Metal-ligated induced structural interconversion between Pd23(CO)20(PEt3)10 and Pd23(CO)20(PEt3)8 possessing highly dissimilar Pd23 core-geometries

A 31P{1H} NMR study has conclusively established that Pd23(CO)20(PEt3)10(1) and Pd23(CO)20(PEt3)8(2), which differ by only two phosphine ligands, can be chemically induced in solution to interconvert reversibly into each other despite their having highly different metal-core geometries: viz., a centered hexacapped cuboctahedral Pd19 kernel (pseudo-Oh) with four wingtip Pd atoms in 1 versus a highly deformed centered hexacapped cubic Pd15 kernel (pseudo-D2d) with eight capping Pd atoms in 2. A structural diagram is given that shows a plausible hypothetical pathway for the geometrical transformation of 1 into 2(or 2 into 1) upon removal (or addition) of the two phosphine ligands. Although there is no experimental evidence indicating whether these chemically induced conversions are intermolecular or intramolecular, the proposed intramolecular interconversion emphasizes the major structural differences that exist between 1 and 2. Complete interconversions of 2 into 1(and 1 into 2) were accomplished by 31P{1H} NMR-monitored reactions carried out within NMR tubes. Addition of free PEt3 to 2 rapidly converts it into 1; if an excess of PEt3 is added, product 1 slowly transforms into the icosahedral-based Pd16(CO)13(PEt3)9(3). Addition of O2(air) to 1 converts it into 2 and the phosphine oxide byproduct (Et3PO), but the rate of this reverse chemical reaction is not nearly as fast; an excess of O2(air) also slowly converts 3, if present in the reaction mixture, into 2; the relatively slow rates of reactions involving 3 are attributed to the icosahedral-based Pd16 nuclearity in 3 being unlike the identical Pd23 nuclearities in 1 and 2. In contrast to the chemically induced interconversion reactions between 1 and 2, both the reaction of 1 with excess PEt3 to form 3 and the reaction of 3 to form 2 are not quantitative. These facile interconversions provide a striking illustration concerning the abnormal capacity of ligated palladium clusters to undergo major changes in metal-core geometries upon addition/removal of ligands. This exceptional behavior may be readily attributed to the markedly weaker M–M and M–CO bonding interactions in palladium carbonyl clusters compared to those in nickel and platinum carbonyl clusters.