Ammoniacal Synthesis of Lanthanum Aryloxide Complexes and Observation of a Unique Interconversion between Oxygen- and .eta.-Arene-Bridged Dimeric Species. X-ray Crystal Structures of La2(OAr)6(NH3)n (n = 0, 2), La(OAr)3(NH3)4, and La(OAr)3(THF)2 (Ar = 2,6-i-Pr2C6H3)