Ligand Coordination in Bis(β-diketonato) d Metals: The Mn(II) Case of D2 h versus D2 d Symmetry.

The structure of free manganese(II) bis-acetylacetonate [Mn(acac)2] was determined experimentally by gas-phase electron diffraction. The vapor at 197(5) °C is composed of a single conformer of Mn(acac)2 in D2 d symmetry with a central structural motif of an elongated MnO4 tetrahedron with a Mn-O distance ( re) of 2.035(5) Å and a bond angle in chelate rings (∠O-Mn-O) of 89.4(6)°. This result contradicts the predicted planar structure of the MO4 moiety ( D2 h) with a short Mn-O distance ( re) of 1.771 Å from a MC(5i5)QDPT2 calculation. From these findings and by comparison with data from the literature, we conclude that in general in bis(β-diketonato) coordination compounds of d metals there is a perpendicular arrangement of the two ligands for d0, d5, and d10 coordination centers and a planar arrangement ( D2 h) for others.