Intramolecular and Intermolecular Bonding in Crystalline Clusters of

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Organometallics 1995, 14, 5350-5361

5350

Intramolecular and Intermolecular Bonding in Crystalline Clusters of the Type (CpR)sMs(CO)s[M = Co, Rh, Ir; CpR = C6&, C5Me5, CsHmel Dario Braga* and Fabrizia Grepioni Dipartimento di Chimica G. Ciamician, Universita di Bologna, Via Selmi 2, 40126 Bologna, Italy

Hubert Wadepohl* and Stefan Gebert Anorganisch-Chemisches h t i t u t der Universitat, 69120 Heidelberg 1, Germany

Maria Jose Calhorda" Znstituto de Tecnologia Quimica e Biolbgica, R . da Quinta Grande 6,2780 Oeiras, and Instituto Superior Ticnico, Lisboa, Portugal

Luis F. Veiros Centro de Quimica Estrutural, Instituto Superior Ticnico, 1096 Lisboa Codex, Portugal Received June 1, 1995@ The relationship between molecular and crystal structures of (CpR)3MdC0)3clusters [M = Co, Rh, Ir; CpR = C5H5, C5Me5, C5H4Mel has been investigated by a combined use of

extended Hiickel molecular orbital calculations, empirical atom-atom painvise packing potential energy calculations, and computer graphics. The indenyl derivative (Ind)3Ir&CO)3has also been investigated. The molecules present several isomers of similar energies, and the number of bridging carbonyl groups decreases along the family cobalt, rhodium, iridium, as the metal-metal antibonding character of the orbitals populated in consequence of their formation increases. Hydrogen-bonding networks of the C-H- - -0 type involving the carbonyl oxygen atoms have been detected in all of these crystalline complexes. Facial and edge bridging CO's are observed to form preferential interactions with respect to terminal ligands. The intermolecular and intramolecular interactions in each case are discussed and compared in crystal isomers and polymorphs.

Introduction The molecular structure features of flexible organometallic molecules in the solid state depend on the subtle interplay between intramolecular interactions (the bonding between atoms and the steric interaction between nonbonded atoms within the molecule) and intermolecular interactions (the "bonding" between molecules responsible for crystal structure stability and cohesion).l It is often difficult to grasp how the process of global energy minimization, which leads to the ultimate isolation of a molecule in its crystalline form(s), determines the molecular and crystal structure features. When the molecule is structurally nonrigid,2 viz. when some geometric or bonding features at the molecular level can vary along a smooth potential energy hyper~urface,~ there can be several alternative ways to attain comparable crystal cohesion with the same molecule in one or more of its isomeric forms. This is the basic idea behind the existence of conformational polymorphs of a given organic molecule, a field widely Abstract published in Advance ACS Abstracts, September 1,1995. (1)Braga, D.; Grepioni, F. Acc. Chem. Res. 1994,27,51. (2)(a) Cotton, F.A.; Hanson, B. E. Rearrangements in Ground and Excited States; De Mayo, P., Ed.; Academic Press: New York, 1980;p 379. (b)Faller, J. W. Adv. Organomet. Chem. 1978,16,211.(c) Band, E.; Muetterties, E. L. Chem. Rev. 1978,78,639. (3)(a) Crabtree, R. H.; Lavin, M. Inorg. Chem. 1986,25,805. @

