Ni(III) Mott–Hubbard Compounds - Material Designs and New Physical Properties in MX-and MMX-Chain Compounds - page 34

where e 2 Qq , e 2 Qq atom , i , and s are the observed coupling constants, given twice the
observed resonance frequency (2 hn ) by assuming an axially symmetric EFG,

the coupling constant for atomic 81 Br given by 643.032 MHz [ 14 ], the degree of
ionicity of the Ni-Br bond, and the s -character of the bonding orbital in Br ions,
respectively. The s -character, which is the contribution of the s -orbital in the sp -hybrid
orbital of Br ions,wasassumedtobe0.15byDaileyandTownes[ 12 , 13 ] in cases
when the halide ion is bonded to atoms that are 0.25 more electropositive than the
halide ion. Thus, partial electron-transfer values ( i ) from Ni to Br in comparison
with the value (0.0) for a neutral Br atom were approximated to be 0.491 (high-
temperature phase) and 0.521 and 0.459 (low-temperature phase). The high-
temperature phase e 2 Qq ( (0)). The differences

between the i values in the high- and low-temperature phases (0.03) are comparable
to those for [MBr 6 ] 2 containing M-Br bonds ( i ¼

n (0) was used to determine 2 n

0.37 [ 15 ], 0.38 [ 16 ], and 0.39
[ 17 ] determined in octahedral complex ions where M

Pd 4+ (4 d 6 ), Pt 4+ (5 d 6 ), and
Re 4+ (5 d 3 ), respectively), suggesting that a marked change in the population of the
electrons of Br atoms takes place during the spin-Peierls transition in the present
complex.

¼

3.4 Optical Properties

Figure 3.4 shows the polarized optical conductivity spectrum ( E // b ) for [Ni(chxn) 2 Br]Br 2
obtained by using the Kramers-Kronig transformation of the reflectivity spectrum, which
corresponds to the absorption spectrum [ 18 ]. A sharp and intense absorption band was
observed at 1.3 eV. Intense absorption spectra have also been observed for Pd and Pt
complexes, which were assigned to charge transfer (CT) excitation from the fully
occupied d z 2 orbital of the M II site to the unoccupied d z 2 orbital of the nearest neighbor
M IV site. In the case of [Ni(chxn) 2 Br]Br 2 , on the other hand, the origin of the absorption
band ought to be different from the Pd or Pt cases because the ground state of Ni
compounds is a Ni III MH state.
Okamoto et al. have determined the band structure of [Ni(chxn) 2 Br]Br 2 by using
X-ray photoelectron (XPS), Auger electron (AES), and optical conductivity
spectroscopies [ 19 ]. From XPS and AES, the U value was estimated to be ca.
5.5 eV [ U ¼ E 2p (XPS)

E k (AES)]. In the optical conductivity
spectra, an intense CT band was observed at 1.3 eV. Although there has been
controversy in assigning the intense absorption band, it has been concluded that this
band is a bridging ligand ( p z band of Br ) to metal (UH d z 2 band of Ni) charge
transfer (LMCT) band, meaning that this compound is not an MH insulator but a CT
insulator (Fig. 3.5 ).
Photoluminescence is a powerful probe for investigating the electronic structure
of excited states and their dynamics. Although photoluminescence, i.e., relaxation
processes, of Peierls-distorted (mixed valence) MX chains have been extensively
studied [ 20 ], the processes in [Ni(chxn) 2 Br]Br 2 are not well understood. We have
studied the luminescence properties of the MX-chain system [Ni(chxn) 2 Br]Br 2 [ 21 ].

2 E valence (XPS)