Ni[C
2H
8N
2]
2NO
2BF
4 (
NENB) is isostructural to Ni[C
2H
8N
2]
2NO
2ClO
4, a prototypical Haldane compound. We report the polarized optical transmittance of NENB as a function of applied magnetic field (
H) up to 30T. Electronic absorption intensities become modified above the critical field (
Hc7.5–10.5T, as determined from weak kinks in the magnetization data). For
H>Hc, the intensities of Ni
2+ d–d excitations in both polarizations are diminished, whereas the charge transfer transition along the nickel chain is enhanced.