how to "!" in a binding in xaml

i.e.

`

    <Button Clicked="OnLoginClicked" 
        Image="Images/Login/loginWithFacebookButton.png" 
        IsEnabled="{!(Binding IsLoggingIn)}"  <<<<<<< this is the bit that I don't know how to do.

`

Do I just have to add a waiting variable too? (hope not).

Clicked answer on the wrong one!! - this is my preferred answer:

<Button Clicked="OnLoginClicked" 
            Image="Images/Login/loginWithFacebookButton.png" 
            IsEnabled="{!(Binding IsLoggingIn.Equals(false))}"