Dimerized segregated stack

In the salts based on segregated stacks of metallocenium-based cations with [M(mnt)2)] (M = Ni, Pd and Pt) anions, the anionic stacks are isolated from each other by the cations in a few compounds these stacks are regular (at high temperatures) but in most cases the [M(mnt)2)] units form dimers. The magnetic behavior of these compounds is dominated by the AF interactions between the anion units. 

The crystal structure of [Fe(Cp)2]2[Ni(mnt)2]2[Fe(Cp)2] is composed by segregated stacks of pairs of cations, [Fe(Cp)2]+, and zig-zag dimerized anions stacks, with a neutral [Fe(Cp)2] molecule laying beside each anionic dimer [65]. In the case of [Co(Cp)2][Ni(dmit)2], the crystal structure consists of layers composed by two types of chains formed by the [Ni(dmit)2] anions, through short S—S contacts. Cation pairs are located between the anionic stacks [68], In the crystal structure of [Co(Cp)2][Ni(dmit)2]3 2MeCN, the partially oxidized anions form... 

The classification in Fig. 2 contains other oversimplications. For example, the asymmetry of MP cations results in a disordered MP-TCNQ structure Such disorder leads to characteristic power laws in thermodynamical properties as illustrated most completely for quinolinium (TCNQ)2. Chemical disorder provides another possibility. Miller and Epstein have exploited phenazine doping " in segregated-stack alloys of the type P,(MP) jTCNQ for 0 g x 0.5. Phenazine doping of MjP-TCNQ, which has a mixed dimerized stack, restores segregated stacking and high conductivity in P(M2P)(TCNQ)2, a system that closely resembles MP-TCNQ. The MgP-TCNE complex in Section 4.4. probably shows yet another kind of disorder involving preferential TCNE-allyl interactions. 

The CT integral t, Eq. (2), reflects the detailed geometry of the n-n overlaps in either mixed or segregated stacks. Different overlap patterns in TCNQ salts, however, yield only qualitative information. For example, ring over external-bond overlap is favored and presumably results in large t. Theoretical treatments are still more oversimplified and focus primarily on the plane-to-plane separation and tilt of simple n-donors and acceptors. As discussed in Section 3.3, the jt-dimers of TCNQ" in PP-TCNQ and BP-TCNQ have close face-to-face geometry. On changing the DA dimer in Fig. 3 to an A"A" dimer, we find... 

Later on, Torrance et al. [89] have proposed another model for a segregated stack of radical ions causing a ferromagnetic spin alignment. In this respect, they consider a pair of adjacent radical anions Mj and M, each species having an unpaired electron delocalized in a tt orbital over an aromatic molecule. The ground state of the two molecule dimer may be written as Mj Mj and may be either a singlet or a triplet. The excited state is the one induced by... 

S = segregated stacks M = mixed stacks L2 =lone dimers... 

The dimerized segregated ds) stack has inversion symmetry only between molecular radicals. The An phonon mode is Raman-active and unperturbed, the anti-phase Au mode is shitted in frequency and vibronically enhanced in infrared. Thus no frequency coincidence between infrared and Raman spectra is to be expected in this case. 

The crystal structure of [Fe(C5H4R)2][Ni(mnt)2] ([Fe(C5R)2]+ = l,l -bis[2-(4-(methylthio)-( )-ethenyl]ferrocenium) is based on segregated chains of cations and anions. The stacks of anions are isolated by the cations and consist of a packing of dimers. In this compound yT decreases cooling, which indicating the presence of dominant AF interactions [66]. 

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