Suggested answer:
(1) Rather than opposite meanings, impassive and passive may be said to resemble each other in meaning. However, as we shall see, their meanings are clearly different, albeit not opposite.

(2) It is quite clear from the corpus results that impassive and passive differ in meaning and are used in different contexts. Impassive typically has to do with emotion, or the reluctance to reveal emotion; collocates are parts of the body that typically show emotion, e.g. face and eyes — face and eyes are emotionless/expressionless. An example from the corpus is:
Her face remained impassive as she went from half-furnished room to half-furnished room...

In comparison, passive is not linked to emotion in the same way, but suggests inactivity, i.e. not taking active part in something. This is underlined by the opposition between "passive" and "active" in the following example from the corpus:
But the atmosphere was one of passive acceptance .

(3) While we have seen that impassive means to show no emotion, impassioned can be seen to have almost the opposite meaning, i.e. it expresses intense emotion. An example from the corpus is:
...in her most glowing and impassioned hues, their virtue and magnanimity...

The fact that impassioned is used together with glowing suggests that the hues are passionate rather than impassive.

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