The 8051 modern chips, for example the AT89 family, are static. This means that there is no lower limit to clock frequency. This can be useful in low power designs or to get a step by step execution.

However, the internal oscillator circuitry, as Peter said, can only accept some crystals. If your AT89C2051 oscillator do not run with a 32 kHz crystal, you can build an external osillator. I have built a 4060-based osillator using a 32kHz crystal and works properly. The 4060 CMOS I.C. includes the oscillator circuitry and an asynchronous counter with many outputs. In my application, one of these outputs was utilized to wake-up a microcontroller.

Regards,

Alfredo del Rio.