Trigonal planar arrangement

Section 1 10 The shapes of molecules can often be predicted on the basis of valence shell electron pair repulsions A tetrahedral arrangement gives the max imum separation of four electron pairs (left) a trigonal planar arrange ment is best for three electron pairs (center) and a linear arrangement for two electron pairs (right)... 

Answer Three cr-bonds formed from F2/7c-orbitals and B2s/r hybrids in a trigonal planar arrangement]... 

The only reported structure of a stannanimine (entry 4, Table V), shown below, was found to have a trigonal-planar arrangement of the three nitrogen atoms about the tin atom.88 The Sn=N double bond is significantly shorter than the two Sn—N single bonds (1.921(2) versus 2.015(2) and 2.030(3) A). 

For the coordination number 3, two different environments of tin(II) can be distinguished. One is the trigonal planar arrangement which is realized when the non-bonding electron pair at the tin atom is engaged in bonding, with tin acting as a Lewis base. The first example of this kind characterized by X-ray structural analysis is compound 4 30) (for the structure see also Chapter 5). 

The first coordination sphere is a special case. It can be generated from the trigonal planar arrangement by adding a further ligand, resulting a tin atom which simultaneously acts as an acid and a base. An illustrative example for this kind of bonding is compound 733) in which the tin atom receives electrons from pyridine and transfers electrons to the chromium atom (see also Chapter 6). 

To minimise repulsion, the three sp hybrid orbitals adopt a trigonal planar arrangement, l.e. they lie in the same plane with an angle of 120° between them. 

An X-ray crystal structure determination of the [f-Bu3Zn] anion present in 21 has an almost perfect trigonal planar arrangement [Zn—C bond distances 2.080(7), 2.059(7) and 2.057(8) A C-Zn-C angles 118.1(2), 120.7(3) and 120.6(3)°]. Compound 21 was prepared by reacting l,3,4,6,7,8-hexahydro-2//-pyrimido[l,2]pyrimidine, Me2Zn and f-BuLi (equation 9). It is notable that the methyl groups initially bound to zinc all become... 

Use of the Tbt group has also resulted in the successful isolation of the elusive 2-stannanaphthalene system, that is, 43, which is quite stable in the solid state and in solution <2006JA1050>. X-Ray diffraction studies on 43 reveal an essentially planar aromatic ring system with a completely trigonal planar arrangement about the Sn-atom. 

The ylide-ylene carbon atom is found to be the center of a trigonal-planar arrangement of ligands. This planar configuration may be part of a it system, thus giving rise to restriction in bond rotation and to the existence of geometrical isomers. 

Note that in borane the hydrogen atoms form a trigonal planar arrangement around the boron atom whereas in diborane they are arranged tetrahedrally. The BHB bonds in diborane are examples of three-centre two-electron bonds. 

The configuration of most R2P-NR2 compounds is represented by F2P-NMe2, which features a short P-N distance (162.8 pm) and trigonal planar arrangement (sp2 hybridization) at the N atom. The plane defined by the C2N unit bisects the F-P-F angle. This configuration minimizes steric repulsion between the substituents on P and N and also orients the lone pairs on the P and N atoms at a dihedral angle of about 90°, as shown in Fig. 15.5.1(a). 

In alkyl cations R3C , n = 3 and m 0. The substituents of the trivalent central atom lie in the same plane as the central atom and form bond angles of 120° with each other (trigonal planar arrangement). This arrangement was confirmed experimentally by means of a crystal structural analysis of the ferf-butyl cation. 

The X-ray structure analysis 7 shows that each nickel atom has a trigonal planar arrangement of ligands (14). Presumably, this is also the case for the nickel atoms in the anion of 6, for which a linear or bent 2e-3c bond Ni—H—Ni can be assumed. 

One last exception to the octet rule lies in the bonding of the atom boron. Boron prefers six electrons in its outermost principal energy level. This allows compounds containing boron to make three bonds in a trigonal planar arrangement. Two examples are BH3 and BF3 as shown in Figure 3.13. 

Fig. 2. Proteins that bind Cu(I). (a) Saccharomyces cerevisiae metallothionein (Cupl, pdb code laqr). Cupl binds up to seven Cu(I) ions (medium gray spheres) using 10 cysteine sulfur atoms (light spheres) in a polythiolate cluster (Peterson et al., 1996). All bonds shorter than 2.8 A are shown as dotted lines, (b) Cucumis sativus stellacyanin (pdb code Ijer). Both Cu(l) and Cu(ll) are bound by a pseudo-trigonal planar arrangement of (His)2Cys residues with an axial Gin ligand (Hart et al., 1996). In other cupredoxins such as plastocyanin, a Met residue is the axial ligand (Adman, 1991).

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