J. CHEM. SOC. FARADAY TRANS., 1990, 86(2), 361-369 361

Crystal and Electronic Structures of New Molecular Conductors Tetramethylammonium and Tetramethylarsonium Complexes of Pd (d m it)*

Akiko Kobayashi, Hyerjoo Kim and Yukiyoshi Sasaki Department of Chemistry, Faculty of Science, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113, Japan Keizo Murata Electrotechnical Laboratory, Umezono, Tsukuba , lbaraki 305, Japan Reizo Kato and Hayao Kobayashi* Department of Chemistry, Faculty of Science, Toho University, Funabashi, Chiba 274, Japan

The synthesis, structural characterization and electrical conductivity measurements of a-, j?-[(CH,),N)[Pd(dmit),], and [(CH,),As][Pd(dmit),], are reported. They have similar crystal structures made of stacked Pd(dmit), dimers. The mode of the intra-dimer molecular overlapping is that of the eclipsed type. Pd atoms deviate from the planes formed by ligand S atoms by 0.07-0.08 A to approach to each other. Simple tight-binding band calculations were made on these three Pd(dmit), salts. Each energy band is made up of two energy branches separated by a large mid-gap. The lower energy branch is a narrow half-filled band which seems to be consistent with the weakly metallic or semiconducting behaviour of these compounds around room temperature.

Recently, two molecular superconductors a- and a’-TTF [Pd(dmit),], (TTF = tetrathiafulvalene; dmit = 43- dimercapto- 1,3-dithiole-2-thione) have been found by Cassoux et al. at high pressure.’ They are Pd analogues of the first high-pressure molecular superconductor based on Ni(dmit), , TTF[Ni(dmit),], ,, which has a stable metallic state down to low temperature at ambient pressure. Note that a- and a’-TTF[Pd(dmit),], become insulators at low temperature despite the close similarity of the structures. As reported earlier, tight-binding band examination of (TTF)[Ni(dmit),], has revealed the one-dimensional nature of the system., In fact ‘H NMR experiments by Bourbonnais et al. have shown the 1D properties of TTF c01umns.~ To our knowledge, this may be the only one with a stable 1D metal- lic state among many molecular metals containing TTF columns.

As is well known, a normal 1D metal has a pair of plane Fermi surfaces and exhibits a metal instability when a 2k, periodical perturbation develops. However, unlike normal 1 D metals, (TTF)[Ni(dmit),], has a ‘multi-Fermi surface’ where a single periodical potential cannot open an energy gap all over the Fermi surface. We have proposed that the stability of the metallic (TTF)[Ni(dmit),I2 may be ascribed to the multi-Fermi surface nature of the system. However, recent investigations by Brossard et al. have shown that the Pd ana- logue transforms to an insulating state at low temperatures.’

The multi-Fermi surface nature is diminished with decreas- ing interaction between M(dmit), columns interrelated by an inversion centre.’ There might be some possibility that this interaction in (TTF)[Pd(dmit),], becomes small at low tem- peratures. However, it seems to be more natural to consider that the simple tight-binding band picture based on the fron- tier orbitals of the component molecules, which has been revealed to be very useful for qualitative analyses on the elec- tronic properties of various types of molecular conductors, is not sufficient in the Pd complex.

[(CH,),N][Ni(dmit),], has been found to be another high- pressure superconductor based on Ni(dmit), .’ In this complex, Ni(dmit), molecules are stacked to form columns in two directions and two pairs of plane Fermi surfaces appear.

We have examined the structures and electrical properties of two tetramethylammonium salts (a and f l ) and a tetra- methylarsonium salt of Pd(dmit), to compare them with those of the Ni complex.

Experimental Preparation of a-[( CH,),N][Pd( dmi t),] ,

The (dmit)2- ligand and the [(CH,),N],[Pd(dmit),] com- plexes (n = 1, 2) have been prepared following the procedure described in ref. (6). We have obtained two different types of Pd analogue of [(CH,),N][Pd(dmit),], , a triclinic form (a) and a monoclinic form (j?). The black plate-type crystals of the a form of [(CH,),N][Pd(dmit),], were obtained by elec- trochemical oxidation from [(CH,),N][Pd(dmit),] (2.3 mmol dm-,) and [(CH,),N]ClO, (32 mmol drn-,) in acetonitrile (30 cm3) for 6 weeks using 0.6 pA constant current. Good crystals of the j? form were obtained by leaving the mixture of [(CH,),N][Pd(dmit),] (1 mmol dm- ,) and [(CH3),N]C10, (1.2 mmol dm-3) in acetonitrile (40 cm3) for 4 months. These experiments were carried out under an inert atmosphere. The a form was also obtained directly by electrochemical oxida- tion of [(CH 3)4N] , [Pd(dmit),].

