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Cycloalkylated biimidazolium dications (A") of reduction potentials from -0.4 to -1.4 V form ion pair charge-transfer complexes (A2+[M(mnt)z]2-) with dithiolene metalates, M = Zn, Ni, Pd, Pt, mnt2-= maleonitrile-2,3-dithiolate. X-ray analyses of (PzBBImZf[Ni(mnt)2]2-) [P2BBImZ+ = 1,1',3,3'-bis(propane-l,3-diyl)bibenzimidazolium] and (B2BIm2+ [Pd(mnt)2]2-) [B2BImZ+ = 1,1',3,3'-bis(butane-l,4-diyl)biimidazolium] reveal that the solid-state structure is largely determined by the geometry of the acceptor. When the latter is strongly twisted, the usually observed mixed donor-acceptor columns are modified to a chain-like arrangement. In the case of the bibenzimidazolium dication P2BBIm2+ the otherIon-pair charge-transfer (IPCT) complexes consisting of redox-active components provide ideal conditions for the systematic investigation of supramolecular CT interact i o n~ [~,~] .The combination of dianionic transition metal dithiolenes [ML2I2-with organic dicationic bipyridinium compounds (A2+) leads to IPCT complexes of the type (A[ML2]} showing an IPCT band in their UV-Vis-NIR spectraL41. Application of the "Hush model" affords thermodynamic and kinetic data about the relation between optical and thermal electron transfer in Flash photolysis reveals the formation of the primary redox products A+' and [ML2]-followed by a fast back-electron transferr61. Moreover, on the basis of the redox potential differences of the components the solid-state electrical conductivity can be quantitatively tunedL71.In the course of our previous investigations only bipyridinium compounds and closely related systems were used as acceptors. The first reduction potentials of these are found in the range of -0.4 to -0.8 V (MeCN, vs SCE). In the following we report on the chemical, structural, and spectroscopic properties of IPCT complexes containing biimidazolium acceptors R2BIm2+ which have unusually low first reduction potentials from -1.1 down to -1.4 V. These potentials qualify them as excellent candidates in photoreduction reactions including the photoreduction of water. Ad-

lengths and bond angles of [Pd(mnt)2] 2-given in Table 2 compare well with the data reported for the mono-an- (Domingos et al., 1988). Relative to the monoanion, the Pd--S and C--S bonds of the dianion are elongated, in their arithmetic mean, from 2.26 to 2.30 and from 1.71 to 1.73 A, respectively, as expected by molecular orbital theory (Schrauzer, 1968). The C1--C2 bond is almost unchanged according to values of 1.39 and 1.38 ,~, respectively. The aromatic core of the bipyridinium component is planar and the bond lengths and angles agree well with those reported for MV[Ni(mnt)2], and the methylviologen dihalides (Russell & Wallwork, 1972). AbstractThe structure of the ion-pair charge-transfer complex N,N'-dimethyl-4,4'-bipyddinium bis(c/s-1,2-dicyanoethene-l,2-dithiolato)palladate(II), MV[Pd(nmt)2], consists of columns of alternating planar methylviologen dications and planar dithiolene palladate dianions along the a axis. The shortest intrastack and interstack distances of 3.42 and 3.34 A, respectively, are in the range of van der Waals contacts. The compound is isomorphous with the corresponding Ni homologue. CommentIn recent years we have been investigating ion-pair charge-transfer (IPCT) complexes of dianionic transitionmetal dithiolenes with the methylviologen dication MV 2+ and related bipyridinium acceptors. These compounds are of particular interest since they show photoinduced electron transfer from the dithiolene to the bipyridinium component, which might qualify them as photosensitizets for the photoreduction of water with visible light. In the solid state these composite materials are semiconductots and their electrical conductivities can be quantitatively tuned by variation of the redox potentials of the ions (Kisch et al., 1991). In the diffuse reflectance spectra of the solid platinum-group complexes MV[M(mnt)2] (nmt = c/s-l,2-dicyanoethene-l,2-dithiolate), the IPCT band shifts from 920 to 785 and 905 nm when nickel is replaced by palladium and platinum, respectively. This refleets the different reducing powers of [M(mnt)2] 2- (McCleverty, 1968). In the following, we report on the crystal and molecular structure of MV[Pd(mnt)2].The coordination geometry of the hitherto structurally uncharacterized [Pd(nmt)2] 2-dianion is square planar but the cyano groups C3-----N1 are markedly bent out of the mean plane containing Pd, S1, $2, C1 and C2 by 0.38 (N1) and 0.21 A (C3) (Fig.

Ion Pair Charge-Transfer, Optical Electron Transfer, X-rayThe dication of tetrakis(dimethylamino)ethene (TDAE2+) forms ion pair complexes with [M(mnt)2]2~, mnt2-= maleonitrile-2,3-dithiolate, M(II) = Ni, Pd, Pt, and Zn. According to X-ray analysis the structure of {TDAE2+[Pt(mnt)2]2~ } consists of a chain-like arrangement o f planar dianions and twisted (71°) dications. Shortest interionic contacts are 11 pm longer than the corresponding van der Waals radii. In the UV-VIS diffuse reflectance spectra of the complexes with a d8 central metal there is no unambiguous evidence for the presence of an ion pair CT band. This band is observed, however, at 450 nm when the d 10 metal Zn(II) is present. Application of the Hush-Marcus model affords a reorganization energy of 110 kJ / mol for electron transfer from [Zn(mnt)2]2-to TDAE2+. This value is 40 kJ / mol higher than the one reported for the analogous complex wherein TDAE2+ is replaced by a 2,2'-bipyridinium acceptor of similar reduction potential.

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