Pi-acceptor ligands

Technetium(III) is one of the more accessible oxidation states and has been shown to complex with a variety of ligands in several coordination geometries. In general, the trivalent state of technetium appears to require the presence of good pi-acceptor ligands such as triphenylphosphine, diarsines, or carbonyl for stabilization. 

FIGURE 10-22 Energies of d Orbitals in Octahedral Complexes Sigma-Donor and Pi-Acceptor Ligands. H- Ae. Metal r... 

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. 

The importance of the work lies in the fact that (a) the reaction has been studied in an aqueous medium, (b) the reaction has been studied at pH (7.4) which is the physiological pH of the human body, (c) the aqua-amine complex is chosen, (d) ruthenium(ll) than ruthenium(III) is chosen, as ruthenium(III) is a prodrug which is reduced in the cell to ruthenium(II), and (e) the title complex maintains its +2 oxidation state even at pH 7.4 due to the presence of a strong pi-acceptor ligand tap (tap= 2-(m-tolylazo)pyridine ), where most of the other ruthenium(II) complexes are oxidized to ruthenium(III). 

The bonding between the transition metal and the group 13 element has been studied extensively by computational methods. Because RM compounds have a valence electroitic structure consisting of a filled orbital of a symmetry and two unfilled orbitals of n symmetry, they are analogous to carbonyl ligands in that they can act as sigma donors and pi acceptors see tv-Acid Ligand). Such an electronic structure... 

Pi bonding in coordination complexes is possible when the ligand has p or tt molecular orbitals available. Because the effects are smaller for occupied orbitals, we will first treat the more important case of ligands with empty tt orbitals, or TT-acceptor ligands. 

Assign the symmetry labels of the d orbitals (labels of the irreducible representations). Repeat the calculations in Part a for a ligand that can act both as a sigma donor and a pi acceptor. 

In an octahedral complex with six 7r-acceptor ligands, the and orbitals do not engage in pi interactions with the ligands in positions 1 through 6 (their parameters in the table are all zero). However, the and dy orbitals all have total interactions of 4 ... 

As a further illustration of the dependence of n i 7t pi-backbonding interactions on metal and ligand character, we may compare simple NiL complexes of nickel with carbonyl (CO), cyanide (CN-), and isocyanide (NC-) ligands, as shown in Fig. 4.41. This figure shows that the nNi 7rL pi-backbonding interaction decreases appreciably (from 28.5 kcal mol-1 in NiCO to 6.3 kcalmol-1 in NiNC-, estimated by second-order perturbation theory) as the polarity of the 7Tl acceptor shifts unfavorably away from the metal donor orbital. The interaction in NiCO is stronger than that in NiCN- partially due to the shorter Ni—C distance in the... 

In accord with the expected trans influence of the 71-acceptor aUenylidene unit [212], substitution of the chloride ligand by different anionic nucleophiles in complexes frans-[MCl(=C=C=CR R )(Pi-Pr3)2] (M = Rh, Ir) is favored, affording new aUenylidene derivatives frans-[MX(=C=C=CR R )(P/-Pr3)2] (X = I, F, OH, N3, etc.) (see reactivity studies below). Of particular interest is the behavior of the Rh(I) species frans-[RhCl(=C=C=CPh2)L2] (L = Pf-Pr3, f-Pr2AsCH2CH20Me) towards NaCsHs since the reactions lead to the clean formation of complexes 40 (Scheme 13), the only half-sandwich-type Group 9 allenylidenes presently known [206, 209]. 

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. 

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