[(CH,),As][Pd(dmit),], was obtained from an acetone sol- ution (40 cm3) containing [(CH,),As],[Pd(dmit),] (0.03 g) and acetic acid (1-5 cm3). These experiments were carried out in air. Crystal data and crystal sizes for the diffraction studies are listed in table 1.

&[(CH,),N)IPd(dmit)212 and I(CH,), AslIPd(dmit)212

Crystal Structure Determinations

Intensity data were measured on a Rigaku automated four- circle diffractometer (Rigaku, AFC-6) using graphite- monochromated Mo K , radiation. Lattice parameters were determined from the least-squares refinement of 20 reflections with 20 < 28/O < 35. All of the reflections were collected using the 0-28 scan method. Three standard reflections were measured after each 100 reflections. No correction was made for absorption owing to the small values of the linear absorp-

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362 J. CHEM. SOC. FARADAY TRANS., 1990, VOL. 86

Table 1.

formula wt crystal size/mm space group

4 b/A C I A a/o Bl" Y l o VIA3 Z p/cm - PA3 cm - no. of reflections collected independent reflections R R2 weighting scheme, w - g.0.f. scan speed, o r m i n - scan width recycling scan range, 28,J X-ray monochrometer scan technique no. of parameters h,kJ

standard reflections

1072.3 0.33 x 0.30 x 0.04 triclinic

35.48q12) 7.807(3) 6.32q2) 112.10(3) 94.2 l(3) 92.7q 3) 1612.5 2 23.7 2.21 5927 5440 0.055 0.10 o2 + 0.01 IF 12, IF I 2 10.0 0.95 4 1.04 + 0.5 tan 8 3 50 Mo K , graphite o-scan 353 - 42,42, - 9,9, 037, 2,0,1,4,2,0, 2,2,0,

p i

1072.3 0.13 x 0.20 x 0.04 monoclinic c2/c 14.523( 3) 6.320(1) 35.134(6)

90.94(2)

3224.2 4 23.5 2.20 6508 2630 0.068 0.12 o2 + 0.01 IF 12, I F I 2 15.0 1.23 4 2.39 + 0.5 tan 8 2 50 Mo K, graphite o-scan 182

0,47

0,272,

- 18,18,0,8,

2,2,0,4,2,0,

1133.3 0.38 x 0.33 x 0.05 monoclinic c2/c 14.340(4) 6.344(2) 36.528( 8)

97.8 5( 3)

3292.0 4 33.54 2.29 4380 2617 0.073 0.13 a2+0.0031F12,1F1210.0 1.39 4 1.39 + 0.5 tan 8 3 55 Mo K, graphite o-scan 182 - 18,18,0,8, 0,47 1,1,7,1,1,7, 4,0,2,

tion coefficients p and irregular shapes of the crystals. Inde- pendent reflections [20 < 60°, I F, I 2 3 4 I F, I)] were used for the structure calculations. The structure was solved by the direct methods and refined by a block-diagonal least-squares method. Anisotropic thermal parameters were adopted for all the non-hydrogen atoms. All the hydrogen positions in the methyl groups could not be identified on the difference Fourier maps and they were not included in the calculations. The atomic scattering factors were taken from the Znter- national Tables for X-Ray Crys t~ l lography .~ All the calcu- lations were performed on a HITAC M-680H computer at the Computer Centre of the University of Tokyo using the program UNICSIII.~ The experimental details are summarized in table 1

Results and Discussion Crystal Structures of a-[(CH,),N][Pd(dmit)2]2 (a Form)

The atomic coordinates, bond distances and angles are listed in tables 2 and 3. The molecular structures of Pd(dmit), with the atomic numbering schemes are shown in fig. 1. The crystal structure of the a form is shown in fig. 2, which is similar to, but is not isomorphous with [(CH,),N][Ni(dmit),],, which has been found to be a molec- ular superconductor. There are two crystallographically inde- pendent molecules of Pd(dmit), named A and B. Each of the two forms weak metal-metal dimers with Pd-Pd distances of 3.17 and 3.13 A. The shortest Pd-S distances within the dimers are 3.917 and 3.842 A. The average Pd-S distance and S-Pd-S angle is 2.293 A, 89" in molecule A and 2.292 A, 89" in molecule B.

