Semiconductor Properties of Transition Metal Chelates of Ligands

3132

E. J. ROSAAND G. N. SCHRAUZER

Semiconductor Properties of Transition Metal Chelates of Ligands Derived from a-Dithiodiketones by E. J. Rosa and G. N. Schrauzer’

Downloaded by UNIV OF NEBRASKA-LINCOLN on August 28, 2015 | http://pubs.acs.org Publication Date: September 1, 1969 | doi: 10.1021/j100843a058

Shell Development Company, Emeryville, California (Received March 1.9, 1959)

Transition metal complexes derived from a-dithiodiketones have semiconducting properties comparable with large organic aromatic hydrocarbons and charge-transfer complexes. The observed resistivities lie between lo8 and 1Ol6 ohm cm (at 25’). They depend on the chelate structure, the metal, and the ligand substituents. M = Ni, Pd, and Pt, with the same substituents R, a linear correFor the square-planar complexes M(SZCZR& lation between the resistivity and the first polarographic half-wave potential was observed which indicates that electron transport takes place in the lowest unoccupied T MO’s of the complexes. This is in accord with the proposed electronic structures of the complexes and the result of Hall-effect measurements which identify the majority charge carriers as negative. Measurements of the thermoelectric power coefficient, furthermore, revealed the presence of trapped negative centers of charge. Exposure of polycrystalline NiS&rPha to a 1.5-MeV electron beam produced a potential persisting for several weeks. Among the trigonal-prismatic complexes M(SzCzR2)s, the V, Cr, Mo, and W complexes have resistivities of between 10l2and 1014 ohm cm. The rhenium complexes are the best conductorsof all, indicating that the unpaired electron must be in a molecular orbital delocalized over the whole molecular complex.

Introduction The current interest2r3in electron-transport phenomena in organic molecular crystals, charge-transfer complexes, coordination compounds, and radical-anion salts prompted this report of the results of a study on the electrical conduction properties of recently discovered sulfur ligand chelates. It is well known that the semiconducting properties of organic compounds, in a large number of cases, correlate with the energy difference between the highest occupied and the lowest empty ?r molecular orbitals, suggesting that the transport of electrons in these systems involves the antibonding, or nonbonding, empty T molecular orbitals of the compounds. Recently, semiconducting sulfur-containing polymers have been obtained4 from the reaction of sulfur with aromatic hydrocarbons. Although the detailed constitution of these polymers is not known, this demonstrates that unsaturated carbon-sulfur systems are capable of facilitating electron transport. It thus appeared likely that coordination compounds of certain unsaturated sulfur ligands might have interesting electrical properties. The study to be reported in this paper deals with the electrical conductivity of solid chelates of composition M(SzCzR2), (n = 2 or 3), MzSz(SzC2R&, n12S2(S4C4R4)2, Mz(S4C4R4)z,and related compounds (structures are shown in Table I) which constitute novel delocalized ~ y s t e m s . ~The ligands in these complexes are derived from dithiodiketones

s s

II il

R-C-C-R

which on complex formation attain a state intermediate between dithiodiketones and dithiolato dianions. The The Journal of Physical Chemistry

compounds possess unusually high electron affinities and low optical excitation energie~.~~’ For example, the first T-T* transition in most of the complexes occurs in the near-infrared region. The comparative study of the semiconducting properties of these complexes appeared particularly promising in view of the large number of available compounds of most transition metals with unsubstituted and various substituted ligands.

