For Want of a Nail: The Single Structure That Halted an Interesting Project. Lee M. Daniels, F.A. Cotton, Steven C. Haefner, and Xiaoping Wang, Laboratory for Molecular Structure and Bonding, Department of Chemistry, Texas A&M University, College Station, TX 77843

The model refined from a dataset collected at -100(C has displacement factors several times larger than expected, and the identity of one of the ligands is elusive. The problem has persisted through several datasets collected at different temperatures over a period of six years. The monoclinic (P2₁/n) crystals are the result of the transformation of a dimolybdenum acetonitrile complex in the presence of a phosphine:

PR₃[(CH₃CN)₄Mo-Mo(CH₃CN)₅][BF₄]₄ [(CH₃CN)₄Mo(CH₂CN?)₂Mo(CH₃CN)₄][BF₄]₄ CH₃CN

where the (CH₂CN?) group bridging the two metal atoms in the product represents the unknown ligand.

The two ligands bridging the two metal atoms in the centrosymmetric structure refine well as C-C-N. The distances between the atoms suggest a structure such as C=C=N. These three atoms appear to be coordinated side-on, oriented perpendicular to the metal-metal vector. However, the largest peak in the final difference map is found about 1.9A from the center atom of this group, and refines well as a carbon atom with occupancy 0.5.

The (well-behaved) structures of the starting material and of two isolated intermediates are known and will be presented for comparison. Possible explanations for the large displacement parameters, such as twinning or a phase transformation, will be put forward, and discussion of these and other explanations are invited.