Low-valent M atoms

In the Rieke process for the reduction of salts, magnesium is suitable only in exceptional cases as a substitute for potassium. However, the ready formation of the anthracene complex 1 can also be exploited for this purpose. As shown in Scheme 3. not only activated metals can be generated by a catalytic cycle, but also, in the presence of suitable ligands, complexes with a zero- or low-valent central atom, such as [M(cod)2] (M = Ni, Pt), [CP2M] (M = V, Co, Fe), and [Pd(PPh3)3], and [Mo(butadiene)3] can be prepared [45]. 

Of these the direct reaction with N2 to displace H20 is perhaps most notable. Despite early reports to the contrary, no way of reductively transforming the coordinated N2 in [Ru(NH3)5N2]2+ into NH3 has yet been found. In fact, reduction of coordinated N2 to NH3 in a stable complex has not yet been reported. However, there are several systems (page 345) in which reduction of N2 to NH3 is catalyzed37 by low-valent metal atoms, presumably via transient M-N2 complexes. 

Darkowski and Cocivera [94] investigated trialkyl- or triarylphosphine tellurides, as low-valent tellurium sources, soluble in organic solvents. They reported the cathodic electrodeposition of thin film CdTe on titanium from a propylene carbonate solution of tri-n-butylphosphine telluride and Cd(II) salt, at about 100 °C. Amorphous, smooth gray films were obtained with thicknesses up to 5.4 p,m. The Te/Cd atomic ratio was seen to depend on applied potential and solution composition with values ranging between 0.63 and 1.1. Polycrystalline, cubic CdTe was obtained upon annealing at 400 C. The as-deposited films could be either p- or n-type, and heat treatment converts p to n (type conversion cf. Sect. 3.3.2). 

Let us turn now to the nature of the bond that is normally found in M—CO groups. In most cases, it is adequate for practical, everyday purposes to regard a ligand simply as an electron pair donor and to think of the bond to the central atom simply as L—>M. However, there are important classes of compounds for which this simple concept is seriously inadequate. The most prominent examples are the metal carbonyls, metal nitrosyls, and compounds of low-valent metals containing phosphines or isonitriles as ligands. 

These results suggest that the transition state features an incipient free radical (note the trends with R), and that the metal atom has begun to make a bond to the halogen of appreciable strength. The trend with halogen substitution is a particularly pronounced one, since the trends in BDE(R-X) and BDE(M-X) are in the opposite direction (that is, the low-valent metal center is a soft acid). 

Elements of this group do not form stable carbonyls. M(CO) compounds, where M = Pr, Nd, Gd, Ho, Eu, Er, Yb, and U, may be prepared by condensation of metal atoms with carbon monoxide at low temperatures.Some low-valent lanthanide... 

Norton has thoroughly investigated half-sandwich chromium carbonyl derivatives, inspired by literature reports of a radical pathway for the hydrogenation of certain unsaturated substrates by low-valent hydride complexes (Figure 28), that is, anthracene/HCo (CO)4, a-methylstyrene/HMt L (Mf L = Mn (CO)s, or CpM (CO)3 with M = Mo or W ), and styrene/HCo (CO)4. Note that the initial equilibrium corresponds to the p-H atom transfer process in CCT and that all above-mentioned substrates lead to highly stabilized radicals. 

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