Experimental Section The synthesis and properties of the compounds under study have been described in numerous papers and will not be detailed here.* They are of composition M(S2C Z R Zor)8,~ MzSZ(SZCZRZ)Z, ~ T z S Z ( S ~ C ~ [MSdCdRJ R ~ ) Z , 12, etc. Most compounds M(SzCzRz)z or 3 are air-stable and can be heated without melting or decomposition up (1) To whom correspondence shorild be addressed at the Department of Chemistry, University of California, San Diego, Revelle College, La Jolla, Calif. 92037. (2) For general references, see articles in (a) H. Kallmann and M. Silver, Ed., “Symposium on Electrical Conductivity in Organic Solids,” Interscience Publishers, New York and London, 1961, (b) T. T. Brophy and T. W. Buttrey, Ed., “Organic Seniiconductors,” The Macmillan Co., New York, K. Y., 1962, and (e) F. Gutmann and L. E. Lyons, “Organic Semiconductors,” John Wiley and Sons Inc., New York, N. Y., 1967. (3) (a) D. D. Eley, G. D. Psrfitt, M. T . Perry, and D. H. Taysum’ Yrans. Faraday SOC.,49, 79 (1953); (b) A . Many, E. Harnik, and D. Gerlich, J . Chem. Phys., 23, 1733 (1955). (4) H. Akamutu and H. Inokuchi, ref 2a, p 277. (5) G. N. Schrauaer, 5‘. P. Mayweg, and W . Heinerich, J . Amer. Chem. Soc., 88, 5174 (1966), and references cited therein. (6) G. N. Schrauaer and T’. P. Mayweg, ibid., 87,3585 (1965). (7) D. C. Olson, V. P. Mayweg, and G. N. Schrauser, J . Amer. Chem. SOC.88, 4876 (1966). (8) See, e.g., G. N. Schrauzer, Transition Metal Chem., 4, 299 (1968) ; Accounts Chem. Res., 2, 72 (1969).

SEMICONDUCTOR PROPERTIES OF TRANSITION METALCHELATES

3133

Table I: Structure and Solid-State Electric Properties of Sulfur Chelates (Resistivity, ohm cm, at 25' Shown in Parentheses)

Figure 1. Conductivity apparatus

M = Re structure I (105-107)


M = Ni, Pd, Pt structure 111 (107-1011)

M = Co, Fc Structure N ( 10'-1012)

to 180". NiSCaPh4 (Ph = phenyl) was available in large enough crystals for single-crystal measurements. The remaining compounds were measured as compressed powders, samples being prepared in the following way. Approximately 0.2 g of the powdered material was placed in a stainless steel die and compressed at 44,000 kg/cm2. The resulting disks had a diameter of 0.43 cm and a thickness from 1to 2 mm. Measurements on compressed-powder samples are subject to various experimental difficulties. For example, the large number of grain boundaries may obscure effects due to the bulk conductivity. Because of the large surface area of a powdered sample which is usually exposed to air or other gases, pressing the sample with the adsorbed gases must be considered as a potential source of error. The magnitude of the grain-boundary effect was determined by observing the changes in conductivity as a function of pressure and the effect of repeated applications of the same pressure. The conductivity increased with increasing pressure and

asymptotically reached a limiting value. Repeated applications of a given pressure gave a hysteresis which approaches constant conductivity after several compression cycles. Comparisons were made of the conductivity of compressed samples with the conductivity of single crystals of the same complex. It was observed that the conductivities of the crystal and the limiting conductivities of the repeatedly compressed samples were the same. For this reason, the effect of grain boundaries was assumed to be negligible. I n attaching electrodes, contact problems can arise. The type of material and the method used in making an electrical contact to a sample has been found to have a significant effect on the magnitude of the measured conductivity. After several tests, it was concluded that gold-plated copper electrodes attached with aquadag (a water suspension of carbon) produced satisfactory electrodes. These electrodes provided an ohmic contact up to a field strength of 5000 V/cm. For the Hall-effect measurements, a silver-impregnated epoxy resin was used to give the needed mechanical strength. Conductivity Measurements. The dc conductivity was measured using the apparatus outlined in Figure 1. The samples were subjected to a constant dc potential from a Hewlett-Packard H P 500 regulated-power supply, using a Keithley 110 A electrometer to measure the resulting current. All current measurements were made a t an applied potential of 50 V and were found to be linear with respect to potentials up to 500 V. No lower voltage limit of conductivity could be observed down to approximately 10 mV. For convenience and safety 50 V was chosen as the potential for all measurements. The samples were placed in a cell where they could be subjected to controlled environments. Conductivity data for two of the complexes were collected in atmospheres of N1,02, H2,He, and Ar or under vacuum. It was found that the conductivities of NiS,CrPh, and ReSeCs(CH& remained unchanged in the various environments. Activation. Energy. The conductivity measurements were made at temperatures between 25 and 100" by placing the sample cell in a thermostatically controlled oven. Sample temperatures were continuously monitored by an iron-constantan thermocouple and recorded with a chart recorder. The first current measurement on each sample was made a t room temperature. The circuit was opened, the temperature was raised and Volume 75, Number 9 Seplember i9E.9


