  We present a $^{209}$Bi and $^{17}$O NMR study of the Mn electron spin
correlations developed in the charge ordered state of
Bi$_{0.5}$Sr$_{0.5}$MnO$_{3}$ and Bi$_{0.5}$Ca$_{0.5}$MnO$_{3}$. The unusually
large local magnetic field $^{209}H_{loc}$ indicates the dominant $6s^{2}$
character of the lone electron pair of Bi$^{3+}$-ions in both compounds. The
mechanism connecting the $s$ character of the lone pairs to the high
temperature of charge ordering $T_{CO}$ is still not clarified. The observed
difference in $^{209}H_{loc}$ for Bi$_{0.5}$Sr$_{0.5}$MnO$_{3}$ to
Bi$_{0.5}$Ca$_{0.5}$MnO$_{3}$ is probably due to a decrease in the canting of
the staggered magnetic moments of Mn$^{3+}$-ions from. The modification of the
$^{17}$O spectra below $T_{CO}$ demonstrates that the line due to the apical
oxygens is a unique local tool to study the development of the Mn spin
correlations. In the AF state the analysis of the $^{17}$O spectrum of
Pr$_{0.5}$Ca$_{0.5}$MnO$_{3}$ and Bi$_{0.5}$Sr$_{0.5}$MnO$_{3}$ prompts us to
try two different theoretical descriptions of the charge-ordered state, a
site-centered model for the first manganite and a bond-centered model for the
second one.