investigated by J. Bernstein and other^.^ The possibility of structural reorganization, as a manifestation of structural nonrigidity, directly in the solid state should also be ~onsidered.~ Systematic studies of systems in which the very same organometallic molecule is present in different crystal forms are very rare. Few examples are known and were mainly reported by some of US.^ The practical reason for this paucity is mainly due to the difficulty of handling organometallic systems in different crystal(4)(a)Bemstein, J. In Organic Solid State Chemistry; Desiraju, G. R., Ed.; Elsevier: Amsterdam, 1987;pp 471. (b)Bernstein, J.; Hagler, A. T. J.Am. Chem. SOC. 1978,100,673,(c) Destro, R.; Gavezzotti, A. J. In Structure and Properties of Molecular Crystals; Pierrot, M., Ed.; Elsevier: Amsterdam, 1990;pp 161-210. (5)Braga, D. Chem. Rev. 1992,92,633. (6) (a)Braga, D.; Grepioni, F. Acta Crystallogr. 1989,B45, 378. (b) Braga, D.; Grepioni, F.; Johnson, B. F. G.; Dyson, P.; Frediani, P.; Bianchi, M.; Piacenti, F.; Lewis, J. J. Chem. SOC.,Dalton Trans. 1992, 2565. (c) Hong, F. E.; Coffy, T. J.; McCarthy, D. A.; Shore, S. G. Inorg. Chem. 1989,17,3284. (d) Draper, S.M.; Housecroft, C. E.; Keep, A. K.; Matthews, D. M.; Song, X.; Rheingold, A. L. J. Organomet. Chem. 1992,423,241. (e) Adams, R. D.; Babin, J. E.; Tasi, M. Organometallics 1988,7,503. (f) Braga, D.; Grepioni, F. J. Chem. SOC.,Dalton Trans. 1993,1223. (g) Della Pergola, R.; Garlaschelli, L.; Martinengo, S.; DeMartin, F.; Manassero, M.; Sansoni, M. Gazz. Chim. Ital. 1987, 117,245. (h)Brown, M. P.; Bums, D.; Harding, M. M.; Maginn, S.; Smith, A. K. Inorg. Chim. Acta 1989,162,287.(i) Braga, D.; Grepioni, F.; Sabatino, P.; Dyson, P. J.; Johnson, B. F. G.; Lewis, J.; Bailey, P. J.; Raithby, P. R.; Stalke, D. J. Chem. SOC., Dalton Trans. 1993,985. (i) Dyson, P.J.; Johnson, B. F. G.; Lewis, J.;Martinelli, M.; Braga, D.; Grepioni, F. J. Am. Chem. SOC.1993,115,9062.

0 1995 American Chemical Society Q276-7333/95/2314-535Q~Q9.QQIQ

Bonding in Crystalline Clusters

Organometallics, Vol. 14, No. 11, 1995 5351

Table 1. Molecular and Crystal Structure Parameters for Cluster Species of the Type [M= Co, Rh,Ir; CpR= CaHa, C&Iea,C&Me) (CpR)sMs(CaRa)s(COh formula

CO type

FE2 E3 Cp3Rh3iuz-C0)3.O.24(acetoneP E3 C~~R~~@Z-CO)Z(CO) TE2 Cp3Ir3(COh T3 cp’3c03@3-c0)@2-c0)2 FEZ cp*zcp’co3@3-co)@2-co)z FE2 C ~ * ~ R ~ ~ @ ~ - C O ) @ Z - C O ~ FEZ CpzCp*CozIr@z-C0)3 E3 cP3co3@3-co)@2-co)z CP~R~~~Z-CO)~

a

ref code CPTCCOlO CPNCRHlO

space group

Pi

CABBEM

P21lm p3ml

CPRHCTOl BEZMAU BIKBAY CMNCOD VENPEJ FOHFOX

Pi P21 P211n P2 1Ic p 21

P212121

(A3)

vmOl

291.8 311.3 313.6 315.5 324.9 344.1 417.9 560.4 384.6

PC

PPe

ref

0.73 0.76 0.64 0.74 0.76 0.73 0.71 0.71 0.72

-60.32 -64.73 -52.64 -68.08 -70.00 -65.16 -69.91 -77.20 -64.11

13 14 15 16 17 13 18 19 20

No coordinates of acetone given.

lization conditions. One way around this difficulty it is to make use of the large body of structural information deposited in the Cambridge Structural D a t a b a ~ e , ~ (CSD). Many organometallic systems belong to fairly populated families or classes of compounds. The differences and analogies in chemical and structural behavior of complexes and clusters in which, for instance, the metal atom changes along a group of the periodic table, can be used to correlate molecular and crystal structure features with the idea in mind that the electronic characteristics, as well as the crystal environment, can be changed by modifying the nature of the metal atoms and/or with the bonding mode of the ligand(s1. Some of us have previously investigated the relationship between molecular and crystal structures of ruthenium clusters carrying benzene bound in terminal- or facial-bonding mode,8aas well as the relationship between the presence of intermolecular hydrogen bonds and the bending of C-H bonds in bisarene complexes of the type [(arene)2Ru2C131+(arene = benzene, toluene).8b The problem of the existence of different isomeric forms for M4 clusters (M = Co, Rh, and Ir) and their derivatives with a variety of ligandsgaas well as the intramolecular and intermolecular bonding in Ru3(C0)12and its derivatives with facial ligands, have also been addressed.gb The relationship between intramolecular and intermolecular bonding can be conveniently tackled with the aid of two complementary tools, namely, molecular orbital calculations of the extended Huckel typelo (to address specific bonding problems) and packing potential energy calculations’l and packing analysis (to study the intermolecular bonding). In this paper, we report on the family of compounds of general formula (CpR)3M3(C0)3,where M = Co, Rh, and Ir and CpR = C5H5 (Cp), C5H4Me, (Cp’), CsMes (Cp*). Several compounds of this type have been characterized mostly over the past decade, and the molecular structures and the chemistry of these and related cluster complexes have recently been reviewed.12 Formulae, crystallographic information, and refere n c e ~ are ~ ~grouped - ~ ~ in Table 1. The CSD7was used to extract the atomic coordinates. Sketches of all the molecules under investigation are grouped in Figure 1. The main features of the molecular structures can be described as follows: (i) CO ligands are present in all three main bonding modes: terminal (hereafter mode T), edge-bridging 0 1 2 coordination, hereafter mode E), and face-bridging 013coordination, hereafter mode F). (ii) Three terminal ligands (T3) are shown only by the Ir complex Cp3Ir3(CO)3[BEZMAU].