3134 allowed to come to equilibrium before the potential was reapplied to the sample, and a current reading was taken. The current through the sample was continuously recorded by a strip-chart recorder. It was observed that the currents in some samples were greater immediately after the potential was applied than at a later time. For these samples, the initial current was used to obtain the conductivity. It is now believed that in some cases the time-dependent currents are a manifestation of carrier trapping. The precision of the conductivity measurements was limited by the uncertainty in the resistance and the thickness of the aquadag layer; the voltage was measured to better than 0.1 V with a Fairchild digital voltmeter. The current readings were accurate to 1% and thickness measurements made with a Starrett micrometer gauge were also accurate to 1%. Plots of log of conductivity us. the reciprocal of the absolute temperature yielded an activation energy with a value of about 0.5 eV for each of the samples. This insensitivity of activation energy with respect to conductivity is not understood, but it clearly does not represent the same quantity described by a band-theory model. Xeebeck Coeflcients. The Seebeck or thermoelectric power coefficients were determined by placing the samples in a temperature gradient and measuring any electric potential developed across the sample. The apparatus was constructed to hold a sample, provide the temperature gradient, and to measure any potential that developed. The sample was held between goldplated copper electrodes. The top electrode was in thermal contact with a cartridge heater which was electrically isolated from the electrode by a thin mica sheet. The bottom electrode was part of a large copper block which served as a heat sink; it was assumed that the temperature of the lower electrode would remain at the temperature of the block. The temperature of the upper electrode was monitored by an iron-constantan thermocouple imbedded just beneath the electrode surface. The temperature gradient was obtained by measuring the thickness of the sample and distributing the temperature differences across it. The potential drop across the electrodes was measured by a Keithley 110 A electrometer. The potentials observed by this technique were not used to obtain the usual semiconductor parameters because of the appearance of an interfering potential of comparable magnitude. This Electret effect behavior is thought to be a result of carrier trapping. Hall Voltage Measurements. To determine the mobility and charge sign of the carriers responsible for electrical conduction, Hall voltage measurements were made on one crystalline and two compressed-powder samples. The measurement of Hall voltages in such high-resistance samples presented special problems and necessitated the construction of a unique apparatus to perform the measurements. This equipment will be The Journal of Physical Chemistry

E. J. ROSAAND G. N. SCHRAUZER described elsewhere. In each sample, Hall potentials were measured at room temperature and at least five different current levels. The magnetic-field strength was essentially fixed because its source was a permanent magnet. However, through the use of removable poleface shims, a choice was possible between a high- and a low-field value. From the observed Hall potential a Hall constant ( R H )was obtained for each sample from the expression

where VH is the measured Hall potential in volts, I is the sample current in amperes, H the applied magnetic-field strength in gauss, and d the sample thickness in centimeters. To obtain the Hall mobilities, the assumptions were made that the measured conductivity u was intrinsic and that the electron mobility was much greater than the hole mobility. When minority and secondary charge carriers are not considered, the expression for Hall mobility reduces to the simple form

8

p~ = -

3n

RHUcm2/V sec

The sign of the majority charge carriers was obtained by comparing the sign of the Hall potential in the unknown samples to that in a semiconductor of known majority carrier sign. All of the complexes tested exhibited n-type conductivity. Hall measurements were made on the complex NiS4C4Ph4in two forms, compressed powder and single crystal. Another complex investigated was ReS6C6(C&)6, the second most conductive of the entire series. The Hall mobilities measured for each case are about 10 cm2/V sec. Other Measurements. Some measurements were also carried out on samples doped with various amounts of other samples; for instance, nfoS6Cd'h6 was doped with ReS&&Ph6to introduce species with a mobile electron. This only had the effect of diluting the conductivity of the rhenium compounds in a simple way.

