Despite the slower transformation at lower temperatures, we observed no additional cluster species. The formation of the 9-or 11-mer would require additional μ 3 -bridging O and OH, which are disfavoured at lower temperatures particularly once a cluster is saturated with terminating coordinating groups, whereas the μ 2 -OH bridging in the Hf12 cluster could occur via a much more straightforward nucleophilic attack of OH 37 from one cluster to another. Thus at both 150°C and 120°C, the double cluster is obtained preferentially to other high-nuclearity clusters.