Synthesis, molecular and electronic structure of the first homoleptic complex of platinum with a secondary phosphine

ELSEVI ER

InorganicaChimicaActa 264 ( 1997~ 185-191

Synthesis, molecular and electronic structure of the first homoleptic complex of platinum with a secondary phosphine Piero Leoni a.h.. GiuseppinaChiaradonnaa Marco Pasquali a Fabio Marchetti a Alessandro Fortunelli ~'*, Guido Germano a " Dipartimento di Chimica e Chimica Induxtriale. Unive~siti~ di Pi.sa. 1-56126 Pisa. haly "Scuokt Normale Superiore, Piaz2a dei Cavalieri. 1-56126 Pisa. Italy • Istituto di Chimica Quantistica ed Energetica Molecolare. CNR. 1-56126 Pisa, haly

Received20 February 1997; revised 5 May 1997:accepted27 May 1997

Abstract

The reaction of CpPt(rlLC3H5) with PBu'2H in acetone gives the Pt(0) mononuclear derivative Pt(PBu'2H)~ (!). As demonstrated by variable temperature 3;P{ =H } NMR spectra, complex I dissociates one of the phosphines in toluene solution affording the I'4e Pt( PBu'_,H)2. The crystal and molecular structure of I was solved by X-ray diffraction which reveals a trigonal planar disposition of the phosphine molecules (C2.~Hs7P3Pt: hexagonal, space group P6~hn, a = h = 10.4150( I I ), c = 16.263(3) A, Z=2). The P-H h~drogen atoms were located in the Fourier map and were found to lie in the coordination plane, with the t-butyl substituents in an eclipsed configuration. Ab initio and molecular dynamics calculations essentially confirm the structure obtained by X-ray diffraction. An agostic Pt-H-P interaction was excluded and the geometry was explained in terms of steric interactions. © 1997 Elsevier Science S.A. Keywords:

Secondaryphosphinecomplexes:Platinumcomplexes;Homolepticce-mplexes:Crystal structures:Electronicstructure

1. Introduction M

Owing to the tendency o f the P-H bond to oxidatively add to the metal center, secondary phosphine complexes o f late transition metals in low oxidation states are frequently very reactive and may be considered advantageous precursors of polynuclear phosphido-bridged complexes [I I. We are currently investigating the reactivity of some derivatives of palladium 121 and platinum [31 belonging to this latter class. The reactions of CpM( ~¢-C3Hs ) ( M = Pd [ 1,2a 1, Pt [ 3a I ) with PBu*2H represent a convenient access to [ Pd{/lP(t-Bu)2}{PCt-Bu)_.Hll2 and [Pt{/z-P(t-Bu)2}CH){P(tBu)_,H} 12, the precursors o f a series of di- and tri-nuclear derivatives with the metals in the oxidation states + I or + 2 [ 2,3 l- Bo~h reactions were supposed to proceed ( Scheme I ) through the formation of an intermediate homoleptic complex o f the metal with only P(t-Bu)_.H ligands, followed by P-H oxidative addition and dimerization to [M{/z-P(tBu ) 2 }( H ) {P( t-Bu )_,H } ] 2; the latter is stable in the case o f platinum [3a] and reductively eliminates H2 in the case o f palladium I 1,2a]. With suitable modifications o f experimental conditions the mononuclear palladiumCO) complex * Correspondingauthors. 11020-1693/97/$17.00 © 1997 ElsevierScienceS.A, All rights reserved PII SOU20-1693 ( 97 ) 05679- X

3 PBut2H .,

M(PBut2H)3

.~

M = Pt, Pd

- PBut~H

MCPBu~zH)2

+ PBuaH

(1) M = Pt (2) M = Pd

| I

H~-"7~..p-pd-Pd-p-.~

,p, ,t4,~" H

•

-H~

--~ xM M ..~-

ff "P" "~"F" .-/,)~ H

(3) M = Pt (4) M = Pd

CS)

Scheme I. Pd[ PC t-Bu )2H ] 3 was isolated, characterized and found to give quantitatively [Pd{kt-P(t-Bu).}{P(t-Bu)zH} 12 [ 1 ]. We now report the synthesis, spectro~opic and structural characterization o f Pt [ PCt-Bu) zH Is ( 1 ). In the solid state complex 1 assumes a trigonal planar configuration, with the P - H bonds in the coordination plane and the t-butyl in an eclipsed configuration: a theoretical analysis es~ntially confirms the X-my structure and excludes the p~esence a Ft-H

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P. Leoni et aL / hmrganica Chhnica Acta 264 (1997) 185-191

bonding interaction, explaining the structure in terms of steric interactions.

