When using the 'metal' shader, you cannot manipulate the material's IOR. What IOR value is used internally by the shader?
The underlying calculation is based on measured spectrally varying complex indices of refraction. I.e. gold has IOR of about 5.9 + 0.2i at 885 nm light (red) and 1.9 + 1.8i at 300 nm light (blue). Is is not a simple index of refraction for all wavelengths. This also affects how the thin-film behaves when you enable the anodized option.
Søren
Very interesting, and thank you for sharing the details. It's good to know that the reflection properties are grounded in real data.