Metal-ligand bonding orbital overlap

Four-coordinate complexes exhibit lower isomer shifts than six-coordinate compounds. Metal-ligand bonds are shorter and more covalent if the coordination number is smaller because of less steric hindrance and less overlap with antibonding 2g orbitals in the case of four as compared to six bonds. 

It appears that the metal-ligand bonds in carbonyl and dinitrogen complexes are similar but somewhat weaker in the dinitrogen complexes. Carbon monoxide is no only a better cr donor but also a better ir acceptor. This is 10 be expected on the basis of whatever polarity exists in the CO molecule (see Fig. 5.18) and the fact that the ir antibonding orbital is concentrated on the carbon atom (see Fig. 5.20), which favors overlap with the metal orbital. The superior r accepting ability of CO also accounts for the instability of carbonyl dinitrogen complexes. Both Cr(CO)sN2 and cis-... 

Figure 11.3 Diagrammatic representation of metal-ligand bonds, (a)

Steric factors probably prohibit simultaneous rotation of the olefin and alkyne C2 units which would crowd all four metal-bound carbons into the same plane. Separate rotation of each unsaturated ligand was explored theoretically using the EHMO method. Rotation of the olefin destroys the one-to-one correspondence of metal-ligand tt interactions. Overlap of the filled dxz orbital with olefin n is turned off as the alkene rotates 90°, creating a large calculated barrier for olefin rotation (75 kcal/mol). Alkyne rotation quickly reveals an important point the absence of three-center bonds involving dir orbitals allows the alkyne to effectively define the linear combinations of dxy and dyz which serve as dn donor and dir acceptor orbitals for 7T and ttx, respectively. Thus there should be a small electronic barrier to alkyne rotation (the Huckel calculation with fixed metal... 

Figure 2.3 Schematic presentation of orbital overlaps for metal-ligand bond formations.

Figure 23. View looking down the threefold axis of a TP tris(dithiolene) complex, (a) The HOMO orbital is comprised of the ligand Uy orbitals centered about the dz2 metal orbital. (b) Energy minimization of the HOMO occurs if ligand bending takes place improving the bonding orbital overlap. [Figure adapted from (372).]...

The intensity associated with charge-transfer excitation of an electron from these filled ligand orbitals into the half-occupied Cu dx2.y2 orbital also reflects metal-ligand bonding. Charge-transfer intensity is proportional to (RS)2, where S is the overlap of the donor and acceptor... 

Syntheses and properties of cyclobutadiene-transition metal complexes have been discussed in detail by Maitlis 167). Brown 168), and others 169) have reviewed the metal-ligand bond in terms of the MO approximation. The main bonding in these complexes is due to an overlap of the two degenerate nonbonding cyclobutadiene orbitals with spd hybrid metal atomic orbitals. 

---