My new wi-fi thermostat requires a common wire coming from the 24 volt transformer. Rather than running an additional wire can I ground the common side of the transformer to the building steel ( this is a commercial application) and pick it up at the thermostat 100 feet away?
No. If you do as you propose, you'd be using the structure of the building as a current carrying conductor. This is not allowed by any building code that I'm familiar with.