Jr-donor ligand

Os(VI) complexes of type [Os(VI)NX5]2- or [0s(VI)NX4H20]-(X = Cl, Br, CN, ox) are made by the action of the appropriate acid HX on potassium osmiamate (6, 11). The tendency to lose the ligand trans to the nitride and to substitute a water molecule is some evidence of a trans labilizing effect of jr-donor ligands. X-ray studies show a distorted octahedral structure with the osmium atom slightly out of the plane of the equatorial X atoms towards the nitride (1). The Os=N stretching frequency is around 1080 cm-1 (11) in such complexes. 

Na Mg Ca Metal clusters Metal clusters with jr-donor ligands] with -acceptor ligands Al Si P s Cl Ar... 

Four coordination for nF is relatively common, with tetrahedral and square-planar geometries representing the two extreme possibilities. Good jr-donor ligands such as halides tend to stabilize tetrahedral geometries, for example, [NiCLt] (3), while TT-acceptor ligands such as PR3 and CN afford square-planar geometries, for example, [Ni(CN)4] (4). 

AUcoxy groups are rather hard jr-donor ligands. They stabilize the highest oxidation number of the metal. Therefore alkoxides of main group elements and d° transition metals are well known, while those corresponding to soft d late transition metals have been much less studied. The number and stability of metal alkoxides decrease from left to right across the periodic table. 

The value of Aq, and therefore the order of the Ugands in the spectrochem ical series, does not depend only on cr interactions. We shall see in Chapter 3 ( 3.3.4.2) that the presence of rr interactions can either decrease the value of Aq ( jr donor ligands such as 1 or Br ) or increase it ( n -acceptor ligands such as CN or CO). 

Figure 3.1. Sketch of the interaction between the doubly occupied orbital on a jr-donor ligand (X) and an empty 4 orbital on the metal centre.

Figure 3.2. Interaction diagram showing the perturbation of the d block of an octahedral complex (a interactions only, left-hand side) by the two orbitals of a double-face jr-donor ligand (Cl, for example, right-hand side). The electronic occupation shown corresponds to a dP electronic configuration.

Figure 3.8. Perturbation of the d block (in the centre, a interactions only) of an octahedral complex MLs by the jr interactions with double-face jr-donor ligands (L = Cl, for example) on the left, and double-face jr-acceptor ligands (L = CO, for example) on the right.

Chisholm, M. H., ed. (1995). Early Transition Metal Clusters with jr-Donor Ligands, VCH, New York. 

Rather limited information is available on how the nature of R affects the rate of CO insertion, all other factors being constant. A generalization that ethylmetal complexes react faster than the corresponding methyl carbonyls derives from investigations on four systems RIr(CO)2(AsPh3)Cl2 (92), RMn(CO)5 (51), CpFe(CO)2R (98), and CpMo(CO)jR (80). When R = Et the reactions with CO or P and As donor ligands proceed at least 6 times faster than when R = Me. 

Hydroxide attack on complexes of the type cis-[(NO)L(bpy)2Ru ] also yields the corresponding nitro complex [118]. The equilibrium constants for these reactions are a strong function of the cis-ligand and increased by 10 on changing L from a jr-donor (Cl ) to a rr-acceptor (py) [98]. The ability of the nitro complexes to transfer an oxygen atom to a reductant such as triphenyl-phosphine is also a function of the cis-ligand [98]. 

The vast majority of MoO + core complexes are six-coordinate and mostly of distorted octahedral structure. Some non-octahedral complexes are known. In the octahedral structure the two Mo—O, bonds are invariably cis to each other. The strong o- and jr-donor nature of the oxo ligands makes it favorable for them to avoid competing for the same p and d orbitals. If the oxo groups were tram they would be forced to share two d orbitals and one p orbital. By residing on adjacent coordination sites they are forced to share only a single d orbital. 

The series of complexes Mo02X2 (X = F, Cl, Br) can be used to prepare M0O2X2L2 complexes where is py2, bipy, (DMSO)2 or other ligands. Examples of these complexes, which are formally adducts of Mo02X2, are given in Table 1. The structurally characterized complexes have the jr-donor halides trans to each other and cis to the Mo—O, bonds. 

In square-pyramidal MB 8 complexes of the type ML, DAB, where D is a jr-donor trans to an A 7r-acceptor, the NO or S02 ligand will bend in the DMA plane towards the Ji-aoceptor. 

For coordinatively unsaturated carbene complexes ligated by strong (o, Jr) donor co-ligands, with the metal in a higher formal oxidation state, the carbene carbon typically shows nucleophilic character. By far the majority of these involve substituents with no 7i-dative capacity (H, CRV SiR3). 

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