Tetrahedral coordination with lone pairs

In the examples we have discussed so far, the shape of the molecule is defined by the coordination geometry thus the carbon in methane is tetrahedrally coordinated, and there is a hydrogen at each corner of the tetrahedron, so the molecular shape is also tetrahedral. 

You will recall that C02, lacking any lone pair electrons on the central atom, is a linear molecule there two bonded atoms and the coordination number around the carbon is also two. 

In the water molecule, the central atom is O, and the Lewis electron dot formula predicts that there will be two pairs of nonbonding electrons. The oxygen atom will therefore be tetrahedrally coordinated, even though the number of atoms bound to it is only two. The two nonbonding orbitals are very much like the bonding ones, except that there are no other atoms at their far ends. 

Notice the two non-bonding electron pairs in the schematic diagrams at right and center. The computergenerated picture on the right shows the higher electron density around the oxygen atom. 

The bent shape of the water molecule can be a source of confusion if you are not careful the oxygen atom in water is tetrahedrally coordinated, but the molecule itself has a shape defined by its atoms rather than its orbitals. The two hydrogen atoms are situated near the corners of a tetrahedron that is centered on the oxygen atom, but these three points define only a bent shape, not a complete tetrahedron. 

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The length of the coordinative bond (2.86 A) corresponds to a normal Sb-Sb single bond. The coordination geometries of the donor or acceptor antimony atoms are distorted tetrahedral for the former and pseudo trigonal bipyramidal for the latter with the iodine atoms in axial (I-Sb-I 169.71°) and the lone pair, the Me3Sb and the methyl group in equatorial positions. 

The molecular structure determination of bis(0-ethylthioacetoacetato)zinc76° shows the metal to be at the centre of a deformed tetrahedron, with Zn—S = 2.25 A and Zn— O = 2.01 A. The molecular structure of the related complex bis(0-ethylthioacetothio-acetato)zinc, in which the bonding is totally via sulfur, has also been determined.761 Coordination is approximately tetrahedral, with Zn—S = 2.30 A. In both complexes, the bond distances in the chelate ring indicate aromatic character, with a lone pair on the ethoxy oxygen being involved in delocalization. 

The local coordination environment at each Mn is approximately tetrahedral. If we had a discrete tetrahedral Mn complex, e.g., Mn(PR3)4, we might expect a qualitative bonding picture such as 45. Four phosphine lone pairs, Zi + t2 in symmetry, interact with their symmetry match, mainly Mn 4s and 4p, but also with the t2 component of the Mn 3d set. Four orbitals, mainly on P, P-Mn o bonding, go down. Four orbitals, mainly on Mn, P-Mn o antibonding, go up. The Mn d block splits in the expected two below three way. 

Thus, simple tetrahedral complexes with ancillary ligands incorporating donating groups (with a lone pair of electrons able to coordinate to the metal center) allow the selective design of elastomeric polypropylenes. 

Bromination of (RC5H4)2Mo2FeTe2(CO)7 (R = H, Me, i-Pr) forms (RC5H4)Mo(CO)3Br (13-15) and (RC5H4)MoFeTe2Br(CO)5 (16-18). The structure of 16 (Fig. 7) consists of a MoFeTe2 tetrahedral core with Br atom attached to one Te atom. A Te-Te-Br angle of 120° indicates the presence of a stereochemically active lone pair at the four-coordinate Te center. 

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