Results The dc bulk conductivity of compressed powder samples was determined by measuring the current resulting from the application of a known potential. The resistivity at 25" was then calculated from the expression

where i is the current in amperes, E , the applied potential in volts, t the sample thickness in centimeters, and A the sample area in square centimeters. The roomtemperature resistivities of the various complexes measured are given in Table 11. Along with the resistivities are listed the first electronic transition energies and first polarographic half-wave reduction poten-

3135



Table I1 : List of Resistivities, First Half-Wave Potentials, and of First Intense Absorption Transition Energies Formula

M

R

MS~C~RI

Pd Ni Pt Pd Ni Pt Pd Ni Pt Pt Pd Ni Fe Fe Fe co co

CHI CH3 CH3 Ph Ph Ph PhOCHs PhOCHa PhOCH3 PhCHa PhCHs H PhCHs Ph PhOCHs Ph PhCHa Ph Ph Ph PhCHa CH3 H H CH3 Ph CH3 PhOCHs PhOCHs Ph Ph PhCH3 PhOCHa CH3 PhOCHa Ph PhOCHa PhCHa

MSeCeRs

M (CO)zSnCcR4

M~SZ(&CIR~)Z

Fe~Sz(SzC~R2)2

os

Re W W Re Re Mo Mo V Mo Mo W W Mo Mo NI 0 MO W Fe Fe Fe

tials. I n some cases, the transition energies were obtained both from solution measurements and reflection spectra of solid samples. I n these cases, there were no obobserved differences between the values of A,, served in solution or in the solid, and no new bands or unexpected spectral phenomena were observed in the reflection spectra, indicating a weak-coupling solid.

Discussion Conductivity and Structure of the Complexes. The range of observed resistivities (Table 11) can be compared with the structures of the complexes. The fact that the rhenium complexes Res&& (I in Table I) are much better conductors than the essentially isostructural vanadium, molybdenum, and tungsten compounds shows that the number of electrons in the available orbitals of the complexes strongly affects their conductivities. On the other hand, the conductivity of the vanadium complexes VSeC6R6 is about that of the molybdenum and tungsten compounds, indicating that

p

ohms om

9x 6X 9 x 7 x 5 x 4x 9x 2 x 3x

107 lo8 109 107 108 10'0 10'0 101' 10"

>io14

3 x 108 107 8 x 108 7 x 108 nJ10'0 -10"

2 x 4x 6X 4x 3 x 4 x

10'2 1014 10' 10'8 1014 106

-103

1 . 4 X 10l2 3 x 1014 1 x 1014 5 x 1016 2 . 3 x 1013 5 x 10'8 7 x 107 7 x 1018 3 x 1014 2 x 1012 1 . 5 x 1013 1.3 x 1014 3 x 107 2 x 108 2 x 10'0

El,*, V

AX, mp

-0.060 -0.107 -0.133 -0.182 0.134 0.090 0.086 0.035 -0.004 f0.043

800 763 735 885 866 802

+o. 120 ... ...

...

925

...

823 916 720 ,..

... 716

fO.01

... ...

$0.151 -0.04 -0.138 -0.136

...

-0.09 -0.307 f0.33

0.022

...

735 555 710 660 650 630 603 640 69 1 710 740 560 547 546

880

the presence of holes in the system has little if any effect on the mobility of charges. The sulfur-bridged iron complexes Fe2S2(S4C4R4)are somewhat better conductors than the bis complexes MS4C4R4 of Xi, Pd, and Pt. I n the series of the sulfur-bridged molybdenum compounds the conductivities are about equal to those of the tris complexes. This demonstrates that intermolecular 1\/I-S-M bridging groups do not inhibit charge transport. This, of course, is not unexpected in view of the known semiconducting properties of metal sulfides. However, in the dimeric complexes of iron and cobalt, the conductivity is somewhat lower than in either the planar bis complexes or the iron complexes Fe2Sz(S4Cr RJ. The X-ray analysis of a CFs-substituted cobalt complex indicates that the formation of the Coz[S4C4(CF,)4I2 dimer weakens the conjugation in the CoszC2R2 chelate rings involved in the dimer f o r m a t i ~ n . ~ T. H. Enemark and W-. N. Lipscomb, Inorg. Chem., 4, 1729 (1965). (9)