Pd(dmit), dimers form segregated 1D stacks along [OlO] in molecule A and [ O l l ] in molecule B. In the a form, the Pd(dmit), anions in the dimer are eclipsed, though in [(CH,),N][Ni(dmit)2]2 the corresponding anions are

slipped sideways with respect to each other. In [(CH,),N][Ni(dmit),], , the intra- and inter-dimer separa- tions within the stack are very similar (3.53 and 3.58 A) whereas they are very different in [(CH,),N][Pd(dmit),], (3.32 and 3.69 A, molecule A; 3.28 and 3.86 A, molecule B). Fig. 3 shows the mode of the molecular overlapping and the side view of Pd(dmit), dimer stacks. The two Pd(dmit), mol- ecules which form a dimer curve away from each other to lower the ligand-ligand repulsion between two monomers. The displacement of the Pd atom from the plane of the four ligand S atoms is 0.07-0.08 A. Hoffmann et al. have pointed out that metal dithiolene complexes of Pt and Pd have a greater tendency to form metal-metal dimers than do Ni complexes, which can be explained by theoretical calculation of their M-M and M-S dimerization energies and ligand- ligand repulsions.' The geometry of the PdS, group, which is

SI11) n

S(12) S(16)

Fig. 1. Molecular structure of Pd(dmit), with atomic numbering schemes.

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J. CHEM. SOC. FARADAY TRANS., 1990, VOL. 86 363

Fig. 2. Crystal structure of a-[(CH,),N][Pd(dmit),],.

composed of the central Pd atom with four S atoms bonded to it, is square planar, while that of the Ni analogue is similar, but elongated along the long-axis of the molecule, shown in fig. 4.

The tetramethylammonium cations lie between the stacks of molecule A and those of molecule B. Therefore, similar to the case of [(CH,),N][Ni(dmit),], , the layers of Pd(dmit), dimers stacking along the [OlO] and [ O l l ] directions are arranged such that the anions are displaced about the a axis

Table 2. Atomic parameters of a-[(CH,),N][Pd(dmit),] ," atom X Y Z Beqb

981q< 1) 304q < 1) 5188( < 1) 3232( < 1)

9754(< 1) 2.2 8496(< 1) 2.1

10 14q 1) 10275( 1) 9333(1) 9@8( 1)

10893( 1) 11013(1) 8556(1) 8657(1)

11611(1) 7916(1) 5533(1) 5681( 1) 472q1) 4849( 1) 633q1) 6459( 1) 3987( 1) 4105(1) 7091(1)

7499(2) 10545(2) 105 89(2) 9ow2) 9049( 2)

1 1200(2) 835q2) 5947(2) 6012(2) 4413(2) 4469(2) 6649(2) 3815(2) 7348(3) 76 19( 3) 7 163(2) 7822(4)

3404(1)

30743) '

1525(3) 4373(3) 2861(3) 1704(3) 359(3)

51 5 l(3) 3882(3)

103(4) 5215(4) 3304(3) 4892(3) 1397(3) 2938(3) 4799( 3) 6223( 3)

873(3) 6527(3)

787(9)

1372( 10)

3754(10) 70q 10)

476q 12) 450919) 5 129(9) 945( 10)

154q10) 5856(10)

-41 l(3)

- 1464(3)

2000(9)

43 5 5( 9)

- 369( 10) - 9 19( 14) 241 8( 12) 1268( 1 3) 243( 18)

12991(3) 7608(3)

1 1942( 3) 6493( 3)

13754(3) 8959(4)

106 3q4) 5842(4)

12376(5) 7437(5)

1 1740(3) 7786(3) 9115(3) 5118(3)

13706(3) 10203(4) 6457(3) 281 l(3)

1374q4) 2506(4) 1 63 6( 1 3)

1 19 18( 12) 9632( 12) 9877( 12) 757 1( 1 1)

11692(14) 7931(15)

1 1679( 12) 10003(12) 6801( 13) 5104(11)

12596(15) 3885( 13)

1056(19) 3063( 17) 2875(2 1)

- 577( 18)

2.8 2.9 2.8 2.6 3.2 3.2 3.4 3.3 4.3 5.1 2.6 2.7 2.6 2.8 3.0 3.2 2.8 3.0 4.1 3.6 3.2 2.2 2.4 2.1 2.4 2.8 2.4 2.3 2.3 2.6 2.3 3.3 2.7 4.7 4.0 4.0 6.0

~

" Positional parameters are multiplied by lo4. Thermal parameters are given by the equivalent temperature factors A'. Be, = (4/3)(B,,a2 + B,, b2 + B,, c2 + B,, ab cos *J + B, , ac cos B + B,, bc cos a). ' CN(i) (where i = 1, . . . 4) refer to the methyl carbon atoms.

with the cation sheets lying between the layers. The tetra- methylammonium cations are in an ordered state. The average C-N distance in the (CH3)4Nf cation is 1.524 A. Intermolecular S-S interactions (S-S distances are shorter than the sum of the van der Waals radii; 3.7 A) build a quasi- three-dimensional network as shown in fig. 5.

