It seems you need first get position by get_loc and then select by []:
dt = pd.to_datetime("2016-11-13 22:01:25.450")
print (dt)
2016-11-13 22:01:25.450000
print (df.index.get_loc(dt, method='nearest'))
2
idx = df.index[df.index.get_loc(dt, method='nearest')]
print (idx)
2016-11-13 22:00:28.417561344
#if need select row to Series use iloc
s = df.iloc[df.index.get_loc(dt, method='nearest')]
print (s)
b 1.0
c 132.0
Name: 2016-11-13 22:00:28.417561344, dtype: float64