Volume 73, Number 9

September 1969


3136

E. J. ROSAAND G. N. SCHRAUZER

This could, at least in part, also account for the lower conductivity in these compounds. The compounds N!o (C0)2S4GR,, finally, are rather poor conductors, most likely due to packing and diminished conjugation. Electronic Structure and Bonding. In previous papers, it has been shown that the complexes n/r(S2CzR 2 ) 2 0 r 3 and many of the related species are novel delocalized systemsn6g8 The most realistic description of the complexes results if the ligands are initially considered to be dithiodiketones. On interaction of the metal orbitals with the butadiene-like a MO’s of the dithiodiketone systems, new molecular orbitals result, which are delocalized over the whole molecular complex.6b6’8 In view of the high degree of covalency in the AT-S bonds it is no longer permissible to simply distinguish the “central metal” from the “ligands.” The complexes must be regarded as individual entities, not unlike the polycyclic aromatic hydrocarbons, e.g., quinones. The chemical similarity, mainly with the latter group of organic compounds, is particularly noteworthy. Planar Complexes of Ni, Pd, and Pt. For the present purposes it is sufficient to consider the behavior of the set of lowest unoccupied R ligand 140’s in a planar complex, h!tSdCdRd,on interaction with the appropriate metal orbitals. The relevant ligand orbitals transform as B1, and BZg,which may interact with the 4p, and the 3d,, orbital, respectively. Calculations have shown that the B1,-type 140’s are sufficiently stabilized on interaction with the 4p, orbital to become occupied.6 The Bzgcomponent, on the other hand, becomes slightly antibonding and remains empty in the neutral complexes. The system 1111S4C4Rdthus contains two electrons in predominantly ligand-based R MO’s, causing the ligands to attain a state intermediate between dithiodiketones and dithiolato dianionsS6 This is best represented in terms of the two limiting resonance structures I and 116

I

I1

The Bzgorbital is the lowest unoccupied a MO in the system. As it is only about 1.5 eV above the ground state, it is responsible for the pronounced electron affinity of the complexes. I n addition, this orbital also causes the appearance of an intense T-T* transition (2bl, + 3b2,) in the near-infrared region. The 3b2, MO is delocalized over the whole molecule of the complex but is predominantly ligand in character.6 It thus would appear to be involved in the processes of electrical conduction. Prismatic Complexes. I n the prismatic tris complexes, the bonding situation is very similar to that in the planar group VI11 bis complexes, except that for reasons of symmetry the ?r interactions are no longer T h e Journal of Physical Chemistry

strictly separated from the u interactions.8f10 For the present purposes, it is sufficient to describe the behavior of the set of lowest unoccupied ligand a orbitals on interaction with the metal orbitals. I n the prismatic arrangement, the lowest unoccupied ?r orbitals of the isolated ligands transform as E’ and A’z. The A’z set cannot interact with any metal orbitals and is nonbonding in the complex. The E’ orbitals interact with the metal p and d orbitals of E’ symmetry producing new orbitals of which the 4e’ is occupied and significantly ligand in character. Orbital 5,’ remains empty in the neutral complexes of V, Cr, RIo and W, but is occupied on reduction to the anions, or in the rhenium complexes. This orbital is delocalized over the whole complex but is still significantly ligand a. Being only about 1.5-2.0 eV above the ground state it gives rise to the high electron affinity of the complexes as well as to the intense low energy a - ~ *transitions observed in the near-infrared region. It thus is the orbital equivalent to the 3bz, MO in the planar complexes, and it must be assumed to be involved in the process of conduction. A schematic molecular-orbital diagram describing the behavior of the most important orbitals is shown in ref 11. I n terms of valence-bond theory, the result of the M O description for complexes of Cr, Mo, or W can be expressed by three limiting resonance structures III-VlO

I11

IV

V

Sulfur-Bridged Species. I n the sulfur-bridged iron l 2whose struccomplexes of cornpositionFe~S~(S~C~R~)~, ture is as yet unknown, the electron transfer evidently occurs across the Fe-S-Fe bridges. The same is probably the case in the molybdenum complexes6M O ~ S ~ ( S ~ C ~ Owing R ~ ) to ~ . the unsaturated nature of the ligands, there are available low-lying empty orbitals for electron transport, just as in the case of the complexes M(S2C2Rz)z or 3 . The electron structure of the dimeric Fe and Co complexes M2(S4GR4)2 may be understood qualitatively by bringing together two hypothetical monomeric species MS4CdR4 in a fashion which produces the structure of the dimers. Owing to the similarities in the orbital energies and overlap integrals, model calculation^^^ for the hypothetical planar complexes I\iISqCdR4indicate that the electronic structures should be analogous to those of the com(10) G. N. Schrauser and V. P. Mayweg, J. Amer. Chem. Soc., 88, 3235 (1966). (11) D. C. Olson, V. P. Mayweg, and G. N. Schrauzer, ibid., 88,4876 (1966).