Crystal Structures of p-[(CH,)4N][Pd(dmit)2], (p Form) and [(CH3),As][Pd(dmit)J2 (TMAs Salt)

Although Pd(dmit), molecules form dimeric structures, the crystal structures of the p form and TMAs salt are iso- structural with [(CH 3)4N] [ Ni(dmi t)J , except for the cat ion sites. The molecular structures of the p form and TMAs salt with the atomic numbering schemes used are shown in fig. 6. The atomic coordinates, bond distances and angles are listed in tables 4 and 5. The crystal structure of /3-[(CH3),N][Pd(dmit),], is shown in fig. 7. The unit cell contains eight Pd(dmit), molecules and four cations. Thus, one Pd(dmit), molecule and half of the tetramethyl- ammonium and arsonium cation are crystallographically independent. The average Pd-S distances and S-Pd-S angles of the p form and TMAs salt are 2.295 A, 89.95", and 2.295 A, 90.05", respectively.

The Pd(dmit), molecules form 1D stacks along the [ l l O ] and [ 1701 directions forming dimeric structures. Within these dimers the two anions adopt an eclipsed configuration but between dimers the anions are displaced sideways with respect to each other. In [(CH,),N][Ni(dmit),], , Ni(dmit), molecules form apparently uniform stacks. Thus, the intra- and inter-'dimer' separations within the stack are very similar (3.53 and 3.58 A) whereas they are very different in the fJ form (3.28, 3.82 A) and TMAs salt (3.31, 3.71 A) which is similar to the case of c1 form. The two Pd atoms of the dimers are displaced towards each other from the ligand plane. The Pd-Pd distances within the dimers are 3.116 ( p form) and 3.167 8, (TMAs salt), respectively. The shortest Pd-S dis- tances within the dimers are 3.888 (p form) and 3.875 A (TMAs salt). The out-of-plane displacement of the Pd atom from the plane composed of four sulphur atoms is 0.07 A. By this pyramidization effect, the Pd-Pd distances can be short- ened without an increase of the ligand-ligand repulsion caused by the proximity of the two molecules. Tetra- methylammonium cations and tetramethylarsonium cations lie between the Pd(dmit), anion layers. Although they are on the two-fold axis and are in an ordered state, the y coordi- nates of the N atom and As atom differ from one another by ca. 25%. The four methyl groups are orientated in different directions. The average bond distance of C-N is 1.504 8, (p form) and that of C-As is 1.906 (TMAs salt). Unlike the case of the c1 form, the Pd(dmit), anions have two orienta- tions, which appear alternately along [Ool].

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364 J. CHEM. SOC. FARADAY TRANS., 1990, VOL. 86

Table 3. Bond distances and angles in a-[(CH,),N][Pd(dmit),], ~ ___ ~~ ~~~

bond distances bond angles

bond distance/A bond distancelA bonds angle/" bonds angle/"

2.282( 2) 2.297(2) 1.73 1( 8) 1.682( 8) 1.7 1 418) 1.766(8) 1.736(9) 1.7 19( 10) 1.632( 9) 1.364111) 2.29q2) 2.290(2) 1.71 9( 8) 1.672(8) 1.743(8) 1.650(10) 1.770(8) 1.72 1( 8) 1.349( 1 1) 1.56q 10) 1.523(12)

2.29412) 2.300(2) 1.676(8) 1.682(8) 1.740(8) 1.736(8) 1.720(9) 1.732(10) 1.63 1 ( 10) 1.377(11) 2.299(2) 2.297(2) 1.709(8) 1.689(8) 1.73 7( 10) 1.75 5( 8) 1.706(8) 1.66q8) 1.346(11) 1.502( 14) 1.509(16)

89.76(8) 88.95(8)

102.1 (3) 100.2( 3) 12 1.3(6) 125.6(6) 119.6(4) 119.2(4) 98.2(4)

119.1(6) 96.2(4)

112.415) 124.6(6) 12 1.3( 5) 115.2(6) 89.95( 8) 89.00(7)

101.3( 3) 101.413) 1 23.8( 6) 124.2(6) 120.1(5) 120.1( 5 ) 97.2(4) 97.q 4)