2. Results and discussion As briefly described in Section I and graphically illustrated in Scheme 1, the homoleptic secondary phosphine Pt(0) and Pd(0) dzrivatives M(PBu'2H)3 ( M = P t (1), Pd ( 2 ) ) were supposed to be the key intermediates in the transformation, in the presence of PBu'_~H,ofCpM( 'r/3-C3H5) into the dinuclear platinum ( II ) [ Pt (p,-PButz) ( H ) ( PBut2H ) ] z (3) [3al or palladium(1) IPd(#-PBu'2)(PButzH)I2 (5) [I,2a] derivatives. This was demonstrated in the case of M = P d [ I 1. The assumption that the corresponding platinum reaction follows the same mechanism was, at the time of our first note 13a], only a working hypothesis, although quite reasonable. Our attempts to isolate complex 1 were successful when we operated the same experimental procedure [ I ] which allowed us to isolate its palladium analogue 2. These are: (i) CpPt(r/3-C3Hs) must be added to an excess of the phosphine ligand, this minimizes the concentration of Pt(PBu'2H)~_, which is the real precursor of the dinuclear derivative 3; ( ii ) acetone, where I has a low solubility, was used as the solvent; ( iii ) the product was isolated as a solid as quickly as possible, while keeping the temperature below 0°C. Under these conditions the expected Pt(PBu'_~H)3 ( 1 ) was isolated as a yel!ow crystalline solid in satisfactory yield (see Section 3). Complex I is air-, light- and thermally-stable when in the solid state, in contrast to its palladium analogue, which is photolabile and easily oxidized in the air in the same conditions I I ]. If dissolved in toluene and heated 24 h at 60°C, complex 1 is quantitatively transformed into the dinuclear Pt(ll) derivative 3. When dissolved in C6D~,, complex 1 nearly completely dissociates one of the phosphine ligands to yield Pt(PBut2H)2. in fact, the 3~p{ ~H} NMR spectrum shows a strong singlet at 19 ppm (~Jp)~= 198 Hz from the corresponding proton coupled spectrum ) for the free secondary phosphine, and a strong singlet ( with ~')-~Ptsatellites, d, ~JePt = 4 2 4 0 Hz, ~JPH= 230 from the H-coupled spectrum) at 83 ppm attributable to Pt( PBut_,H ) 2; a weak singlet at 53 ppm ( ~'~-sPtsatellites, d, Ljp~ = 3740 Hz, IJl, H = 330 from the Hcoupled spectrum, intensity ~ 10% of the signal at 83 ppm) was assigned to complex 1. The spectrum clearly shows that the equilibrium settles slowly compared to the NMR time scale. As can be predicted, and as was shown before, the corresponding equilibrium for Pd( PBu'~H )~ is much faster, and only a broadened average resonance was observed in the same conditions for the three phosphorus containing species. These gave well separated signals only at low temperatures

