Transition metals, second row

Molybdenum because of its unique chemical versatility and unusually high bio-availability has been incorporated widely into biological systems. It is the only second-row transition metal that is essential for most of living organisms and belongs to elements (along with Cu, Cd, Hg, Pb and Cr) potentially hazardous to humans. 

The study of molecular systems containing metal atoms, particularly transition metal atoms, is more challenging than first-row chemistry from both an experimental and theoretical point of view. Therefore, we have systematically studied (3-5) the computational requirements for obtaining accurate spectroscopic constants for diatomic and triatomic systems containing the first- and second-row transition metals. Our goal has been to understand the diversity of mechanisms by which transition metals bond and to aid in the interpretation of experimental observations. 

We have also observed competition between products resulting from C-C and C-H bond activation in reactions of Y with propene,138 propyne,143 2-butyric,143 four butene isomers,138 acetaldehyde,128 acetone,128 ketene,144 and two cyclohexadiene isomers,145 as well as for Zr, Nb, Mo, and Mo with 2-butyne.143 In this chapter, we use the term C-C activation to describe any reaction leading to C-C bond fission in which the hydrocarbon reactant is broken into two smaller hydrocarbon products, with one hydrocarbon bound to the metal. It is important to note, however, that C-C activation does not necessarily require true C-C insertion. As will be shown in this chapter, the reaction of Y, the simplest second-row transition metal atom, with propene leads to formation of YCH2 +C2H4. The mechanism involves addition to the C=C bond followed by H atom migration and C-C bond fission, rather than by true C-C insertion. 

Among the early second-row transition metals, the chemistry is richest for yttrium. In our early studies of Y + C2H2123 and C2H6,124 several competing product channels were observed, while only H2 elimination was observed for Zr, Nb, or Mo.122 Likewise, in reactions with larger molecules (containing three or more carbons for example), yttrium always led to the greatest number of unique product channels. A likely cause for this result is the presence of an exit-channel barrier for H2 elimination in reactions with Y. Because H2 elimination is usually the most thermodynamically... 

One of the consequences of the lanthanide contraction is that some of the +3 lanthanide ions are very similar in size to some of the similarly charged ions of the second-row transition metals. For example, the radius of Y3+ is about 88 pm, which is approximately the same as the radius of Ho3+ or Er3 +. As shown in Figure 11.8, the heats of hydration of the +3 ions show clear indication of the effect of the lanthanide contraction. 

Figure 4. Magnetic moment in Bohr magnetons of an iron atom dissolved in various second row transition metals and alloys (one atomic per cent iron in each metal and alloy) as a function of electron concentration (11)...

CID methods have proven to be very useful in measuring the stabilities of clusters of bare transition metal atoms, providing many more thermochemical values than photodissociation methods. In our laboratory, we have used CID to study the cationic clusters of ten different transition metal elements, including TiJ (x=2-22),VJ (x=2-20), CrJ (x=2-21),Mn, FeJ (x=2-19),CoJ (x=2-18),NiJ (x=2—18), and CuJ the second row transition metal clusters of NbJ (x=2-ll) and the third row transition metal clusters of Taj (x=2-4). These results have been summarized and trends analyzed previously [176,177]. CID methods have also been used by Ervin et al. to measure the stabilities of anionic clusters of the coinage metals Cu (x=2-8) [178], Ag (x=2—11) [179], and Au (x=2-7) [180] and group 10 metals Pd [181] and Ptx (x=3-6) [182]. A multiple collision-in-... 

All the results above Indicate a scale of acidities In which the second row transition metal complex Is the most acidic. We do not have an explanation for this at hand although it appears that a similar order has been found for some similar complexes in the I ron triad. UJ... 

With light elements (e.g. first-row and second-row transition metals), to which principal consideration is given in this book, the Russell-Saunders (or IS) coupling scheme suffices. Every microstate is designated with a term symbol of the general form where 25 -I- 1 is the spin multiplicity (5 = 5,), L is the total orbital angular momentum... 

For second row transition metals, the corresponding Is 4d transition requires very high energies. Low monochromator resolution and short core-hole lifetimes hinder the ability to detect this transition in many cases. Other options to obtain information for these metals rely on intensity estimations of the 2p 4d (allowed) transitions at the f Cn.iii) edges. Another promising approach, mentioned in... 

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