Synthesis, X-ray Structure and Bonding of Tris(2,2-6,6-tetramethylheptane-3,5-dionato) Bismuth(III)

The title compound [Bi(dpm)3] has been synthesized and its molecular structure determined by single-crystal X-ray diffraction. Two different crystalline forms have been found: [(Bi(dpm)3]·H2O (1) and [(Bi(dpm)3]·3H2O (2). 1 crystallizes in the space group P21/n with a=12.426(5), b=19.565(11), c=15.820(9) Å, beta=94.31(4)°, V=3835 Å3, Z=4. 2 crystallizes in the space group Pbcn with a=20.953(5), b=19.619(6), c=19.475(3) Å, V=8006 Å3, Z=8. The coordination around the Bi atom consists of a distorted pentagonal pyramid with two ligands approximately lying in the basal plane while the third one is in a vertical mirror plane. Molecules are associated in dimer units because of the presence of weak interactions in the crystal lattices. The bonding of the monomer compound has been investigated by means of quasi-relativistic quantum-mechanical calculations on the simpler acetylacetonate complex. The metal-ligand interaction is found to be dominated by ionic interactions with a significant pulsion between the Bi 6s lone pair and the symmetric n+ ligand based combination of the oxygen lone pairs. Such electronic repulsion is reduced, but not eliminated, by relativistic effects. Optimized geometrical parameters obtained by carrying out calculations in the C1 symmetry point group are in good arreement with experiment. Interestingly, however, the pseudo-octahedral D3 structure is found to be lower in energy, suggesting that the observed pyramidal molecular structure cannot be explained on the basis of simple stereochemical arguments.