(12) G. N. Schrauzer, V. P. Mayweg, H. W. Finck, and W. Heinrich, ibid., 88,4604 (1966). (13) Unpublished results.

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c

pounds MS4C4R4,where 14 = Ni, Pd, or Pt. Approaching the monomeric species impairs the resonance 7 Phenyl in the i19SzCzRzchelate rings which are involved in the __ formation of the dimeric structure. I n agreement with Pd (855 ) observation, these complexes should, therefore, be poorer conductors than the Xi, Pd, and Pt complexes. Radical-Anion Salts. We have also determined the conductivity of some radical-anion salts, e.&, N(CzH5)4+, (NiS4C4Ph4-). The resistivities of these compounds were in the order of 1013 ohm cm. They thus appeared to be too poorly conducting to warrant further study. Although one would expect the unpaired electron in the anions to be mobile, the intramolecular barriers resulting from the presence of the cation are probably too large. Mechanism of Conduction. The conductivity of organic molecular compounds is known to increase with the electron affinity of the organic 7r s y ~ t e m . ~ - ~ > ~ ~ Accordingly, the activation energy of conduction as well as the optical threshold energy for photoconduction decrease with increasing conductivity of the compounds. Figure 2. Correlation between resistivity and half-wave potential for three ligand families. It has, therefore, been proposed16that the first step in the conduction process is the excitation of a 7r electron from the uppermost filled n- orbital to the lowest empty cules, and the conduction process would involve ex7r N O . The electron is then assumed to tunnel to the clusively electrons hopping across the intermolecular equivalent empty level of the neighboring molecule in barriers. It is furthermore conceivable that the poputhe direction of the anode whereas the positive hole is lation of electrons in the lowest unoccupied n- MO moving to a molecule in the opposite direction, toward should be proportional to the absolute energy of the the cathode. It is possible to adopt this model for the lowest empty 7r orbital in the complexes, at least for discussion of the mechanism of conduction in the sulfur complexes with the same crystal structure, substituents, chelates, but it must be pointed out that there are some and identical electrode contacts. The conductivity difficulties necessitating modification of the model to should thus correlate with the first polarographic reducsome extent. If initial excitation is invoked for the tion potential. Polarographic potentials may be used complexes, it is no longer necewary that the electron in as measures of the absolute energy of the lowest unthe uppermost n- level comes from the highest occupied occupied R MO because the intermolecular interactions r 1\10. It could also arise from an orbital essentially in the crystal are small. This follows from the similarlocalized on the metal atom, and this would result in the ity of the absorption spectra in solution with the solidgeneration of an essentially immobile positive center. state reflection spectra. For a number of complexes of If the electron is assumed to arise from the highest 7r Nil Pd, and Pt with different substituents a linear corre110,it must be placed in a state of different multiplicity lation with the first polarographic half-wave reduction to become sufficiently long-lived. However, the first potential was observed (Figure 2). The vanadium excited singlet as well as the first triplet state will be complexes are poor conductors in spite of the ease with almost certainly too far above the ground state to allow which they are reduced to monoanions. In this case, thermal population. Hence, it must be assumed that the addition of the electron produces a closed-shell conthe necessary excitation energy is at least partially offset figuration rather than a species with a mobile electron in by some other force, possibly of the charge-transfer the next higher 7r 140. The rhenium complexes, on the type. Although we do not dispute the contention that other hand, are the best conductors of this series, evisome molecules will be initially present in an excited dently because the extra electron occupies the 7r MO state, we believe that the majority of the negativeinvolved in electron transport. The complexes of MO charge carriers are formed by the direct injection of and W are poor conductors since they are less easily electrons into the lowest unoccupied r 110 of the molereduced than the rhenium compounds and possibly becules nearest the cathode. This process appears to incause the closed-shell configuration causes some extra volve only a small energy of activation, and experiments stability of the orbital system. Mixtures of ReS6C6Ph6 in which the conductivity was measured with varying potentials indicate that the threshold voltage for this (14) D. D. Eley arid M. R. Willis, ref 2a, p 257. process is small. The charge carriers that would result (15) D. D. Eley, G. D. P. Parfitt, M . T. Perry, and D. H. Taysum, thus would be anions rather than excited neutral molel'rans. Faraday Soc., 49,79 (1953). Volume 78, Number 9