(iii) Only edge-bridges (E3) are present in two forms of Cp3Rh3@z-C0)3[CPNCRHlO and CABBEMI, in the mixed-metal species CpzCp*CozIr@2-C0)3[FOHFOXI, and in the indenyl derivative of Ir (see below). (iv) One facial p3-bridge is present in Cp3Co3@3CO)(p2-C0)2 [CPTCCOlO], in C ~ ’ ~ C O ~ @ ~ - C O ) @ Z - C O ) ~ [BIKBAYI, and in Cp*~Cp’Co3@3-CO)~p-C0)2 [CMNCOD]; the other two CO’s are p2-bridging in all three complexes. (v) The fourth type of CO-ligand distribution is characterized by the presence of one terminally bound CO and by two bridges; such is the arrangement in Cp3Rh3(~&-20)2(CO) [CPRHCTOl]. If the labels T, E, and F are used, these CO-ligand distributions can be called T3, TE2, E3, and FEZ, respectively. The trend toward a decreasing importance of bridging ligands on descending through the group from Co to Ir, which is observed in the family of M4(C0)12clusters and of their derivatives,loais also clear in the Cp3M3(C0)3 systems. Cobalt is preferentially involved in p3 or p2 bridges, whereas iridium atoms prefer to carry terminal COS. The indenyl derivative (ind)3Ir3012-C0)3is exceptional in this respect because the cluster bears three (7) (a) Allen, F. H.; Bellard, S.; Brice, M. D.; Cartwright, C. A,; Doubleday, A.; Higgs, H.; Hummelink, T.; Hummelink-Peters, B. J.; Kennard, 0.;Motherwell, W. D. S.; Rodgers, J. R.; Watson, D. G. Acta Crystallogr. 1979,B35,2331. (b)Allen,F. H.; Davies, J . E.; Galloy, J. J.; Johnson, 0.; Kennard, 0.; Macrae, C. F.; Watson, D. G. J . Chem. Inc Comput. Sci. 1991,31, 204. (8)(a) Braga, D.; Dyson, P. J.; Grepioni, F.; Johnson, B. F. G.; Calhorda, M. J. Inorg. Chem. 1994,33,3218. (b) Grepioni, F.; Braga, D.; Dyson, P.; Johnson, B. F. G.; Sanderson, F. M.; Calhorda, M. J.; Veiros, L. Organometallics 1996, 14, 121. (9) (a) Braga, D.; Byme, J. J.; Calhorda, M. J.; Grepioni, F. Submitted for publication. (b) Braga, D.; Grepioni, F.; Calhorda, M. J.; Veiros, L. Organometallics 1996, 14, 1992. (10) (a)Hoffmann, R. J . Chem. Phys. 1963,39,1397. (b) Hoffmann, R.; Lipscomb, W. N. J. Chem. Phys. 1962,36, 2179, 3489. (c) Thorn, D. L.; Hoffmann, R. Inorg. Chem. 1978, 17, 126. (d) Albright, T. A,; Whangbo, M.-H.; Burdett, J. K Orbital Interactions in Chemistry; John Wiley & Sons: New York, 1985. (11)(a) Kitaigorodskii, A. I. Molecular Crystals and Molecules; Academic Press: New York, 1973. (b) Pertsin, A. J.; Kitaigorodskii, A. I. The Atom-Atom Potential Method; Springer-Verlag: Berlin, 1987. ( c ) Gavezzotti, A.; Simonetta, M. Chem. Reu. 1982, 82, 1. (12)Wadepohl, H.; Gebert, S. Coord. Chem. Rev., in press. (13) Bailey Junior, W. I.; Cotton, F. A.; Jamerson, J. D.; Kolthammer, B. W. S.; Inorg. Chem. 1982,21, 3131. (14)Mills, 0. S.; Paulus, E. F. J . Organomet. Chem. 1967, 10, 331. (15) Faraone, F.; La Schiavo, S.; Bruno, G.; Piraino, P.; Bombieri, G. J . Chem. SOC.,Dalton Trans. 1983, 1813. (16) Paulus, E. F. Acta Crystallogr., Sect. B 1969,25, 2206. (17) Shapley, J. R.; Adair, P. C.; Lawson, R. J.; Pierpont, C. G. Inorg. Chem. 1982,21, 1701. (18)Cirjak, L. M.; Huang, J.-S.; Zhu, 2.-H.; Dahl, L. F. J . A m . Chem. SOC.1980,102, 6623. (19) Brunner, H.; Janietz, N.; Wachter, J.; Neumann, H.-P.; Nuber, B.; Ziegler, M. L. J . Organomet. Chem. 1990, 388, 203. (20) Horlein, R.; Herrmann, W. A.; Barnes, C. E.; Weber, C.; Kruger, C.; Ziegler, M. L.; Zahn, T. J . Organomet. Chem. 1987, 321, 257.

