Add two UIBarButtonSystemItemFlexibleSpace items to your toolbar, to the left and right of your items
UIBarButtonItem *flexibleSpace = [[UIBarButtonItem alloc] initWithBarButtonSystemItem:UIBarButtonSystemItemFlexibleSpace target:nil action:nil];
[toolbar setItems:[NSArray arrayWithObjects:flexibleSpace, settingsButton,deleteButton,aboutButton, flexibleSpace, nil]];
Adding these as you would any other toolbar items will distribute space evenly between the two of them.