Complexes sixteen-electron

For many species the effective atomic number (FAN) or 18- electron rule is helpful. Low spin transition-metal complexes having the FAN of the next noble gas (Table 5), which have 18 valence electrons, are usually inert, and normally react by dissociation. Fach normal donor is considered to contribute two electrons the remainder are metal valence electrons. Sixteen-electron complexes are often inert, if these are low spin and square-planar, but can undergo associative substitution and oxidative-addition reactions. 

X-ray crystallographic structures of the cw-dioxo osmium(VI) esters derived from dihy-droquinine-4-chlorobcnzoate/Os04/( )-2,2,5,5-tetramethyl-3-hexene and dihydroquinidine 4-chlorobenzoate/Os04/(i )-2,5-dimethyl-3-hexene have also been reported 22. Inspection of these sixteen-electron complexes reveals that, despite the considerable distance between the ligand stereocenters and the reacting site, the 4-chlorobenzoyl moieties effectively shield one of the two possible modes of alkene approach. 

Electronic absorption spectra of M(CNPh) (M = Cr, Mo, or W) complexes have been reported. n-Conjugation of aromatic ring orbitals with the out-of-plane 7t (CN) function is responsible for the unique electronic properties of these complexes. Cr(CNR) complexes can be oxidized by AgPFg in acetone to give either [Cr(CNR)g]-[PFe] or [Cr(CNR) ][PF ]2 products, depending on the molar ratio of reactants. These are seventeen-electron and sixteen-electron complexes respectively. In the crystalline state they are thermally and air stable, but in solution the compounds decompose within hours. ... 

Addition reactions. Certain square planar c/ (sixteen-electron) complexes such as [RhH(CO)L2] play a particularly important role in catalysis. These are sixteen-electron systems, having two less electrons than needed to complete a noble gas electronic configuration. It should not be surprising that such complexes add one ligand to become eighteen-electron complexes, reaction (18). 

Pd, Pt and Au, however, often show little or no tendency to be converted into 18-electron products. In these cases the 16-electron configuration may often be viewed as being essentially saturated . Sixteen electron complexes are common for (Co), Rh and Ir and for Ni, Pd and Pt. Even lower electron configurations (16,14 and 12) are normal for Ag and Au, for which 18-electron complexes are rare. 

Comparable numbers of radical exchange and polymer propagation events are needed to achieve limiting small PDI. Organo-cobalt porphyrins are five coordinate sixteen-electron complexes and this coordinate and electronic unsaturation provides a highly favorable situation for fast associative radical interchange (Figure 5.9). 

The spd hybridization extends beyond the group of the Pt metals, as can be seen in the complex KAuC14 which has square groups and AuGIt is diamagnetic the electron configuration with sixteen electrons is the same as that in K2PtCl4. 

Otherwise, unusual valency states are often observed in cyanide complexes. A Mn complex K5Mn(CN)6 has been reported here the stable 18-electron configuration causes the valency of manganese to take the very unusual value of one, and the compound is formed in spite of the extremely unfavourable cation anion ratio. Still more remarkable are the complex nickel cyanides. KGN and Ni(CN)2 form a complex K2Ni(CN)4, in which sixteen electrons are involved in the bond formation. The diamagnetism and the square structure of the Ni(CN)4 ion show that the bonding is due to dsp2 hybridization. 

Sixteen-electron ruthenium(O) species of type (rj6-arene)(L)Ru(0) and containing two-electron ligands are probable intermediates for C—H bond activation and formation of metallacyclic complexes (Section II,A,3,c). A variety of 18-electron complexes of general formula (arene)(L1)(L2)Ru(0) have been prepared by H. Werner and co-workers either by deprotonation of hydride ruthenium(II) complexes or by reduction of cations RuX(L)2-(arene)+. Some of these Ru(0) complexes have already been discussed with the formation of alkyl or hydridoruthenium complexes (Sections... 

Finally, many complexes that participate in homogeneous catalytic reactions have electron counts less than 16. This is especially true for high-oxidation-state early-transition-metal complexes such as (C2H5)TiCl3, Ti(OPr )4, etc. Cat-alytically active, late-transition-element complexes with electron counts less than sixteen are also known. An important example is RhCl(PPh3)2, a 14-electron complex that plays a crucial role in homogeneous hydrogenation reactions (see Section 7.3.1). 

We anticipated those results based on our observations concerning the reactions of [Rhi 5(CO)27 " — F°r instance, we found that amines, e.g., n-methyImorpholine or 1,10-ortho-phenantroline, are able to transform this cluster into CRh-j4(CO)2in acetone solution, with an equilibrium similar to those above. It Is quite possible, as suggested by an infrared band at 2060 cm-, that a complex such as [N2Rh(C0)2l]+, a sixteen-electron species, would be generated In these cases. 

Sixteen-electron bis(arene) complexes of formally zero-valent zirconium, hafnium, and titanium have been made by cocondensation of the metal atoms with bulky ligands such as 1,3,5-tri-t-butylbenzene71 ... 

Sixteen-electron square planar complexes are most commonly found for ds metals, in particular those metals having formal oxidation states of 2+ (Ni2+, Pd2+, Pt2+) and 1+ (Rh+, Ir+). Some of these complexes have important catalytic behavior, as discussed in Chapter 9. 

The metal atom in the square-planar complexes of Pd(II), Pt(II), Rh(I), Ir(I) has only sixteen electrons in its valence orbitals. These complexes are easily oxidized by the addition of oxygen or halogens to yield an octahedral... 

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