Braga et al.

5352 Organometallics, Vol. 14,No.11, 1995

(a)c -co-

/b & 3

\ I ......$00 .....""-

ligands is perhaps responsible for the "switch" from E3 to FEZwhen M = rhodium. The reasons for these differences will be explored by extended Huckel calculations. Introduction of methyl groups does not significantly alter the structures: although the series is far from complete, the same structure is observed for Cp with cobalt and for Cp* with rhodium (TEz). Interestingly, when the substituent is an indenyl, the E3 structure is observed for the first time in an Ira cluster.z1 Indenyl leads to specific steric hindrance problems, but there are electronic factors responsible for reverting the normal trend and for forcing the formation of bridges along metal-metal bonds involving the heaviest element of the cobalt group. The second objective of this study is the investigation of how the molecular arrangements of these complexes in the solid state compare with those of other carbonyl clusters in terms of efficiency of volume occupation and packing patterns.z2 The molecules selected for this study also form a particularly well suited sample for the investigation of hydrogen-bondinginteractions of the C-H- -0 typeZ3yz4 because the only acceptors are the CO ligands in their different coordination modes and because the only H artoms are those present on the Cp ligands or on the methyl substituents. In previous studiesz5it was shown that edge-bridging CO's tend to form shorter C-H- - -0linkages than terminal ones in keeping with the different basicity of the oxygen atom in the two bonding modesUz6The simultaneous presence of CO ligands in the three most frequent bonding fashions (uiz. terminal, doubly, and triply bridging bonding modes) can be used to study the different strength of the intermolecular (and intramolecular) C-H- - - 0 C hydrogen-bonding interactions. Although the sample is formed by a limited number of compounds, it is homogeneous and should permit an appreciation of the subtle differences in CO oxygen atom basicity as the coordination mode varies. This all will be put in the light of the choice exerted on the type of CO coordination by the characteristics of the metals. Molecular Structures. The family of clusters represented in Figure 1is formed of trinuclear M3 clusters typically possessing 48 electr0ns.~7It is not suprising, therefore, that these species will in some ways behave like the related M~(CO)IZ species (M = Fe, Ru, Os),28 assuming a [CpRl- ring to replace there carbonyls and

-

P

Figure 1. Sketches of the molecular structures of (CpR)3M3(C0)3clusters (a,b) and of the indenyl derivative (ind)&(C0)3 (c) as determined in the solid state.

edge-bridging ligands (see Figure IC). This will be discussed in detail below. With C5H5 ligands, however, all four types of CO-ligand distributions (FEZ,E3, TEz, T3) are found. The Rhs system shows the greatest structural variability, presenting systems with the combination TEz, E3, and FEZdepending (apparently) on the size of the CpRligand. The increase in the steric bulk of the CpR

