Physical properties of quasi-one-dimensional charge-transfer metal complexes; [M(HBQD)(2)]TCNQ (M=Ni, Pd; H(2)BQD=1,2-benzoquinonedioxime)

We have investigated the physical properties of [M(HBQD)(2)]TCNQ (M=Ni, Pd; H(2)BQD=1,2-benzoquinonedioxime) which consist of alternating -D-A-D-A- type stackings of transition-metal-complex donors and organic TCNQ accepters. The degree of charge-transfer for each sample was estimated to be -0.1 from the results of IR, Raman spectra, and bond length of TCNQ. From XPS measurements, the valence state of Ni or Pd was found to be transformed at low temperature from +II to the mixed valency of +II,+III or +II,+IV, respectively. This behavior can be considered to be due to the charge-transfer from d-orbital of the transition metal to pi-antibonding orbital of the HBQD ligands, which was confirmed by the lower-energy shift of IR stretching modes of C=N in HBQD. This temperature-induced charge-transfer between transition metal and ligand is the first observation in the transition-metal-complex assemblies.