Dimeric and Cyclotrimeric Piano-Stool Vanadium(III) Dihalides with Unusual Differences in V−V Distance and Magnetochemistry. Syntheses, Structures, and Reactivities of (η-C 5 Me 4 R) 2 V 2 (μ-Br) 4 and the Trivanadium Cluster (η-C 5 Me 4 R) 3 V 3 (μ-Cl) 6 , New Mid-Valent Organovanadium Synthons