square planar

Meaning: Square planar describes a central atom with four atoms at the corners of a square around it, all in one plane, with bond angles of 90°. VSEPR explains it. Six electron domains give an octahedral electron geometry. If two of those domains are lone pairs, they sit opposite each other (180° apart) to minimize repulsion. The four bonded atoms are left in the plane between them. This is the AX₄E₂ case, and the central atom is usually described as sp³d². Standard examples are XeF₄ and ICl₄⁻. Many d⁸ transition metal complexes, such as Pt(II) and Pd(II) complexes, are also square planar.

On the MCAT: You may be asked to draw a Lewis structure, count bonding and lone-pair domains, and name the shape. Expect to compare it with tetrahedral AX₄ (109.5°) and seesaw AX₄E (about 90° and 120°). Polarity is also tested. If all four outer atoms are identical, the bond dipoles cancel and the molecule is nonpolar, even though the bonds are polar. In a complex with two different ligand types, cis and trans isomers can exist.

Common confusion: Four bonded atoms does not mean tetrahedral. Lone pairs on the central atom count as domains, so check the total. Also, the two lone pairs are not next to each other. Carbon in organic molecules is never square planar. Its sp³ centers are tetrahedral.