Accurate chemical characterization - fascinating discoveries and proofs From KOALA, the single crystal lane neutron diffractometer at OPAL

Since the first neutron single crystal studies of the nineteen fifties, single crystal neutron diffraction has largely been viewed as a technique of limited application in chemistry due to a range of factors including the availability and accessibility of instruments, extortionate crystal size requirements etc. The revival of the Laue method in the late 20th Century has resulted in this method becoming more readily applicable to crystals of less challenging dimensions - no longer do you need to increase the world supply of a material before you can undertake a single crystal neutron study! We are fortunate that a Laue single crystal neutron diffractometer - KOALA - was among the first instruments built at the OPAL reactor at ANSTO and we are now approaching four years of operation. We have now acquired an evidence based understanding of the parameters within which useful data can be obtained from KOALA and the range of outcomes which can be anticipated for particular structures. It is important to remember that a neutron study will inherently belong to an ensemble of characterization methods applied and the structure derived must be consistent with the demonstrated chemistry and physics of a compound. Indeed, at times, the bulk of the structural information will come from other lab based methods and the contribution of neutron diffraction may be limited to vital observations such as the locus of hydrogen atoms or the identities of atoms with similar atomic numbers. It is important to be clear about what we can and cannot attribute to our data and to think and write clearly - correctly identifying which data support each part of the model that we choose to represent the compounds we are studying. It is vital that as scientists, we do not delude ourselves that a model built to fit X-ray diffraction data well and which has electron density where we might hope to see it if our prejudices are correct constitutes proof of that structure where a chemically different model may fit equally well! Differentiation between such models is frequently found in the complementarity of neutron diffraction. Recent studies which have demonstrated (i) the absence of hydride in an unprecedented low oxidation state metal complex, (ii) remarkable and unexpected chemistry in metal nano-clusters, (iii) clarification of metal atom identities in polynuclear complexes of near neighbour mixed metal transition metal complexes will be presented from the perspective of the forensic analysis of the compound identification and structure verification. Collaborators in each study will be clearly identified on each slide discussing their compound.