Yuka KAGAMI, Fumiyasu IWAHORI, Tachi OHISHI, Tetsuo HAMA, Toshio MANABE (Nagoya Univ.), Masahiro YAMASHITA (Nagoya Univ. and IMS), Hiroshi KITAGAWA, Ken-ichi SAKAI and Tadaoki MITANI (JAIST)
[Synth. Met. 86, 1803 (1997)]
We have investigated the physical properties of [M(HBQD)2]TCNQ (M=Ni, Pd; H2BQD=1,2-benzoquinonedioxime) which consist of alternating -D-A-D-A- type stackings of transition-metal-complex donors and organic TCNQ acceptors. 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 comfirmed by 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.