Group VIII Iron Triad

— The optically active complexes (-)365 [Fe(X)(CO)(PPh2R)( -C5H5)] X=CI orBr, PPh2R=( 9)-(+)8e6-[Ph2PN(Me)CH(Me)Ph] epimerize in solution according to a first-order rate law.  

PFj exhibits preference for an apical site in complexes [Fe(CO)(3-a )(PF3)a -(butadiene)] rather than either of the two basal sites and in as3mimetric methyl-substituted butadiene derivatives with x=2 a secondary preference for the basal position irons to the methyl group is found. Intramolecular exchange of PFs groups occurs in compounds with x=2 or 3 and leads to equivalence of the PF3 F n.m.r. resonances, possibly via Berry pseudorotation. [Fe(CO)(dpe)( j-C4H6)] exists in two isomeric forms assigned structures (29) [basal-basal (bb) phosphine] and (30) [apical-basal (ab) phosphine] on the basis of their n.m.r. spectra. Variable-temperature P n.m.r. spectra provide evidence that interconversion of (29) and (30) 

The free energy of activation for basal-apical CO exchange in tricarbonyl(l-methoxycyclohexa-l,3-diene)iron is 7.3 0.2 kcal mol which is almost identical to that in tricarbonyl(hexa-l,3-diene)iron. Higher barriers are found in n -bonded heterodiene complexes e.g. for compound (31) AG = 14.0 kcal mol whereas the a-n and a-a azadiene derivatives (32 R=Bu ) and (33 R =PrO have barriers of less than 9 kcal moh.  

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Although iron, cobalt, and nickel occur in the same triad in Group VIII., the three elements differ considerably in their ability to form addition compounds with ammonia. Iron forms few ammino-salts, most of which are unstable, and its tendency to complex-salt formation of the ammine type appears in the complex cyanides and not in the ammines themselves. 

Cobalt. The speciation of radiocobalt has been selected for discussion in this chapter because it exemplifies an element for which much information already exists regarding its stable chemical speciation, yet there are additional species which have become environmentally important as a result of the activities of the nuclear industry Cobalt, the middle member of the first triad of group VIII transition metals in the Periodic Table (iron, cobalt, nickel), is most stable in the divalent state when in simple compounds. Studies of radionuclide releases from nuclear power plants under tropical conditions in India seem to indicate that... 

We place hydrogen as the first element in the first period, along with helium. When helium was discovered, Mendeleev put it in the second period. We put the triads of iron, cobalt, and nickel ruthenium, rhodium and palladium and osmium, iridium, and platinum in group VIIIB, in the middle of the table. Mendeleev put them in group VIII. We also have two long groups, the lanthanides and actinides, that were a headache for Mendeleev. 

In Group VIII, each position instead of being filled by a single element is occupied by a group of three elements. Thus there appear in triads iron, cobalt, and nickel ruthenium, rhodium, and palladium and osmium, iridium, and platinum. In this group there is no subdivision into families, but all the members are heavy metals. 

Cobalt, in its properties, is an excellent intermediary between iron and nickel, and, moreover, it is clearly a suitable element to constitute the first of the central vertical triads of Group VIII, namely, Co, Eh, and Ir. Hence, if the Periodic Law holds absolutely, the atomic weight of cobalt should exceed that of iron, but not that of nickel. Either, therefore, the atomic weight of cobalt is slightly too high or that of nickel is slightly too low. 

Group VIII Fe, Ru, Os. Five-coordinated complexes of the iron triad metals M(0) and M(I) undergo oxidative addition with alkyl, aryl and acyl halides to give 6-... 

The running order in this chapter is essentially similar to that used in the preceding one. The main section, dealing with metal-carbon Group VIII triads in the order, iron, cobalt, and nickel, and is followed by a shorter section on carbene and carbyne complexes. Oxidative addition or reductive elimination reactions are included only when they lead directly to the formation or rupture of metal-carbon bonds. 

Carbene Complexes of the Group VIII Metals The Iron Triad.— The electrophilic methylene complex [Fe=CHa(Cp) (dppe)]+ has been characterized in solution by and C n.m.r. The iron cyclohepta-trienylidene complexes [Fe(chpt)(CO)2(Cp)] [PFe] (chpt=(27), (28), and (29) have been synthesized and their crystal structures determined chpt=(27), (28). The complex [Fe(CHPh)(CO)a(Cp)] [PF ] readily undergoes benzyli-... 

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