2. I. Co,stal and molecular structure o f 1

The molecular structure of I (ORTEP projection in Fig. I, bond distances and angles in Table I ) presents a C3h symmetry in the solid state, with the metal, P and P-bonded H nuclei lying on the coordination plane. The phosphine molecules bear a t-butyl substituent above and the second, eclipsing the former, below the mirror plane. The P-Pt-P bond angles are those expected for an undistorted trigonal planar PtP 3 core. Pt-P distances were found at 2.281 (2) ,A; to the best of our knowledge this is the first crystallographic determination of a homoleptic metal complex of a secondary phosphine and this prevents homogeneous comparisons. The Pt-P distance is however dose to the range of those observed in the slightly distorted Pt(PPh3) 3 ( in the range 2.262-2.27 I ,~) [4]; as expected, it is sensibly longer than those found in the dicoordinated Pt(PCy3)2 (2.231(4) ,A) I51 or Pt(PPhBu'2)_, (2.252(I) ,A) I61, and shorter than in the tetracoordinate Pt(dpfpe) z ( 2.284( 2)-2.293 ( 2 ) ,A, dpfpe = 1,2-bis(dipentafluorophenylphosphino)ethane) [7] and Pt(dppp)2 (2.286(I) .A, d p p p = 1,3-bis(diphenyl phosphino) propane ) 18 I. As shown in Fig. I, the methyl groups exhibit relatively high thermal parameters, consistent with a vigorous motion occurring in the crystal at room temperature. Another explanation, which is supported by the theoretical analysis (see next section) and does not exclude the former, is that some degrees of di~;order in the crystal packing are due to the presence of severat slightly different conformations. The P-H hydrogens were located in the Fourier map at a P-H distance of 1.55 A, and a Pt...H distance of 3.22 A. 2.2. Theoretical analysis

From the X-ray analysis in the preceding section, we have seen that complex 1 adopts a solid state molecular structure where the three P-H bonds lie in the PtP3 coordination plane. Keeping in mind the tendency of P-H bonds to oxidatively add to an electron-rich center, an obvious question arises: is this particular conformation induced by an agostic Pt-H-P interaction, the step preceding the P-H oxidative addition? We had no clear evidence of such an interaction from the

C(3)

c¢~2c(1) ~-/r" l , ~ N

(-70~C) I )1. Elemental analyses were in safe accord with structure 1, which was then confirmed by a single-crystal crystallographic study.

Fig. I. Perspectiveviewof the molecularstructureof PI( PBu'~H) +.Thermal ellipseidsare at 50~ probability.The methylhydrogenshavebeenomiued tk)r¢larily.See Table I for explanationof primes.

187

P. L e o n i e t aL / I n o r g a n i c a C h i m i t ' a A c t a 2 6 4 ( 1 9 9 7 ) 1 8 5 - 1 9 1

Table I Experimentalbond lengths (A) and angles I°) for 1 ( the correspondingcalculatedvalues are shown in squarebrackets) J Pt-P P--C( I ) Pt"-H-P Pt...H-C(3) P'-Pt-P

2.28112) 12.281 h 1.87419) [ 1.9521 3.22( I ) [3.2101 3.1214) 13.1361 120 11201

C( I l--C(2) C( I )-C(31 C( I )-C(4) P-H C121-C11 i-CI3)

1.497t 131 11.3431 1.527114) [ 1.5421 1.532113) [ 1.5481 1.55 I 1.4301 107.61 I01 [ 108.81

C( I ) - P - C ( I " )

110.716) [ II 1.41

CI2)-C( I )-C14)

11)9.619) 1109.5 I

C( I )-P-Pt

116.3(3) [ 116.61

C(3)-C( I l-C(4)

106.5(I0) 1107.61

C(2)-C( I )-P

113.0(8) I II I.I I

C13)-C( I )-P C(4I-C( I )-P

105.016) [ 105.11 114.718) [ 114.41

"Symmetry,transformationsused to generateequivalentatoms: ' = h Imposedequal to the experimentalvalue.

I - x + y , I - x , z; " = I - y . x - y . z: " = ~. y . I / 2 - z.

solution NMR spectra, but we have seen above that complex I is largely dissociated in solution, where, in practice, we observed the spectra of the dicoordinated species Pt(PBut,H) z- The P - H hydrogen was located quite far from the Pt center (3.22 ~,), but the uncertainty in the crystallographic location o f hydrogen atoms close to heavy metal centers is well known. A deeper insight into this matter was furnished by a theoretical study. Ab initio molecular orbital and empirical molecular dynamics ( M D ) calculations were performed, aimed at answering these two questions: I. which is the minimum energy configuration o f complex 1 in vacuo and how does it compare with the experimentally determined crystal structure; 2. what is the nature o f the interaction between the Pt atom and the P - H bond in the ground state. In order to answer question I, three different ab initio geometry optimizations were performed. In the first we determined the gas phase structure of the simplified model complex Pt(PMe2H) 3, with Me groups replacing the t-butyls; the lowest energy conformation was shown to be the C3h one, with the three P - H hydrogen atoms lying on the PtP 3 coordination plane, which corresponds exactly to the experimental c ~ s t a l structure of 1. The complete complex I was inspected in the second optimization. The lowest energy conformation resulted slightly distorted from C 3 h and had only C 3 , symmetry, with

