Formation of Transition Metal Complexes

Four molecules of CO coordinate to Ni to form Ni(CO)4, but Ni(CO)5 is never formed. The stoichiometry of complex formation can be understood by the 18-electron rule. According to this rule, a stable complex with an electron configuration of the next highest noble gas is obtained when the sum of d electrons of metals and electrons donated from ligands equals 18. Complexes that obey the 18-electron rule are said to 

Similarly, Pd(0) and Pt(0) form complexes using their d10 electrons. The numbers of d electrons of major transition metals used for the complex formation are shown in Table 2.1. Coordinatively saturated complexes are formed by the donation of electrons from the ligands until total numbers of the electrons reach 18. 

Well-known complexes that obey the 18-electron rule are shown below. Typical ligands, such as CO, phosphine and alkenes, donate two electrons each. The total number of d electrons of Ni(CO)4 can be calculated as 10 + (2 x 4) = 18, and hence Ni(CO)5 cannot be formed. In Co2(CO)8, the number of d electrons from Co(0) is nine and four CO molecules donate eight electrons. Furthermore, a Co-Co bond is formed by donating one electron each. Therefore, the total is 9 + 8 + 1 = 18 electrons, to satisfy the 18-electron rule. The relationship between the coordination numbers and numbers of d electrons of metal carbonyls is shown in Tables 2.2 and 2.3. 

Number of electrons in ferrocene 1 can be counted in the following way. In ferrocene, Fe is Fe(II) and has six d-electrons. The cyclopentadienyl anion donates six electrons (2x2 from two double bonds and two electrons from the anion), and 6 + (4 x 2) + (2 x 2) = 18 electrons satisfy the rule. In another calculation Fe, regarded as Fe(0), offers eight electrons and the cycloptendienyl radical supplies one electron. Therefore, total electron count is 8 + (4 x 2) + (1 x 2) = 18. 

In bis-7T-allylnickel 2, Ni(II) has 8e and the two allyl anions supply four electons each 8 + (4 x 2) = 16. The following calculation is also possible if Ni(0) supplies lOe and the two allyl radicals supply three electrons each, the total number is 10 + (3 x 2) = 16. Therefore, this complex is coordinatively unsaturated. 

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As mentioned above, in contrast to classic antioxidant vitamins E and C, flavonoids are able to inhibit free radical formation as free radical scavengers and the chelators of transition metals. As far as chelators are concerned their inhibitory activity is a consequence of the formation of transition metal complexes incapable of catalyzing the formation of hydroxyl radicals by the Fenton reaction. In addition, as shown below, some of these complexes, for example, iron- and copper-rutin complexes, may acquire additional antioxidant activity. 

Formation of Transition Metal Complexes with Metal-Carbon a-Bonds Properties of Complexes with Metal-Carbon a-Bonds... 

Aromatic groups on the ring carbons stabilize strongly the C-phenylated group 14 metalloles, as in the case with phospholes109, but the reactivity of the dienic system (i.e. Diels-Alder cycloadditions, formation of transition metal complexes) decreases3,4. 

Diazaphospholes 4 and 5 are colorless to pale yellow distillable liquids or crystalline solids that are stable to oxidation by air and do not react with elemental sulfur. They are readily hydrolyzed to give the hydrazone from which they originate and phosphorus acid. While only a few reactions of 1/7-1,2,3-diazaphospholes 4 are reported, the chemistry of the 2//-isomers 5 is well studied. In CHEC-II(1996), the following reactions of 1,2,3-diazaphos-pholes are described in detail N-protonation and alkylation, polar addition to the P=C bond and substitution at C-4, cycloaddition reactions, substituent reactions, and the formation of transition metal complexes <1996CHEC-II(4)771>. 

