginary components of anomalous dispersion. Inspection of the residuals ordered in ranges of sin ( B ) / X , IFo[, and parity and value of the individual indices showed no unusual features or trends. The largest peak in the final difference Fourier map had an electron density of 2.14 e-/A3 and was located only 0.74 A from the iridium atom. All other peaks in the final difference Fourier map had densities of less than 1 e-/A3. The positional parameters of the atoms and the estimated standard deviations are given in Table 8. Structure factor amplitudes were provided as supplementary material in the preliminary comm~nication.~ Acknowledgment.Abstract: The transmetalation reaction of trimethylaluminum with the palladium chloride dimer Pd2C1,(p-dppm), (1) (dppm = bis(dipheny1phosphino)methane) at -78 O C gave an intermediate, Pd2C1Me(p-dppm), ( 2 ) , which disproportionated at -10 OC to yield the trans-face-to-face palladium dimer Pd2C12Me2(p-dppm), (3) and a palladium ...

Ligand substitution reactions on square-planar platinum (11) complexes of the types trans-[PtRXL,], trans-[PtR (4-PADA)L2] [BF4], trans-[PtRX (CL) and trans-[PtR(4-PADA)(cL)][BF4] ( R = H . Me; X=Cl-. I-; L = PEt,, bis(3-trifluoromethyl-pheny1)benzylphosphine (4), C L = the trans-spanning 2,ll-bis [bis (3-trifluoromethylphenyl)phosphinomethyl]benzo [clphenanthrene (3); 4-PADA (= pyridine-4azo-4'-(N, N-dimethy1)aniline) have been studied at 30" using stopped-flow and conventional spectrophotometry, methanol as solvent, and 2.5 x lo-* M ionic strength (LiC104 as supporting electrolyte). 4-PADA was used as indicator ligand, as its absorption spectrum differs from those spectra where it is complexed.The expected steric effects of the bulky ligands, especially of 3, on the rates and mechanisms of all the reactions studied are small. All reactions take place by the usual two-term rate law. Noteworthy, for the complexes with the bulky ligands 3 and 4. the direct reaction path with the entering nucleophile is predominant. There is no preference for a solvent or dissociative path. The reactivity order for the hydride complexes is trans-[PtHX (PEt3)2] < r r a n~-~t H X ( 4 )~] < tmns-ptHX (3)]. However, for the corresponding methyl complexes, there is some retardation by ligand 3. probably due to an interaction between the methyl group and the hydrocarbon moiety of 3, which inhibits the fluxional behavior of this ligand. The results have some relevance for the understanding of olefin-insertion reactions of hydride complexes containing these phosphine ligands.

ChemInform Abstract trans-Complexes of the type PtMeX(P1-P1) such as (I) and (III) as well as PtHX(P1-P1) such as (IV) and (VI) are prepared. Their decompositions to the corresponding trans-hydride complexes (II) and (V), respectively, are studied. Some binuclear complexes are also obtained. Thus, the hydride-bridged one (IX) can be synthesized by reaction of the methanol analogue (VIII) with sodium format (VII) as shown. The hydridomethyl-bis(phosphine) complex (XII) is prepared via the γ-picoline analogue. The structure of the analogous diphosphine complex (XIII), substitution product from the bis(triphenylphosphine) analogue, is determined by X-ray analysis (space group P21/c, Z=4).

A general synthetic route has been developed to the air-stable complexes [(PR3)2Pt(M-H)2PtY(PR'3)2]+

6105kcal/mol at 300 K). This change is small compared with the calculated solvation enthalpy (-23 kcal/mol) at infinite dilution.1°Comparison of the internal entropy differences of the conformers in solution shows that the values are of the same magnitude as those found in vacuum (see Table I). The solvated polyproline conformation appears to have somewhat less freedom than the a-helical conformer, which is very similar in entropy to the C7eq conformer.This study of a peptide model system has demonstrated that molecular dynamics simulations can be used to estimate the configuration entropy differences in conformational transitions and in solvation processes. Such internal entropy changes have to be included in any calculations of the free energy differences between Acknowledgment. We thank B. Brooks, M. Pettitt, W. Reiher, P. Rossky, and S. Swaminathan for helpful discussions. Part of the calculations reported here were performed at the Rowland Institute and Los Alamos National Laboratory. Registry No. CH,CONHCHCH,CONHCH,, 227 15-68-0. ~~ ~~~ (12) Mezei, M.; Mehrotra, P. K.; Beveridge, D. L., private communication. (13) Pettitt, M., Karplus, M., unpublished results. Dinuclear elimination of groups from two different metal atoms involving the formation of metal-metal bonds or bridged metal ( I ) For several examples, see: Collman, J. P.; Hegedus, L. S. "Principles (13) Puddephatt, R. J.; Azam, K. A,; Hill, R. H.; Brown, M. P.; Nelson, C. D.; Moulding, R. P.; Seddon, K. R.; Grossel, M. C. J. Am. Chem. SOC. 1983, 105, 5642. J = 11 Hz, 1 H). The coupling between the hydride and the methyl protons (14) ' H N M R (CD2C12, -25 "C) 5 -0.49 (t, J = 4 Hz, 3 H), -10.92 (t, could not be resolved. (15) Tatsumi, K.; Hoffmann, R.; Yamamoto, A,; Stille, J. K. Bull. Chem. SOC. Jpn. 1981, 54, 1857.( 1 6) For a related Pt complex containing a pdppm and a q'-dppm ligand, see: Braunstein, P.; de Meric de Bellefon, C.; Lanfranchi, M.; Tiripicchio, A.

Die Reaktion einer äquimolaren Menge von AlMe3 mit dem Dimeren (I) führte über ein nicht‐isoliertes Intermediäres (II) (1H‐ und 31P‐NMR‐Nachweis) und dessen Disproportionierung zu (IV) und dem Pd(O)‐Komplex (V), die durch Reaktion mit CH2Cl2 den Brückenkomplex (VI) bilden.

No abstract

Kinetische Daten der Ligandsubstitutionsre‐ Å aktionen der quadratisch‐planaren trans‐PtUU‐Komplexe (l ) und (II) (einzähnige Pyridin‐N‐Koordination von L′ ) in Methanol bei 30°C werden mit Hilfe der Spektrophotometrie ermittelt und im Hinblick auf die sterischen Einflüsse der P‐Liganden L′ und auf die Mechanismen diskutiert.