the P - H hydrogens slightly displaced out of the PtP3 plane: the dihedral angle H - P - P t - P assumed a final value o f 7.2 °. Last, we performed another geometry optimization on complex ! imlmsing a higher C+J, symmetry by fixing the H - P P t - P dihedral angle to 0, thus constraining the F~-H hydrogens on the PtP 3 plane. The energy difference between the last two structures resulted low, only 0.00228 Hartrees = 1.43 kcal mol - ~, and all bond distances and angles were practically the same, with differences below 0.5% for bond distances and below 3% for angles (see Table I ). For comparison, we repeated this last calculation with the Pd homologuc o f complex 1. With the obvious exception o f the metal-phosphorus bond lengths, all distances and angles were nearly the same as in complex 1. Due to this high similarity between the two structures we did not pursue further the Pd track. These calculations show a clear preference o f Pt(PRzH) ~ ( R = M e , Bu t) toward a planar configuration, with the R substituents in eclipsed conformation ( Fig. 2). The enef~,etic preference is however modest, and the steric hindrance o f the t-butyl groups is sufficient to slightly distort the structure to small but non-null values o f the H - P - P t - P dihedral angle. If such a small difference is considered significant with respect to the experimental and theoretical accuracies, the fact that the X-ray data indicate nevertheless a C3h symmetry for complex 1 can be attributed to: (i) packing effects in the crystal, since our calculations were done on a single molecule

Fig. 2. Top view of the experimental(a I and computed(b) structuresof complex I. The P-H hydrogensaxe markedwith a dot. The eclip~d conformationof the t-butylsis clearly visible.The root mean squaredeviationbetweenthe cartesiancoordinatesof the heavy atoms of the two structuresis 0.06 A.

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P. Leoni et aL / huJrgunica Chhnica Acta 264 ( 1997; 185-191

isolated in vacuo: or (ii) an averaging of various slightly distorted C~, structures with the P-H atoms pointing up or down, even within the same molecule, as can be seen from the MD simulations. This second hypothesis could explain the high thermal parameters found in the crystallographic determination, in any case. all distances and bond angles compare nicely with the experimental ones. the mean square root deviations being respectively 0.05 ,A and 0.2 °. In this regard, it is to be noted that the theoretical approach employed in the ab initio calculations has the only remarkable shortcoming of yielding too long a bond distance between Pt and P atoms (see Ref. 19] for a thorough discussion of this item and Section 3.4). Indeed, in complex Pt( PMe2H)3 we found an equilibrium Pt-P distance of 2.43 ,~. versus an experimental value of 2.28 ,~ in complex 1. Therefore we constrained the Pt-P and PdP distances to their experimental values of 2.28 and 2.33 ,~ [ 21 in all subsequent calculations. The X-ray diffraction structure being essentially cokefirmed, we turned to the question whether one can assume t,he existence of a bond interaction between the P-H hydrogens and the metal center. A Mulliken bond population analysis on the C3~,optimized I yielded a bond order of only 0.025 for H-Pt, For comparison, the bond orders were 0.35 for H-P, 0.13 for P-Pt, 0.25 for P C , 0.21 for C-C and 0.38 for C-H ( the same numbers were obtained for the C3, structure, and similar ones for Pt(PMe2H)3). A possible Pt-H-C agostic interaction with the/3-hydrogen of the t-butyl on each phosphine which goes closer to the metal (found experimentally at 3.12(4) A. and at 3.14 ,~ in our calculations) was also excluded: the bond order between Pt and the closer t-Bu proton was in fact found at 0.0032, one order of magnitude less than for H-Pt. Further insights into the bonding interactions among the Pt, P and H atoms can be gained from an analysis of the total electron density in the Pt-P-H plane. In fact, it has been shown 110] that a maximum in the total electron density along directions roughly orthogonal to the lines connecting two nuclei is a characteristic feature of chemical bonds. In our case, an inspection of the total electron density in the PtP-H plane of complex I ( Fig. 3) reveals local maxima along the axes perpendicular to the H-P and P-Pt but not the H-Pt connecting segments: i,~ the latter case, the density steadily decreases along the axes. These facts lead us to the conclusion that there is substantially no bond between the P-H hydrogens and the Pt center. leaving the steric repulsion between the bulky Bu t groups as the principal responsible for the conformation assumed by complex 1. The absence of a specific metal-hydrogen interaction i~l complex 1 and its Pd homologue is supported also by the great similarity of their theoretically optimized structures, in spite of the different affinities of the two metals toward hydrogen. However, a careful inspection of the electron isodensity lines in Fig. 3 shows a weak interaction of the Pt center with the P-H bond: the ridge corresponding to the