(21) Comstock, M. C.; Wilson, S. R.; Shapley, J. R. Organometallics 1994, 13, 3805. (22) (a) Braga, D.; Grepioni,F. Organometallics 1991, IO, 2563. (b) Braga, D.; Grepioni, F. Organometallics 1992, 11, 711. ( c )Braga, D.; Grepioni,F.; Sabatino, P.; Gavezzotti, A. J . Chem. SOC.,Dalton Trans. 1992, 1185. (23) (a) Green, R. D. Hydrogen Bonding by C-H Groups; Wiley: New York, 1974. (b) Taylor, R.; Kennard, 0.J . A m . Chem. SOC.1982, 104, 5063. (24) Desiraju, G. R. ACC.Chem. Res. 1991,24, 290. 125)(a) Braga, D.; Grepioni, F.; Sabatino, P.; Desiraju, G. R. Organometallics 1994,13,3532. (b) Braga, D.; Biradha, K.; Grepioni, F.; Pedireddi,V. R.; Desiraju, G. R. J . Am. Chem. SOC.1995,117,3156. (26) Horwitz, C. P.; Shriver, D. F. Adu. Organomet. Chem. 1984, 23, 218. (27)(a) Lauher, J. W. J . A m . Chem. SOC.1978, 100, 5305. (b) Kharas, K. C. C.; Dahl, L. F.Aduances in Chemical Physics; John Wiley & Sons: New York, 1988; Vol. 70. (c) Evans, D. G.; Mingos, D. M. P. Organometallics 1983,2, 435. (d) Mingos, D. M. P. Inorganometallic Chemistry; Plenum Press: New York, 1992; p 179.

Bonding in Crystalline Clusters

Organometallics, Vol. 14, No. 11, 1995 5353

Table 2. Relative Energies (ev) of Four Possible Isomers of Cp&l~(C0)s(M = Co, Rh, Ir) structural type

cobalt

FEZ

0 0.528 0.635 0.962

E3

TEz “3

rhodium

iridium

0

0

0.588 0.883 1.266

0.275 0.509 0.520

0.220

noting the same metal electron count (ds).29aFrom this point of view, the all-terminal structure Cp3Ir3(CO)3 [BEZMAUI is the analogue of the “all-terminal” Os3(C0112, while the structure exhibited by Fe3(C0)12 is observed in Cp3Rh3(p2-C0)2(CO)[CPRHCTOlI, containing an Rh3 unit. Symmetry is also different: for instance, while Os3(CO)12 has idealized D3h symmetry, Cp3Ir3(CO)3is only C,,not even the expected C3”. Some of these aspects, especially those dealing with the existence of bridges, have been addressed theoreti~ally.2~~ A 0.198 co- 0 182 We can also point out that, while Ru~(C0)12presents no bridging carbonyls, no such structure is observed for the rhodium derivatives addressed here. The rhodium co Rh E type CO > T type CO, which is the well-known trend of decreasing basicity of the CO ligand. (vii) The indenyl derivative is exceptional, in this respect, since the C-H- - -0distance is very short (2.30 A) and comparable to the distances attained by F ligands. (ix)If large Cp* ligands are substituted for the smaller Cp ligands, the CO ligands bound to the metal atoms become segregated within the organic sheath and can no longer form intermolecular H bonds. (x) In these cases, however, the CO ligands establish intramolecular interactions with the surrounding methyl groups. These results represent, in our opinion, a (small) step ahead in the understanding of the relationship between the structure of the isolated (gas-phase) molecule and that of an ensemble of such molecules. The information on the interplay between size of the cyclopentadienyl ligand, type of coordination of the CO ligands, and the network of intermolecular and intramolecular interactions that can be established in the solid state are useful in the engineering of molecules which are able t o participate in predictable intermolecular a ~ s e m b l i e s . ~ ~ Apart from this general outcome, we have provided further evidence, based on a small, though consistent, set of molecular systems of the participation of CO ligands in intermolecular hydrogen bonds. These bonds become particularly important in organometallic cluster systems because of the large number of CO ligands and of the large number of hydrogen atoms present on the organic ligands.

Methodology All the molecular orbital calculations were done using the extended Huckel method’l with modified The basis set for the metal atoms consisted of ns,np and (n - l ) d orbitals. The s and p orbitals were described by single Slater type wave functions, and the d orbitals were taken as contracted linear combinations of two Slater type wave functions. Standard parameters were used for C, 0, and H, while those for the metals were as follows (H”/eV, Co, 4s, -5.29,2.1; 4p, -5.29, 2.1; 3d, -13.18, 5.55, 2.1 (521, 0.5679 (cI), 0.6059 (cz). Rh, 5s, -8.09,2.135; 5p, -4.57,2.1; 4d, -12.50,5.542,2.39 ([d, 0.5823 (cl), 0.6405 (cz). Ir, 6s, -11.3, 2.504; 6p, -4.5, 2.2; 5d, -12.1, 5.796, 2.557 (i3,0.6351 (cl), 0.5556 (cz). Three-dimensional representations of orbitals were drawn using the program

e):