122.7(5) 123.2(6) 113.5(5) 11 5.8(6) 1 15.8(6) 111.0(8) 109.q7) 111.418)

90.23(8) 90.9 1( 8)

10 1.7( 3) 10 1.5( 3) 125.1(6) 122.416) 12 1.7( 5 ) 112.415) 99.0(4)

1 13.1(6) 98.414)

123.0(6) 113.q5) 125.6(6) 115.2(6) 89.55(8) 9 1.22( 8)

101.6(3) 100.8(3) 123.2(6) 124.q6) 120.0(4) 120.4(5) 96.2(4) 96.7(4)

122.5( 5 ) 1 23.3( 6) 114.8(5) 1 16.3(6) 1 l6.1(6) 105.3( 6) 108.7( 7) 1 1 1.3(9)

The Electrical Conductivity

The electrical conductivity measurements were performed by four probe methods using 0.02 pm Au wire with Au paint as a contact. 1-2 mm crystals were used for the conductivity measurements. The temperature dependence of the electrical conductivity of the a form is similar to that of the /? form. The electrical conductivity of the a form is almost constant around room temperature and the resistivity begins to

A

( a

increase rapidly around 100 K. The room temperature con- ductivity of the a form is ca. 50 S cm-'. Thus, in spite of the high conductivity, the a and /? forms are not metallic. Since every second molecule has one electron and electron correla- tion causes the electrons to separate as far as possible, the electrons will tend to localize to form dimeric columns. The pressure dependence of the resistivity of a form was measured up to 12 kbar, which is shown in fig. 8. In the range 0-3 kbar metallic behaviour was not observed and in the high-

B

Fig. 3. Side-view of Pd(dmit), dimer stacks with interplanar distances: A, (a) 3.32 A, (b) 3.69 A; B, (a) 3.28 A and (b) 3.86 A. Mode of intermolecular overlapping of Pd(dmit), molecules in a-[(CH,),N][Pd(dmit)23,.

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J. CHEM. SOC. FARADAY TRANS., 1990, VOL. 86 365

A

Fig. 4. The geometry of MS, square planes of [(CH,),N][M(dmit),], (M = Ni,Pd).

~ ~~~

Ni 3.130(3) 2.969(3) 93.0(1) 87.0(1) Pd 3.24q 3) 3.2443) 89.9( 1) 9QW)

temperature region conductivity becomes almost constant. On the other hand, above 6 kbar a metallic region appears. This is because the pressure increases the band width and strengthens the metallic nature. The metal-insulator (M-I)

Table *a). Atomic parameters of ~-[(CH,),N][Pd(dmit),],"

atom X Y Z Be,

1587( 1) 727(2)

1 502( 2) 1564(2) 2351(2) - 742) 596(2)

21 84(2) 2 89 5( 2) - 470(3) 294q3)

552(7) 886(7)

2079(7) 2409(7)

15(7) 2709(8) 4713(9) 4201(9) 5000(0)

1792( 1) 3451(5)

- 1309( 5 ) 4846(5)

93(5) 19245)

- 2333(5) 5786(5) 1587(6)

4683(8) 148418)

4O89( 18) 2065( 18)

4045(20) 1729(21)

434(24)

- 1282(6)

- 588( 18)

- 607( 19)

-958(22)

189(0) - 275( 1) - 151(1)

537( 1) 681(1)

- 1010(1) -9W1) 1336(1) 1459( 1)

2097( 1) - 1602( 1)

- 5933) - 538(3)

952(3) 1012(3)

1649( 3) 2 8 3 2( 4) 2372(4)

- 1195(3)

25WO)

2.0 2.7 2.6 2.4 2.7 2.9 2.9 3.0 3.1 3.7 4.4 2.0 2.0 2.1 2.1 2.2 2.3 3.0 3.0 2.5

~

" Positional parameters are multiplied by lo4. Thermal parameters are given by the equivalent temperature factors, A2.