$ ~~

~

.°t

Fig. 3. Contourplot of the total electrondensityof comple× | in the Plp~ plane The density holesaround the P! and P nuclei are due to the use of pseudo-potemials,which allow the core electronsto be neglectedin Ihe calculation. local maximum of the total electron density is slighdy displaced from the P-H connecting line towards the Pt cer~,'er. These considerations have been confirmed through MD simu!ations with an empirical force field. The ab ini~io geometries could be reproduced employing force constants in the potentials associated with torsions around the P-Pt bond much smaller than required for an agostic Pt-H-P interaction. We carried out simulated annealing on complex l, first ramping the temperature up to over 1000 K, then running u variable number of picoseconds of dynamics at constant temperature, and finally cooling down again to retrieve the minimum eaergy structure. The ligands oscillated wildly around the Pt-P bonds, but eventually recovered an equilibrium conformation very similar to the C~, ah initio one, also when the energy barriers of the torsional potentials depending on the P-Pt-P-H and P - P t - P - C dihedrals were set equal to zero. Sometimes the system settled down in local minima (only 1.3 kcal tool-~ above the absolute minimum), where one P-H hydrogen atom pointed in the opposite direction to the other two with respect to the PtP3 plane; the deviation of the P-H hydrogens from the coordination plane was always within 8°.

3. Experimental 3. I. G e n e r a l d a m

All manipulations were carried out under a nitrogen atmosphere, by using standard Schlenk techniques. TransPtCI2(SEt) 2 was prepared as previously described 1111.

189

P. Leoni et al. /Inorganica Chimu'a Acta 264 11997) 185-191

Solvents were dried by conventional methods and distilled under nitrogen prior to use. IR spectra (nujol mulls, KBr) were recorded on a Perkin-Elmer FT-IR 1725X spectrophotomcter. 3~p NMR spectra were recorded on a Varian Gemini 200 BB instrument at 80.95 MHz; frequencies are referred to 85% H3PO4. 3.2. Preparation o f Pt( PBut,H)~ (1)

Table 2 Crystal data and structurerefinementfor I Empirical formula Formula weight Temperature ( K ) Wavelength ( A ) Crystal system Space group Unit cell dimensions a(A)

An Et20 !.642 M solution o f CH2=CHCH2MgBr ( 1.2 I ml, 1.99 mmol) was dropped at 0°C into a yellow solution o f trans-PtCI,_(SEt)_, ( 887 mg, 1.99 mmol); a colorless solid precipitated out in a few minutes and was filtered off. A THF ( I 0 ml) solution containing freshly prepared CpNa (2.0 mmol) was slowly added to the filtrate, while keeping the temperature at - 5 0 ° C , the mixture was stirred 30 min at -50°C, and NaCI was filtered off. A small portion of the filtrate was evaporated, the residue was dissolved in C6D~, analyzed by tH NMR spectroscopy, and confirmed to contain only CpPt(r/LC3Hs). The remaining solution was evaporated, the residue was dissolved in acetone (15 ml) and slowly added at O°C to an acetone (5 ml) solution o f PBn',_H (7.95 mmol). A yellow microcrystalline solid precipitated out and was filtered, washed with acetone and vacuum dried ( 361 mg, 29% with respect to trans-PtCI2( SEt )_, ). Anal. Calc. for C24Hs7P3Pt: C, 45.5; H, 9.07. Found: C, 45.1; H, 9.18%. IR (nujol, KBr): 2228 s (~'PH) cm -~- See Section 2 for 3~p{ ~H } NMR spectra. 3.3. X-ray structure determination

