Polynuclear Aggregation of Manganese Dichloride. Syntheses, Properties, and Structures of Polymeric [MnCl 2 (THF) 1.6 ] ∞ , Dimeric [(THF) 4 Mg(μ-Cl) 2 MnCl 2 ], and Ionic [Mg(H 2 O) 2 (THF) 4 ][Mg(H 2 O) 4 (THF) 2 ][MnCl 4 ]