> お土産を気に入ってくれたことに安堵すると、

What is the meaning of ことに ? This is something I see from time to time, but I
don't understand its pattern. Aren't ことは or ことを working here ?
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2017-07-04T07:47:51.900
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2017-07-04T09:14:06.983
2017-07-04T09:14:06.983
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question
particle-に
meaning of ことに?