A light yellow crystal of 1 ( { I 0 O} hexagonal prisms closed by { I 0 1 } hexagonal bipyramids) was glued at the end o f a glass fiber. Data were collected on a Siemens P4 automatic single crystal diffractometer, using graphitemonochromated Mo K a radiation. Cell parameters were calculated on the accurately centered setting angles of 25 strong reflections with 1 2 . 4 < 0 < 13°. Details about the crystal parameters and intensity data collection are smnmarized in Table 2. A redundant set o f data was collected to estimate the collection accuracy. Three standard reflections every 97 measurements showed no decay in the crystal. The possible space groeps on the basis o f systematic absences were P63 or P63/m. T h e collected intensities were corrected for Lorentz and polarization effects and for absorption by the ~-scan method [ 121. The equivalent reflections were then merged, resulting in an internal R value ( R ~ . , = E I Fo-"- Fo'-(mean) I / E ( F o : ) ) o f 0.049. The structure was solved in t h e / 6 3 space group by the automatic Patterson method contained in the SHELXTL [ ! 3 ! program which located all the non-hydrogen atoms o f the asymmetric unit. The methyl hydrogens were introduced in calculated positions, whereas the P - H hydrogens were found in the difference Fourier map. The leastsquares refinement, with anisotropic thermal parameters for non-hydrogen atoms and methyl hydrogens riding on the connected carbon atoms, gave an R factor of 0.052 (94

c(A) a(°) /31 °) 71 ° )

Volume I A' Z Density (ealcA (Mg m - ~) Absorptioncoefficient( mm ~ F( 000 ) Crystal size ( mm ) 0 Range for data collection ( ° ) Index ranges Reflectionscollected Observedreflections1k~.> 4o,(F,) ) Independentreflections Absorptioncorrection Max. and min. transmission Refinementmethod Data/restraints/pararneters Goodness-of-fiton F-" Final R indices ( I> 2o-1I) R indices (all data) Largest differencepeak and hole l e a -~)

C:~HsTP~ 422.46 293f2) 0.71073 hexagonal P6dm 10.4150111 ) 10.4150111 ) 16.26313) 9O 9O 120 1527.714) 2 1.378 4.758 648 0.56×0.12×0.12 2.26-29.99 - I _
3952 1257 1542 ( R( int ) = 0.0491 ) semi-empiricalfrom 4t-.seams 0.2889 and 0.2353 full-matrixleast-squareson F-" 1542/0/48 1.168 R~=0.0521. wR_,= 0.1371 Rs =0.0704. wR,_=0.1466 3.388 and - I. 175

R~=5"IIF,,I-IF, III'Y'IF,,I; wR:={w(F,,'--F,'-)Zlw(E,:):l~"-: w= 1/I0-ZIF,, -~) + 10.0704Q)-'+2.63QI where Q= ltrkxx(F,,'-.O)+2F~'-]/3. goodness-of-fit= [ w(F,,'-- F~'-)'-I ( N - P) I i/'-. where N and P are the hUmhers of observationsand parameters,respectively.

