Minimization of steric interactions between the phosphane ligands as well as the geometry of the bridging ligands are responsible for the selective formation of tetranuclear macrocycle 1 in the reaction of [PdCl2(PEt3)2] with an equimolar quantity of o-diethynylbenzene, though the angle of 60° between the two acetylene groups would be expected to result in a trinuclear complex.
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Onitsuka et al. (1999) studied this question.