Table 4(6). Atomic parameters" of [(CH,),As)[Pd(dmit),],

atom X Y Z Be,

0) 3506(1) 4 188( 2) 3461(2) 364 5( 2) 2923(2) 4668(2) 4049(3) 328 3( 3) 2644(2) 4721(3) 2707(3) 4228( 8) 3 9 2 5( 9) 3287(8) 2971(8) 4484( 10) 2859( 10) - 36 1( 1 3) - 1034( 14)

21 75( 3) 3254(1) 1604(5) 6349( 5 ) 212(5)

49845) 3 176(6) 7392( 5 )

3 578( 6) 6393(8)

3572(19) 565q22) 1039(19) 30541 8) 5662(22) 1 159(25) 3866(33) 378(44)

- 647(6)

5 7 w

25WO) 187(0)

- 269( 1) - 141(1)

53W) 657( 1)

-991(1) - 874(1) 1306(1) 141 3( 1)

203q1) - 1577( 1)

- 576(3) - 529(3)

936(3) 976(3)

160 1( 3) 288q5) 2303(6)

- 1173(3)

3.4 2.1 2.7 2.6 2.6 2.7 3.0 2.8 3.0 3.2 4.2 4.5 2.2 2.5 2.0 1.9 2.9 3.2 5.0 7.2

" Positional parameters are multiplied by lo4. Thermal parameters are given by the equivalent temperature factors, A'. CM(i) (i = 1,2) refer to the methyl carbon atoms.

6

Fig. 5. Intermolecular S-S close contacts (-----) (3.70 A) in a-C(CH,),NI CPd(dmit),I 2 .

transition (TMl z 60 K) becomes sharp. The p form shows semiconducting behaviour similar to that of the a form. The room temperature conductivity is ca. 50 S cm-'. TMAs salt also shows semiconducting behaviour, and its room tem- perature conductivity is ca. 1 S cm-' and activation energy 0.07 eV. The temperature dependence of their conductivities are shown in fig. 9.

Band Energy Calculations

The intermolecular overlap integral (S) between the LUMO (lowest unoccupied molecular orbital) of Pd(dmit), was calcu- lated by the extended Huckel method using Slater-type orbitals and the semi-empirical parameters in ref. (9) and (10). Fig. 10 shows the intermolecular overlapping of the LUMOs of the a form, /3 form and TMAs salt of Pd(dmit), complexes, which are the most important MOs in the formation of the conduction bands. The band structure calculations were per- formed by the simple tight-binding method. The intra-dimer overlap integral is the largest. The second largest overlap is

Fig. 6. Molecular structures of Pd(dmit), with atomic numbering schemes in P-[(CH,),N][Pd(dmit),], .

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366 J. CHEM. SOC. FARADAY TRANS., 1990, VOL. 86

Table qa). Bond distances and angles in /?-[(CH3),N][Pd(drnit),1,

bond distances bond angles

bond distance/A bond distance181 bonds angle/" bonds angle/'

2.29q3) 2.287( 3) 1.693( 1 1) 1.696( 1 1) 1.7291 1 1) 1.728( 11) 1.73 1( 12) 1.725( 12)

1.41q 16) 1.488( 15)

1.640(11)

2.297( 3) 2.307( 3) 1.676(11) 1.704(11) 1.732(11) 1.74q11) 1.7 16( 12) 1.713( 13) 1.654( 12) 1.38 1 (1 6) 1.5 19( 16)

85.8( 1) 89.q 1)

102.2(4) 100.2(3) 1 2 1.3(6) 125.6(6) 119.1(6) 119.6(4) 119.2(4) 1 15.2(6) 98.2(4)

112.4(5) 124.6(6)

90.1( 1) 90.8(1)

102.3(4) 1014 3) 125.1(6) 122.4(6) 113.1(6) 1 2 1.7( 5 ) 1 12.4(5) 115.2(6)

123.0(6) 109.2(7)

99.0(4)

Table qb). Bond lengths and angles in [(CH,),A~][Pd(dmit)~], ~

bond lengths bond angles

bond length/i( bond length/A bonds angle/" bonds angle/' ~~ ~

2.295( 3) 2.296(3) 1.684(12) 1.716(12) 1.739( 12) 1.7 17( 14) 1.629( 14) 1.756(11) 1.69 1 ( 16) 1.405( 19) 1.882(20)

2.297( 3) 2.292( 3) 1.704(12) 1.686( 1 1) 1.704( 13) 1.723( 14) 1.724( 12) 1.73 7( 1 5 ) 1.657( 14) 1.370(17) 1.93q23)

89.8( 1) 90.0(1)

1 0 1 4 4 ) 101.6(4) 125.1(9) 12 1.3( 9) 1 13.4(9) 98.6(6)

113.0(7) 124.8(9) 114.0(8) 98.q6)

122.7(9) 12 1247) 121.4(7) 109.7( 9)

90.3( 1) 89.7( 1)

102.9( 5 ) 100.7(4) 12039) 126.0(9) 116.9(9)

1 22.1 (9) 117.3(9) 96.6(6)

114.1(8) 123.2(9) 1 22.5( 8) 120.0(7)

