Stabilizing the [RSn(μ 2 -O)SnR] Motif through Intramolecular N→Sn Coordination. Synthesis and Characterization of [(RSn) 2 (μ 2 -O)(μ 2 -FcCOO) 2 (η-FcCOO) 2 ]·THF and {(RSn) 2 (μ 2 -O)[( t- BuO) 2 PO 2 ] 2 Cl 2 }·THF·2H