を契機(けいき)にLead to, As a good opportunity to, As a result of, As a trigger

Both are formal expressions describing one event influencing another, but differ slightly in usage. を契機に emphasizes a turning point or opportunity that led to change, as in 入院を契機にタバコをやめた (I gave up smoking on the occasion of my hospitalization). に伴って・に伴い often appears in news reports to describe cause-and-effect relationships.

Both indicate events that lead to change, but differ in register. を契機に is formal and often marks major turning points, as in 結婚を契機に転職した (Marriage became the occasion for changing jobs). きっかけ is casual and broad, used for any kind of trigger, as in 友達の紹介がきっかけで知り合った (We met thanks to a friend’s introduction).

を契機に is used when an event serves as a trigger or catalyst for change, often formal and neutral/positive, as in 留学を契機に視野が広がった (Studying abroad became a catalyst for broadening my horizons). をいいことに means taking advantage of a situation for one’s own benefit, usually with a negative nuance, as in 留守をいいことに泥棒が入った (A thief took advantage of the absence to break in).