Trigonal planar molecules radicals

The electron-deficient molecule BF3 has just three electron pairs in the valence shell. It will therefore have a trigonal planar configuration. Likewise, the carbene radical CH2, a rare case of divalent carbon, will be a bent molecule, since the lone pair occupies one of the vertices. 

In the polymeric structures of the first members of the homologous series of trivalent chromium derivatives, the metal atoms possess octahedral coordination [1366], while the derivatives of branched radicals are often monomeric [Cr(OR)2L2] molecules with square planar coordination [1571] or dimeric [Cr(OR)2]2 with triangular cordination of the metal atoms [795, 1185]. The monomeric [Cr(OR)3L2] are trigonal bipyramids [1571], while the derivatives of chromium (IV) and (VI) are tetrahedral [167, 1573, 1527]. 

The methyl radical, CH3, with a planar trigonal structure, looks the same if it is rotated by 120, 240, or 360° about an axis (C3 axis in Fig. 5.18) perpendicular to the plane containing three equivalent carbon atoms. An operation that leaves a molecule (object) looking the same (or sending into itself or a position indistinguishable from the original) is a symmetry operation. Symmetry operations include seven symmetry elements in Table 5.6 which are commonly possessed by molecular systems. 

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