Bond, directiveness strength

The ultimate covalent ceramic is diamond, widely used where wear resistance or very great strength are needed the diamond stylus of a pick-up, or the diamond anvils of an ultra-high pressure press. Its structure, shown in Fig. 16.3(a), shows the 4 coordinated arrangement of the atoms within the cubic unit cell each atom is at the centre of a tetrahedron with its four bonds directed to the four corners of the tetrahedron. It is not a close-packed structure (atoms in close-packed structures have 12, not four, neighbours) so its density is low. 

In the early discussions of hybrid orbitals4,5 it was pointed out that the maximum strength (the maximum value in the bond direction) of a bond orbital formed from completed subshells of orbitals is associated with cylindrical symmetry of the orbital. In order to simplify the analysis of spd hybridization Hultgren5 decided to discuss only orbitals with cylindrical symmetry. He pointed out that no more than three d orbitals with cylindrical symmetry can be formed in a set of five d orbitals, and that each of these three is equivalent to the function d2 (see Table 1), except in orientation. 

The value of this function in the bond direction, called the strength of the bond orbital, is... 

It is found on analysis of the problem that when only two d orbitals are available for combination with the s and p orbitals six equivalent bond orbital- of strength 2.923 (nearly as great as the maximum 3 for the best spd hybrid) can be formed, and that these six orbitals have their bond directions toward the corners of a regular octahedron. We accordingly conclude that complexes such as [CoCNHs ], [PdClc]—, and [PtCle] — should be octahedral in configuration. This conclusion is of course identical with the postulate made by Werner to account for isomerism in complexes with different substituent groups,1 and verified also by the x-ray examination of Co(NH8)el3, (NHOaPdCU, (NH4)2 PtCl , and other crystals (see Fig. 5-1). 

This orbital, as written above, has its maximum (strength 3.000) along 2. Its nodal zones are at 73°9 and 133°37 with the bond direction. Three of these best bond orbitals (mutually orthogonal) can be constructed their bond directions make the angles 73°9 or 133°37 with one another, with each of the three bond angles having independent choice between these two values (except that three bonds at 133°37 cannot be formed). 

The two bond orbitals J( + pB) and ( — pB) make complete use of the s orbital. They have opposed bond directions and strength 1.95. 

Little metal-ligand orbital interaction Little preference in bond direction Bond strengths correlate with electronegativity, decreasing in the order F, OH-. H20,... 

Strong metal-ligand orbital interaction Strang preference in bond direction Bond strengths determined by orbital interaction, normally decreasing in following order CN-, NHj. R,0. OH-, F" Often covalent covalent complexes may exchange slowly... 

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