In all calculations idealized models were used, based on the experimentally observed structures. The following distances (A) were used: Co-Co, 2.45, Rh-Rh and Ir-Ir, 2.65, M-C(bridging carbonyl), 2.0, M-C(termina1 carbonyl), 1.8(M = Co, Rh Ir), Co-Cp(centroid1, 1.75, M-Cp(centroid), 2.0 ( M = Rh, Ir), C - 0 (bridging carbonyl), 1.18, C - 0 (terminal carbonyl), 1.15, Ir-C(allyl), 2.2, C-C, 1.4, and C-H, 1.08 A. The bond angles of the ligands relative to the M3 plane were optimized. The definitions are pictured in Chart 2. Thus, a is the angle between the bridging CO bond and the M3 plane, /3 is the angle between the normal to the Cp ring and the M3 plane, and y measures the “sideways” deviation of each M-Cp (35) (a) Desiraju, G. R. Crystal Engineering, The Design oforganic Solids; Elsevier: Amsterdam, 1989; p 47. (b) Inorganic Materials; Bruce, D. W., O’Hare, D., Eds.; John Wiley & Sons: Chichester. U.K.,

1992. (36)Ammeter, J. H.; Burgi, H.-B.; Thibeault, J. C.; Hoffmann, R. J . A m . Chem. Soc. 1978,100,3686. (37)Mealli, C.; Proserpio, D. M. J . Chem. Ed. 1990, 67, 39.

Bonding in Crystalline Clusters

Organometallics, Vol. 14, No. 11, 1995 5361

Chart 2 M

Table 4 structures

ddeg Pldeg yldeg

FE2 121.0 15.0 15.0

Cp3MdCOh TE2 E3 103.0 126.0 42.0 30.0 21.0 0.0

T3 90.0 36.0 0.0

(allyl)3In(C0)3 E3 T3 132.0 90.0 56.0 62.0 0.0 0.0

bond around the symmetry unequivalent bond of the M B triangle. The optimized values are compiled in Table 4. Crystal structure analysis was carried out with the aid of the computer program OPEC,3s which allows, within the atom-atom potential energy method, the calculation of packing potential energies as well as molecular volumes and packing coefficients. The molecular volumes were estimated by the integration method. The light atoms were attributed van der Waals radii available in the literature, while the cobalt, rhodium, and iridium atoms were attributed radii of 2.15, 2.35, and 2.35 A, r e s p e c t i ~ e l y . ~The ~ values of the molecular volumes were in turn used to calculate the packing (38)(a) Gavezzotti, A. OPEC: Organic Packing Potential Energy Calculations; University of Milano: Milan, Italy.

coefficients by means of the expression pc = VmolZiVcell.A Buckingham type potential energy function was used to estimate the cohesive energy of the crystalline species under investigation. The C, 0, and H atoms were given the generalized potential parameters put foward by Gavezzotti and Filippini, while the Co, Rh, and Ir atoms were treated as Kr and Ar atoms.40 The geometric features of the intermolecular and intramolecular hydrogen-bonding networks were investigated by using the graphic program SCHAKAL9241aand the suite of programs PLATON.41bAtomic coordinates and crystal data were obtained from the Cambridge Structural Database. The available coordinates for the hydrogen atoms were normalized by extending the C-H distances along the C-H vectors to the typical neutron-derived value of 1.08

Acknowledgment. D.B., F.G., M.J.C., and L.F.V. acknowledge the CNR (Italy) and the JNICT (Portugal) for joint financial support; D.B., F.G., H.W., and S.G. thank the Deutscher Akademischer Austauschdienst, Bonn, Germany, and the Conferenza Nazionale dei Rettori, Roma, Italy, for a scientific exchange grant within the Programme Vigoni. OM9504112 (39) (a) Bondi, A. J . Phys. Chem. 1964,68, 441. (b) Gavezzotti, A. Nouv. J . Chim. 1982, 6, 443. (40) Gavezzotti, A.; Filippini, G. Acta Crystallogr., Sect. B 1993,49, 868. (41)(a) Keller, E. SCHAKAL92; University of Freiburg: Freiburg, Germany, 1992. (b) Spek, A. L. PLATON; Acta Crystallogr., Sect A 1990,46, C31. (42) Murray-Rust, P.; Glusker, J. P. J . A m . Chem. SOC.1984, 106, 1018.