One example of useful disorder : Structure of Pr(III) complex of 1,4,7,10-tetraazacyclododecane-10-methyl-1,4,7-tris(methylenephenylphosphinic) acid

The structure investigation of [PrL(H2O)n]2. xH2O has resulted in an interesting conclusion. The successful modelling of disorders in this case gave us a picture of “frozen solution” with samples of isomeric species present. Three chemically and four crystallographically different complexes were identified and structurally characterised: [Pr{L(R,S)}]2 with coordination number (CN) 8, [Pr{L(R,R)}(H2O)]2 with CN 9 and two species [Pr{L(R,S)}(H2O)]2 also with CN 9, but with little different geometrical parameters (the symbols L(R,R) and L(R,S) mean the different chirality on phosphorus atom of the ligand labelled P2 in the resulting complex, respectively). Therefore, the title compound should be considered as 0.67 [Pr{L(R,R)}(H2O)]2 · 0.33 [Pr{L(R,S)}(H2O)]2 · 0.67 [Pr{L(R,S)}]2 · 0.33 [Pr{L(R,S)}(H2O)]2 · 27.5 H2O. In our opinion, this paper illustrates that disorder can represent not only a nuisance in structure solving and refinement, it may bring useful chemical information as well.