Pi -donor ligand

R. E. McCarley, Early Transition Metal Clusters with Pi-Donor Ligands , ed. H. Malcolm, Chisholm, VCH Publishers, New York, 1995. 

FIGURE 10-24 Energies of 4 Orbitals in Octahedral Complexes Sigma-Donor and Pi-Donor Ligands. Ao = - Ae.y. Metal s and p... 

FIGURE 10.25 Energies of d Orbitals in Octahedral Complexes Sigma-Donor and Pi-Donor Ligands. 

Complexes of and The effect of complexation on the splitting of d orbitals is much greater in the case of second- and third-than for first-row transition elements, and the associated effects already noted for Ni are even more marked for Pd and Pi as a result, their complexes are, with rare exceptions, diamagnetic and the vast majority are planar also. Not many complexes are formed with O-donor ligands but, of the few that arc, [M(H20)4] ions, and the polymeric anhydrous acetates [Pd(02CMe)2l3 and [Pt(02CMc)2]4 (Fig. 27.10), are the most... 

The phosphaalkyne P=CBu bonded in the mode, has a Pi value (—0.04 V) [18] that is very close to that of CO (Pi = 0 V). Since the former is not expected to be a good cr-donor, this similarity is accounted for by considering that it is also not an efficient 7r-acceptor [18]. A similar case is that of the N2 ligand (Pi = —0.07 V [10]). Since Pi reflects the net tt-electron acceptor minus cr-donor character, similar values can be found for both a strong rr-acceptor/strong a -donor ligand and a weak rr-acceptor/weak cr-donor Kgand. 

Consider an octahedral complex of the type [ML4CI2], with two doubleface r-donor ligands (Cl) in trans positions and four other ligands which only have a interactions with the metal. Three of the symmetry elements in the complex are shown in 3-19 the planes Pi (xz) and P2 (yz) and the inversion centre i, located on the central atom. 

With ligands that behave as both tt acceptors and tt donors (such as CO and CN ), the TT-acceptor nature predominates. Although Tr-donor ligands cause the value of to decrease, the larger effect of the 7r-acceptor ligands causes to increase. The net result of pi-acceptor ligands is an increase in A , mostly because d orbital overlap is generally more effective with tt orbitals than with Tr-donor orbitals. 

As expected, ligands having the largest values of fare those that can pi backbond and therefore lie at the top of the spectrochemical series (CN , bpy) those in the middle (en, NH3, NCS , HjO, 204 ) are tr-donor ligands and those having the smallest values of f (F , d , Br ) are both tr-donor and r-donor ligands. 

Pyridyl functionalized tris(pyrazolyl)borate ligands show some interesting properties including the formation of polynuclear zinc complexes.23,1 Some of these contain extensive H bonding and have potential as models for multinuclear zinc enzymes such as phospholipase C or PI nuclease.235 A bis-ligand complex of the hydrotris(5-methyl-3-(3-pyridyl)pyrazolyl)borate ligand (23) shows octahedral coordination of all six pyrazole nitrogen donors despite the steric bulk. 

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