Coordinate bond molecular orbital

The molecular orbital energy stacking diagram for free acetylene is shown in Fig. 12. It can be seen that all the available bonding molecular orbitals are filled and that the free ligand has formal triple bond character. Upon coordination to a metal atom, the C-C vector lies perpendicular to the er-bonding orbital on the metal and donates electron density from a filled 7i-bonding orbital, as illustrated in Fig. 13. 

To be consistent, we shall adopt the simple orbital representations used above to describe coordinate bonding interactions. Cyclobutadiene, being a bidentate ligand system, is described by four bonding molecular orbitals. They are similar in symmetry to the general bidentate system described earlier (cf. Fig. 5) and are represented in Fig. 7. 

The chief drawbacks to the crystal field approach are in its concept of the repulsion of orbitals by the ligands and its lack of any explanation for bonding in coordination complexes. As we have seen in all our discussions of molecular orbitals, any interaction between orbitals leads to both higher and lower energy molecular orbitals. The purely electrostatic approach does not allow for the lower (bonding) molecular orbitals, and thus fails to provide a complete picture of the electronic structure. 

In the structure of Zeise s salt, the ethylene occupies the fourth coordination site of the square planar complex with the CC axis perpendicular to the platinum-ligand plane. In this compound, the dsp2 hybridised s orbital of Pt overlaps with / -bonding molecular orbitals of ethylene. Simultaneously, the filled dp orbital of Pt overlaps with p orbital of C2H4. 

The allyl cation, radical and anion have the same a framework 1.7, with 14 bonding molecular orbitals filled with 28 electrons made by mixing the Is orbitals of the five hydrogen atoms either with the sp2 hybrids or with the 2s, 2px and 2py orbitals of the three carbon atoms. The allyl systems are bent not linear, but we shall treat them as linear to simplify the discussion. The x, y and z coordinates have to be redefined as local x, y and z coordinates, different at each atom, in order to make this simplification, but this leads to no complications in the general story. 

The classical explanation for the increased coordination or valency of sulphur is its use of atomic d orbitals in molecules to form more hybrid orbitals for bonding than can be formed from just s- and p-type orbitals10. The dominant thinking on this subject today11 is that d-type orbitals provide needed spatial flexibility12 for bonding molecular orbitals that are formed even without the d orbitals (in theoretical descriptions or calculations, for example). The d orbitals in hypervalent sulphur are needed for quantitative accuracy and have not been found to be required for the qualitative electronic structure description13. 

The structure of a millerite crystal from Marbridge Mine, Malartic, Quebec, with the empirical formula Nio,98iFeo.oi6Coo.oo4S has been refined. The hexagonal axes were found to be o = 9.607(1) A and cq = 3.143(1) A. Within the lattice each Ni is coordinated by five S atoms and two Ni atoms. The observed Ni-S bond lengths are comparable to the expected value for a covalent bond. Molecular orbital theory was invoked to show that the millerite structure with five-fold coordination around Ni is more stable than the nickeline structure (a-NiS) with six-fold coordination about each Ni. Thereby it is rationalised that the low temperature phase (3-NiS occurs in nature and the high temperature phase a-NiS does not. 

Divalent late transition metals like cobalt (d ), nickel (d ), and copper (d ) in the first row of the d-block can use five 3d orbitals, one 4s orbital, and three 4p orbitals to form 4-, 5-, or 6-coordinate complexes. As a general rule, if there are N ligands in the first-shell coordination sphere of a transition metal complex, then there should be N bonding molecular orbitals, N anti-bonding molecular orbitals, and 9-N nonbonding molecular orbitals. Exceptions to this rule occur in some square-planar complexes in which three orbitals with the same symmetry properties overlap and form chemical bonds. Usually, some coordination sites in the first-shell of the... 

---