Group 8 iron

Carbides of the Iron Group Metals. The carbides of iron, nickel, cobalt, and manganese have lower melting points, lower hardness, and different stmctures than the hard metallic materials. Nonetheless, these carbides, particularly iron carbide and the double carbides with other transition metals, are of great technical importance as hardening components of alloy steels and cast iron. 

Catalysts for SW tube formation are not confined to the iron-group metals. Some elements of the lanthanide series can catalyze the formation of SW tubes. 

Table 26.2 also reveals a diminished tendency on the part of these elements to form compounds of high coordination number when compared with the iron group and, apart from [Co(N03)4], a coordination number of 6 is rarely exceeded. There is also a marked reluctance to form oxoanions (p. 1118). This is presumably because their formation requires the donation of n electrons from the oxygen atoms to the metal and the metals become progressively... 

The complex cyanides of transition metals, especially the iron group, are very stable in aqueous solution. Their high co-ordination numbers mean the metal core of the complex is effectively shielded, and the metal-cyanide bonds, which share electrons with unfilled inner orbitals of the metal, may have a much more covalent character. Single electron transfer to the ferri-cyanide ion as a whole is easy (reducing it to ferrocyanide, with no alteration of co-ordination), but further reduction does not occur. 

The activation of aromatics by organo-iron groups has been used with two families of complexes [10-13], both synthesized from the arene and A1C13 with variation in the source of iron only [19-20] as shown in Scheme II ... 

Rosenblum M (1965) Chemistry of the iron group metallocenes (part I) Wiley, New Y ork... 

But similar calculations for the iron-group ions show marked disagreement with experiment, and many attempts were made to explain the discrepancies. The explanation is simple in many condensed systems the perturbing effect of the atoms or molecules surrounding a magnetic atom destroys the contribution of the orbital momentum to the magnetic moment, which is produced entirely by the spin moments of unpaired electrons.40... 

This assumption was first made by E. C. Stoner, Phil. Mag., 8, 250 (1929), in order to account for the observed moments of iron-group ions. 

The whole question is clarified when considered in relation to the foregoing quantum mechanical treatment of the electron-pair bond. For the iron-group elements the following rules follow directly from that treatment and from the rules of line spectroscopy. 

Bond eigenfunctions for iron-group atoms are formed from the nine eigenfunctions 3dh, 4s and 4p3, as described in preceding sections. One bond eigenfunction is needed for each electron-pair bond. 

The magnetic criterion is particularly valuable because it provides a basis for differentiating sharply between essentially ionic and essentially electron-pair bonds Experimental data have as yet been obtained for only a few of the interesting compounds, but these indicate that oxides and fluorides of most metals are ionic. Electron-pair bonds are formed by most of the transition elements with sulfur, selenium, tellurium, phosphorus, arsenic and antimony, as in the sulfide minerals (pyrite, molybdenite, skutterudite, etc.). The halogens other than fluorine form electron-pair bonds with metals of the palladium and platinum groups and sometimes, but not always, with iron-group metals. 

Finally, the use of simple valence bond theory has led recently to a significant discovery concerning the nature of metals. Many years ago one of us noticed, based on an analysis of the experimental values of the saturation ferromagnetic moment per atom of the metals of the iron group and their alloys, that for a substance to have metallic properties, 0.72 orbital per atom, the metallic orbital, must be available to permit the unsynchronized resonance that confers metallic properties on a substance.34 38 Using lithium as an example, unsynchronized resonance refers to such structures as follows. 

Fig. 1. Electronic states [or iron-group atoms, showing number of states as qualitative [unction of electronic energy. Electrons in band A are paired with similar electrons of neighboring atoms to form bonds. Electrons in band B are d electrons with small interatomic interaction they remain unpaired until the band is half-filled. The shaded area represents occupancy of the states by electrons in nickel, with 0.6 electron lacking from a completely filled B band. (States corresponding to occupancy of bond orbitals by unshared electron pairs are not shown in the diagram.)...

Values 3.8 to 4.0 Bohr magnetons are observed for iron-group ions with this configuration. 

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