parameters). The refined molecular model showed only approximately a C3h symmetry. A mirror plane was then introduced and the refinement was attempted in the P 6 3 / m space group. The disappearance o f high correlation factors among the parameters, which were reduced to 48 with no increase o f the R factor, made this option preferable. Ti2e residual electron density after the last refinementcycle shows four peaks greater than 1.0 e ,~- 3. Three are clearly due to an insufficiently good description o f the thermal motion of the heaviest atoms: Pt and P. The fourth and larger peak (3.39 e A -3) is near to the phosphine H atom on the P t - P - H plane. We have considered the possibility that this peak can be explained by the presence o f a water molecule, occasionally present near to the phosphine, and we have observed that, refining an oxygen atom with an occupancy factor 0.5 in this position, a "normal" thermal factor is obtained for it together with a reduction o f the R factor. The adoption o f this solution, however, conflicts with at least four observations: (i) the distance H P - O is too short,

191)

P. Leoni et al / I ot~,anica Chimica Acta 264 (1997) 185-191

Table 3 Atomic ct~3rdinates (×10 ~) and equivalent isotropie displacement paramete~ I A:× 10~) lbr l

Pt P CI 1) C( 2 ) CI3~ C14~

x

y

z

U,.,~"

b667 851613 ~ 96951'0) 10621 ( 13 ) 8598( 12) 10678( 15)

3333 575113) 641519} 571X)( 15 ) 5938( 14~ 81031 II )

25110 2501) 3448(6) 3568( 9 ) 4161(6) 35121101

411 I ) 45(l) 62(2) 9314 ) 88(3) 11115~

-' u.,~ is defined as one third of the trace of the onhogonalized U, tensor. 1.095 ,~: ( ii ) w h e n the new "atom" is included, the R factor become significantly lower than the internal R calculated on the equivalent reflections: (iii) a P - H . . - O hydrogen-bond interaction is unlikely for the low electronegativity o f the p h o s p h o r u s atom and, to the best of o u r knowledge, unprecedented in related systems: ( iv ) the presence of water is not confirmed by the results of elemental and spectroscopic analyses. Moreover the intensity of the peak falls if the refinement is made with partial sets of reflections at higher 0. For instance the structure refinement with the 831 unique reflections having sin 0 greater than 0.385 gave a m a x i m u m residual peak of only 0.65 e ,~.- 3. We therefore believe that this residual p e a ~ although high, is the result o f an artifact of the refinement procedure or is possibly due to an unsufficiently reliable absorption correction. The final reliability factors arc s h o w n in Table 2; final fractional atomic coordinates are s h o w n in Table 3. See also Section 4. Molecular d r a w m g was done with the O R T E P I I 1 141 program. 3.4. C o m p u t a t i o m d details

We used Gaussian 94 [ 15] tbr the ab initio calculations and A m b e r 4.1 [ 16] with the 1995 Cornell et al. force field [ 17] for the M D part. There are very few standard basis sets and no force field parameters for the Pt atom. This created s o m e difficulties, limiting the choice of the basis set, and compelling us to develop most force field parameters by ourselves '. We chose the L a n L 2 D Z basis set [ 18], which uses pseudopotentials and incorporates L-S averaged relativistic effects needed for the heavy Pt atom. The lack of polarization functions in this basis set and o f Darwin and mass polarization relativistic corrections in the pseudopotentials has the major drawback of producing [9] an overestimated P t - P bond distance. Moreover, Gaussian 94 does not support a Bader bond population analysis [ 101. so we replaced it with the Mulliken one, nor analytic second derivatives of the ab initio The charges and parameters of the Iorec lield are available on hltp:// www.dcci.unip /~germano/ptphosph ne.htm and, upon request, from alex (~hal.icqem.pi.cnr.it

energy, so we calculated them numerically on the smaller Pt( PMe2H )3 model complex. We used these second derivatives to confirm the hypothesis that the energy barriers associated with torsions around the P t - P bond are fully negligible. We checked that the L a n L 2 D Z basis set is nearly equivalent to the 6-31G* one for the purpose of fitting the RESP [ 191 charges needed in conjunction with the Cornell et al. tbrce field.

4. Supplementary material Crystallographic data have been deposited ( C I F file) with the Cambridge Crystallographic Data Centre ( C C D C L deposition n u m b e r 100620.

Acknowledgements C N R ( R o m e ) and Ministero dell'Universir~ e della Ricerca Scientifica e Tecnologica ( M U R S T ) are gratefully acknowledged for financial support.

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