Square planar arrangement

FIGURE 3.6 The square planar arrangement of atoms taken up in AX4E, molecules the two lone pairs are farthest apart when they are on opposite sides ol the central atom. 

Each heme unit in myoglobin and hemoglobin contains one ion bound to four nitrogen donor atoms in a square planar arrangement. This leaves the metal with two axial coordination sites to bind other ligands. One of these sites is bound to a histidine side chain that holds the heme in the pocket of the protein. The other axial position is where reversible binding of molecular oxygen takes place. 

Reaction of hexafluorobenzene with MeIr(PEt3)3 results in C—F and P—C bond cleavage and P—F bond formation, according to reaction Scheme 28.504 An X-ray crystallographic study of (310) reveals a distorted square-planar arrangement of Ir. 

Another remarkable example of a mercuracarborand halide adduct has been obtained by reaction of the octamethyl tetranuclear derivative 165 with [NMe4]F. This reaction affords complex [165-F]- in which the fluoride is coordinated to the four mercury centers in an approximate square-planar arrangement (Equation (57)). The Hg-F distances, which range from 2.56 to 2.65 A, are slightly longer than those found in [163-/i2-F]2. The 9Hg NMR resonance of the complex indicates coupling to the fluorine atom with 1Jug-F = 698 Hz.215... 

Square-planar arrangement of two terminal and two bridging azide N atoms about each Pd. 

Because Xe has 12 electrons in its valence shell we would expect an octahedral disposition of the six electron pairs, two of which are nonbonded pairs. Because nonbonded pairs have a larger volume requirement than bonded pairs, we would expect the nonbonded pair to be opposite one another, leading to a square planar arrangement of the molecule. 

Figure 13.4 The anion of porphine. Porphyrins are porphine rings with peripheral substituents. The porphine ring is flat, and metal ions can be bound within the square planar arrangement of nitrogen atoms.

Although the formulation as ionic species appears to correlate well with the known crystal structures for the anionic complexes, the situation is much less clear for the cations, and secondary bonding8 between the central atom of the cation and atoms of the anion is important. Thus in the structure of difluorobromine(III) hexaflubroantimonate(V) the bromine atom has two fluorine atom neighbours at 1.69 A and two more, from [SbF6]- units, at 2.29 A, completing a distorted, square planar arrangement of fluorine atoms around the bromine atom.9... 

Similar fluorine bridging in the structure10 of trifluoroxenon(IV) hexafluorobismuthate(V) leads again to a distorted, square planar arrangement, with three fluorine atoms at 1.81, 1.92 and 1.94 A from the xenon atom, and the fourth, from the [BiF6] unit, at 2.25 A. 

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