Seesaw molecular geometry
Disphenoidal or seesaw (also known as sawhorse) is a type of molecular geometry where there are four bonds to a central atom with overall C2v molecular symmetry. The name "seesaw" comes from the observation that it looks like a playground seesaw. Most commonly, four bonds to a central atom result in tetrahedral or, less commonly, square planar geometry. The seesaw geometry occurs when a molecule has a steric number of 5, with the central atom being bonded to 4 other atoms and 1 lone pair (AX4E1 in AXE notation). An atom bonded to 5 other atoms (and no lone pairs) forms a trigonal bipyramid with two axial and three equatorial positions, but in the seesaw geometry one of the atoms is replaced by a lone pair of electrons, which is always in an equatorial position. This is true because the lone pair occupies more space near the central atom (A) than does a bonding pair of electrons. An equatorial lone pair is repelled by only two bonding pairs at 90°, whereas a hypothetical axial lone pair would be repelled by three bonding pairs at 90° which would make it stable. Repulsion by bonding pairs at 120° is much smaller and less important. (Wikipedia).
