Essentially the same as this question, but the solutions there don't seem applicable here. Both isolated and in compounds, I can only find 匙 as either shi⁰ or chi² “spoon”.
Can this character in this context have a specific tone, and if so in what resource can it be found? Or is looking at tone alone misleading; is/was it possible to read 钥匙 as yao⁴chi²? (Where?)