97.9(7)

that of inter-dimer and is ca. 1/9-1/14 the magnitude of the former. There are many S-S contacts which are shorter than the van der Waals distance (3.7 A) along the b and c direc- tions (a form) and the a and b directions (in the /I form and TMAs salt), but intermolecular transverse interactions are small. The weak transverse interaction is a common feature of M(dmit), complexes, which is due to the b,, symmetry of the LUM0.3 Thus, the dmit complexes are essentially 1D. The tight-binding band energies were calculated, and the results for the c1 form are shown in fig. 11. The energy bands of these three Pd(dmit), salts have similar structures with

large mid-gaps, owing to the strong dimeric structures. The lower bands are extremely narrow, half-filled bands, indicat- ing the large electron correlation effect. The one-dimensional plane Fermi surfaces were obtained. The conductivity behav- iour around room temperature is not simply metallic or semi- conducting. Note that the Fermi surface obtained by these simple procedures is not always a real Fermi surface in the usual sense of solid-state physics. Nevertheless, it is very useful from the viewpoint of structural chemistry. The shape of the Fermi surface can be regarded as a visual expression of the anisotropy of the intermolecular interactions through

Fig. 7. The crystal structure of /?-[(CH,),N][Pd(dmit),], .

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2 -

h 0

9 P U

10 100 temperature/K

Fig. 8. The pressure dependence of the electrical resistivity of a-[(CH,),N][Pd(dmit),1,. At (a) 0, (6) 3, (c) 6, (d) 9 and (e) 12 kbar.

P

oo 0"

0 0

5 10 15 20 25 103 KIT

Fig. 9. The temperature dependence of the resistivity of the p form (0) and TMAs salt (a). The resistivity of [(CH,),N][Ni(dmit),], (0, a) is also presented for comparison.

Fig. 10. Intermolecular overlap integrals of the LUMOs of the Pd(dmit), molecules ( x lo3) in the a form, p form and TMAs salt. Left-hand side: ci form, A, -39.964; B, 3.476; 0, - 1.430; p, -2.325; r, -0.866; C, -41.914; D, 2.923; r, -0.502; s, - 1.953; t, -0.575. Right-hand side: p form and TMAs salt. fi-TMA, A, -40.661; B, 2.863; p, - 1.317; q, -0.797; r, -0.722. TMAs salt, A, -39.592; B, 4.196; p, - 1.728; q, - 1.022; r, -0.886.

V /

Fig. 11. (a) Tight-binding band energy of [(CH,),N][Pd(dmit),], and (b) its Fermi surface.

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368 J. CHEM. SOC. FARADAY TRANS., 1990, VOL. 86

a( l+A) tz

t = It,l=lt,l it, I A

Fig. 12. Schematic energy diagram of the dimeric chain; It, I is the intra-dimer interaction, I t , I is the inter-dimer interaction and A is the magnitude of the dimeric distortion.

frontier orbitals. High-pressure conductivity experiments on the a form have revealed the existence of the metallic region above 6 kbar, where the Fermi surface in fig. 11 will have a meaning.

Cs[Pd(dmit),], , which has recently beeen prepared by Underhill, is isomorphous to the /3 form." The Cs salt shows weak metallic behaviour down to ca. 60 K.

It is well known that the tight-binding band energy of the system with a regular chain and the intermolecular inter- action t , is ~ ( k ) = 2t cos(kc). Whereas, the electronic energy of the dimeric chain is given as, ~ ( k ) = & [ I t , 1, + 1 t , 1' + 2 It, I I t , I c~s(kc)]'/~ (see fig. 12). If the intra-dimer inter- action I t , I is much larger than the interdimer interaction I t , I, E = +-I t , I. The state with energy - 1 t , I and that with It, I correspond to the energies of the bonding and anti-bonding orbitals of the M(dmit), dimer, respectively. Since one elec- tron is in a bonding orbital, the dimeric structure is energeti- cally favourable. The electrostatic Coulomb energy (E, )

between conduction electrons is lowest in this case because the electrons are separate with equal spacing 2a (fig. 12). But the dimeric structure is accompanied by the lattice deforma- tion energy (Ed = (k/2)A2, where A is the magnitude of the dimeric distortion and k is the elastic constant). So the dimeric column will be realised when Ed is small and E, is large.

[(CH,),N][Ni(dmit),], has almost equal transfer integrals I t , I and I t , I (where I t , 1/1 t , 1 = l), but I t , [ of the Pd analogue is much larger than 1 t , I, (where 9 d I t , I/[ t , I < 14). The elec- tron motion in the dimeric column of Pd(dmit), may not be so free, compared with that in the Ni(dmit), column. The

simple band picture may be less suitable in the Pd complexes, at least at ambient pressure.