September 1969


3138 and WS&d'h6 were prepared and investigated but only showed diminished conductivity and no special electric properties. Among the remaining compounds that have been studied, the sulfur-bridged iron complexes FezSz(SzC2Rz)z were found t o have conductivities similar t o the planar group VI11 metal bis compounds. It has been pointed out that the complexes of this type have electronic structures related to the group VI11 metal complexes.12 I n addition, it is conceivable that electron transport is facilitated by the greater surface of the molecules which would produce stronger intermolecular interactions and consequently narrower barrrier widths. Substituent Egects. The conductivities were found to be strongly substituent dependent in all cases. On the whole, the conductivities decrease with increasing size of the substituents, evidently because of their effect on the intermolecular separation. For a quantitative discussion, the intermolecular distances of all complexes would have to be known. In NiS4C4H4, the nearest sulfur atoms are separated by 3.04 8 . l 6 I n NiS4C4Ph4 the distance between the nearest sulfur atoms is 4.6 A.l7 The differences in the intermolecular distances thus may well be responsible for the observed differences in the conductivities. However, electronic effects of the substituents must also be considered, since they affect the electron affinities of the compounds. I t is therefore not possible to propose a quantitative theory of the substituent effects without detailed structural information and calculations of the electronic structures in each individual case. For the nickel complexes, the conductivity decreases in the order H < P h < CH3 < C6H4CH3 < CsH40CHa. For palladium the sequence is similar but for platinum compounds it was found to be CH3 < P h < C6H40CH3< C6H4CH3. The overriding effect thus seems to be the size of the subptituents. The single-crystal conductivity measurements on NiS4C4Ph4 and ReS&a(CH3)6indicate that the conductivity is nearly isotropic. Carrier Trapping. The measurements of the thermoelectric power coefficient indicate the presence of trapped negative centers of charge. I n order to study this property of the complexes further a sample of polycrystalline NiS4C4Ph4was exposed to a 1.5-MeV electron

The Journal of Physical Chemistry

E. J. ROSAAND G. N. SCHRAUZER beam. This produced a potential which persisted for several weeks after the exposure. Since this potential could not be discharged by shorting the circuit, it must be concluded that anionic species XiS4C4Ph4-were present in the solid which evidently cannot be rapidly discharged due to their relative immobility. Comparison with Other Compounds. In Table 111,the resistivities of some organic molecular compounds as determined by Akamutu and Inokuchi4 are listed for comparison. I t may be seen that the complexes M(Szare some of the best low molecular weight, Table I11 : Resistivities of Some Organic Compounds Perylene Anthanthrene Anthanthrone Coronene Ovalene Violanthrone Indathrene black Cyananthrone Dimethylaniline chloranil Tetramethylp-phenylenediamine chloranil Per ylene-iodine Metal phthalocyanines M(SzCzRz)zor a

4 x 1018 1 . 5 X 10le 7 . 7 x 101s 1 . 7 X 10l7 2 . 3 x 1015 2 . 3 X 1Olo 2 . 5 X 1OI8 1.2 x 107 1.0 x 109 2 . 4 x 104 1 . 0 x 101 1011-1016

108-1015

neutral-compound semiconductors reported to date. The conductivity of some of the rhenium complexes in fact approaches that of intrinsic silicon. Hence, it might be possible to obtain polymeric complexes with ligands related to the dithiodiketones with even higher conductivities. That such low molecularweight neutral coordination compounds exhibit conductivities comparable to or greater than those of very large carbocyclic n-electron systems is quite remarkable and clearly a further consequence of their unusual n-delocalized electronic structures. (16) A. E. Smith, personal communication, Shell Development Co., Emeryville, Calif. (17) D. Sartain and M. R. Truter, Chem,. Commun., 382 (1966).

Semiconductor Properties of Transition Metal Chelates of Ligands

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