The 1,2,4,3-triazaphospholes are colorless or pale yellow distillable liquids or crystalline solids. They are not oxidized by air and are reluctant to react with sulfur. Three isomeric heterocyclic systems of 277-1,2,4,3-triazaphospholes 15, 177-1,2,4,3-triazaphospholes 16, and 477-1,2,4,3-triazaphospholes 17 are known and they differ considerably in their behavior <1996CHEC-II(4)771>. The synthesis of 1,2,4,3-triazaphospholes and reactivity of different isomers of 1,2,4,3-triazaphospholes in the reactions at a ring nitrogen, in the addition to the P=N bond, oxidative addition to the ring phosphorus, cycloaddition reactions, and the formation of transition metal complexes are systematically covered in CHEC-II(1996) <1996CHEC-II(4)771>. The 1,3,4,2-thiadiazaphospholium ions 18 are only briefly mentioned in CHEC-II(1996) and no new results on their chemistry have been published in the last decade. 

In the realm of all-carbon ligands in the formation of transition-metal complexes, the naked carbon atom holds a special position. Based on the geometry of metal-carbon interaction, these compounds can be divided into four classes terminal carbide (I), 1,3-dimetallaallene (II), C-metalated carbyne (III), and carbido cluster (IV) ... 

Metalloporphyrins exhibit remarkable catalytic properties (38,39,52,122,170, 176,177,183,243-254), which are directly related to the formation of transition metal complexes in high oxidation states (e.g., and species) (255-... 

Structural features of olefins with distorted double bonds have been discussed within the deformation space defined by the eight bond angle deformations. The out-of-plane bond angle distortions are of particular interest because they are involved in addition reactions of the double bond. The symmetrical Blg-type deformation is related to concerted anti-additions, whereas the J3lu-type distortion (cf. Table 1) is appropriate for concerted syn-addition and those reactions that involve three-center intermediates and the formation of transition metal complexes. Twist or torsion is due to the Alu-type oop distortion and may be related to addition reactions, which in principle would lead—in the extreme case of a 90° twist angle—to an eclipsed rather than a staggered arrangement. 

The most interesting reactions of (305) are the thermal insertion reactions of Sg and Se2 into the As—As bonds of the Asj rings, and the formation of transition metal complexes. 

The dicarbollide ions (3)-l,2-BgC2H f and (3)-l,7-B9C2H f act as ligands in the formation of transition-metal complexes which resemble the metallocenes.However the B spectra of the iron(il)... 

From a thermodynamic viewpoint, calculations at different levels of theory have shown that 1,2-additions in Cyo occur preferentially at bonds C(l)-C(2) and C(5)-C(6) [90-92] (for the numbering of Cyo, cf. Fig. 3). However, isomerization of fullerene derivatives is usually not observed under the employed reaction conditions and with few exceptions, e. g. the formation of transition metal complexes [93] and catalytic hydrogenation [92, 94], most additions to fullerenes seem to be kinetically controlled. Based on this assumption, LUMO coefficients [95,96] as well as Mulliken charges [90,97] of the fullerene moiety... 

Table 1.4 Rate constants for the aqueous solution formation of transition metal complexes in...

W-Ethyl-2,6-dimethyl-4-pyri-done, formation of transition metal complexes, a47... 

The formation of dative bonds between a pair of reacting chemical species is the basis of a theory of acidity known as Lewis theory (Chapter 18). Dative bonds are also involved in the formation of transition metal complex ions (Chapter 13). Dative bond formation is often part of organic reaction mechanisms (Chapter 20). Aqueous solutions of acids contain the oxonium ion, H30, a datively bonded species (Chapter 8). 

Figure 13.32 Generalized explanation for formation of transition metal complex ions the transition metal ion can accept lone pairs (ligands) into suitably orientated empty hybridized orbitals... 

The potential formation of transition metal complexes with buckybowls has been of interest since a variety of transition metal complex units were reported to coordinate to buckminsterfullerene C o [54]. Interestingly, in the cases of Ceo complexes the metal is -coordinated to two carbon atoms shared between two 6-membered rings and no -coordinated haptomers were ever reported. 

In contrast, the successful formation of transition metal complexes of buckybowls turned out to be much more challenging. For some time there was only one report of an X-ray characterized buckybowl metal compound (61) the reaction of (Ph3P)2Pt(H2C=CH2) with semibuckminsterfullerene 20 [55]. In this case we... 

Scheme 12.21 Macrocyclization under high-dilution conditions and formation of transition metal complexes of 11-member macrocycles 53...

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