Unable to pair 2009 wireless keyboard with iPad Pro or iMac

I have installed fresh batteries and both the iPad and iMac can see the keyboard, including its unique name from when it was last paired. However, when I try to enter the numeric pairing code, neither device recognizes it. I get a “pairing unsuccessful” message after the pairing times out.

Have tried rebooting everything several times, Bluetooth on other nearby devices is off so there is no interference. I have also tried holding down the power button during pairing. None of this has been successful.

Would appreciate any additional suggestions.