Is Google Cross-client Auth possible with iOS, and if so, how?

Is Google Cross-client Auth possible with iOS, and if so, how?

I'm using oAuth2, and I have a project in my google API console with an
iOS client and a Web client registered.
I'd like to auth in from my iOS app, and have my web backend retrieve an
access token so it can do the heavy processing work with the Google api.
I've tried following these instructions which are for android:
https://developers.google.com/accounts/docs/CrossClientAuth
But I keep getting an invalid scope when I assign the scope as:
oauth2:server:client_id:MY.WEB.CLIENT.ID:api_scope:http://www.google.com/m8/feeds/
https://mail.google.com/ https://www.googleapis.com/auth/plus.login
Anyone successfully implement a similar set up for iOS? If so please
explain or share documentation? What libraries or SDKs did you use to get
the code from google on the iOS app that the web backend could use to
generate a proper access token of it's own?