Low-temperature X-Ray Diffraction Measurements

We examined the temperature dependence of intensities of X-ray reflections within the temperature range 90-300 K. X-Ray photographs of a-Pd(dmit), were taken at 155 and 103 K. Oscillation photographs around the a, b and c axes show that the extra reflections indicating the development of the a x 2b x 2c structure is observed at 103 K (fig. 13), which cannot be found at 155 K. In the /3 form and TMAs salt no change was observed in the photographs around the b axis at 98 K, which is consistent with the X-ray observation of [(CH,),N][Ni(dmit),], with the same space group C2/c. In [(CH,),N][Ni(dmit),], , no structural change was observed at 92 K.? These results seem to show that periodical lattice distortion hardly develops in the system comprising of two crystallographically equivalent molecular stacks with differ- ent stacking directions. The loss of the conductivity of the /3 form cannot be ascribed to simple gap formation due to doubling of the lattice spacings : large electron correlation and small band width will be responsible for it. On the other hand, in the case of the a form, the doubling of the lattice spacings also contributes to the M-I transition. Metallic behaviour at high pressures implies an increase in the band width. The sharpening of the M-I transition suggests that the main reason for the M-I transition is the nesting of the Fermi surface, at least at high pressures. The behaviour of the elec- trical conductivity at higher pressures may be of interest because of the appearance of superconductivity of (TTF) [Pd(dmit),], above 16 kbar. The high-pressure conductivity behaviour of the B form and TMAs salt will also be of special interest because they will become metallic at high pressure. If the existence of the two crystallographically equivalent molecular stacks prevents nesting of the Fermi surfaces, it might be possible that the metallic state existing at high pres- sures will be retained at low temperatures.

The authors are grateful to Dr P. Cassoux, Dr L. Valade and Dr L. Brossard for access to the unpublished data on a- and a'-[(CH,),N)[Pd(dmit),], and also to Prof. S. Kagoshima for his preliminary X-ray work on [(CH,),N][Ni(dmit),], at extremely low temperatures.

t No indication of a structural transition was observed down to at least 15 K. (S. Kagoshima, personal communication).

Fig. 13. The oscillation photographs of a-[(CH,),N][Pd(dmit)2]2 at 155 K (a) and 103 K (b).

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J. CHEM. SOC. FARADAY TRANS., 1990, VOL. 86 369

References 1 L. Brossard, H. Hurdequint, M. Ribault, L. Valade, J-P. Legros

and P. Cassoux, Synth. Metals, 1988,27, B157. 2 L. Brossard, M. Ribault, M. Bousseau, L. Valade and P.

Cassoux, C . R . Acad. Sci. (Paris), 1986, 302, 205; M. Bousseau, L. Valade, J-P. Legros, P. Cassoux, M. Garbauskas and L. V. Interrante, J . Am. Chem. SOC., 1986,108,1908. A. Kobayashi, H. Kim, Y. Sasaki, R. Kato and H. Kobayashi, Solid State Commun., 1987,62, 57. C . Bourbonnais, P. Wzierek, D. Jerome, F. Creuzet, L. Valade and P. Cassoux, Europhys. Lett., 1988,6, 177. A. Kobayashi, H. Kim, Y. Sasaki, R. Kato, H. Kobayashi, S. Moriyama, Y. Nishio, K. Kajita and W. Sasaki, Chem. Lett., 1987, 1819; K. Kajita, Y. Nishio, S. Moriyama, A. Kobayashi, H. Kim, Y. Sasaki, R. Kato, H. Kobayashi and W. Sasaki, Solid

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State Commun., 1988,65, 361; A. Kobayashi, H. Kim, Y. Sasaki, S. Moriyama, Y. Nishio, K. Kajita and W. Sasaki, R. Kato and H. Kobayashi, Synth. Metals, 1988,27, B339.

6 G. Steinmecke, R. Kirmse and E. Hoyer, 2. Chem., 1975,15,28. 7 International Tables for X-Ray Crystallography (Kynoch Press,

Birmingham, 1974), vol. 4. 8 T. Sakurai and K. Kobayashi, Rep. Inst. Phys. Chem. Res., 1979,

55,69. 9 S. Alvarez, R. Vicente and R. Hoffmann, J . Am. Chem. SOC., 1985,

107,6253. H. Basch and H. B. Gray, Theor. Chim. Acta (Berlin), 1966, 4, 367. A. E. Underhill et al., to be published.

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Paper 9/018541; Received 3rd May, 1989

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