validateupdatelistitem using rest to update User field

I am trying to update user and group field using validateupdatelistitem. But it is not updating as well as not throwing any error in the response.

{'formValues': [{'__metadata': { 'type': 'SP.ListItemFormUpdateValue' },'FieldName': 'Title','FieldValue': 'TesChange'},

{'__metadata': { 'type': 'SP.ListItemFormUpdateValue' },'FieldName': 'UserName','FieldValue': '16'}]

,'bNewDocumentUpdate':true}

In this example I am sending the user id after ensuring the user with login name.

I would like to know what is the correct format to update userfield in validateupdatelistitem withrest api.

Thanks.