WordPress pending_post email

I’m trying to make a function that will send me an email when a post is submit for review (pending). I tryied :

function on_post_pending( $ID, $post ) {
    wp_mail(); // etc ...
}
add_action(  'pending_post',  'on_post_pending', 10, 2 ); 

And it doesn’t work. My question is why doesn’t work? And why the next code works? :

function on_post_pending( $post ) {
    wp_mail(); // etc ...
}
add_action(  'draft_to_pending',  'on_post_pending', 10, 1 );

Thank you