The structure of free manganese(II) bis-acetylacetonate [Mn(acac)(2)] was determined experimentally by gas phase electron diffraction. The vapor at 197(5) degrees C is composed of a single conformer of Mn(acac)(2) in D-2d symmetry with a central structural motif of an elongated MnO, tetrahedron with a Mn O distance (re) of 2.035(5) angstrom and a bond angle in chelate rings (ZO-Mn-O) of 89.4(6). This result contradicts the predicted planar structure of the MO4 moiety (D-2h) with a short Mrs O distance (re) of 1.771 A from a MC(5i5)QDPT2 calculation. From these findings and by comparison with data from the literature, we conclude that in general in bis(beta-diketonato) coordination compounds of d metals there is a perpendicular arrangement of the two ligands for d, d5, and dm coordination centers and a planar arrangement (D2h) for others.