Metals valence shells

We now begin to understand how the 7t-interaction can explain the observed spectro-chemical series. The anomalous strong field ligands, such as carbon monoxide, pyridine, 2,2 -bipyridine and CN, all possess vacant orbitals of 7t-symmetry of similar energy to the metal valence shell orbitals (Fig. 1-15). 

Consideration of molecular orbital interactions suggests that it would be ideal to use a transition metal complex having at least two empty and one filled nonbonding metal valence shell orbital, i.e. a 14-elec-tion species, for observing facile bicyclization reactions shown in equations (2)-(4). This is based on assumptions that effective ir-complexation of an alkyne or an alkene with a metal complex requires the... 

Hybridization in an octahedral complex in the valence bond model of coordination ompound the formation of bonds to an octahedral transition metal ion would involve six d sp h /hrid orbitals. In the approach used here the diff( ring syininetry properties of the s, thii p, p and p.. and the d, and d orbitals used in bonding are explicitly taken into account. However, the 3 all of the p and two ol the d metal valence shell orbitals are otill used in o-bond tormation. 

In the valence shell electron count for the complex (Ti -C3Ph3)NiCl(py)2 (py = pyridine) (Fig. 4.1), the cyclopropenyl hgand C3Ph3 supplies three electrons, the chlorine atom one electron, and each pyridine ligand supplies two electrons. Added to the 10 electrons in the valence shell of the nickel atom, this generates a total of 18 electrons. In the case of the sandwich compound dibenzenechromium, (q -C/H jiCr (Fig. 4.2),six electrons from each benzene ligand may be added to the six electrons in the metal valence shell to generate the total of 18 electrons. 

Reactions 2 and 3 represent oxidative addition and reductive elimination sequences. Reactions 1, 2, and 3 all involve a reversible addition/ elimination of a substrate molecule that contributes two electrons to a dinuclear center. However, since the proposed reactions proceed with a change in M-M bond order, the number of metal valence shell electrons... 

There is therefore, no overall change in the number of metal valence shell electrons. Although not many substrates meet the requirement of being carbyne-like, the reactions between nitric oxide and a metal-to-metal triple bond can be viewed as examples of Reaction 6. 

Crystal field theory assumes that all M-L interactions are purely electrostatic in nature. More specifically, it considers the electrostatic effect of a field of ligands on the energies of a metals valence-shell orbitals. To discuss CFT, we need only be aware of two fundamental concepts (1) the coulombic theory of electrostatic interactions and (2) the shapes of the valence orbitals of transition metals—that is, the nd orbitals ( = 3 for the first row of transition metals, etc.). The first concept involves only the familiar ideas of the repulsion of like and the attraction of dislike electrical charges. Quantitatively, Coulomb s law states that the potential energy of two charges Qj and Q2 separated by a distance r is given by the formula shown in Equation (4.3) ... 

It is the total number of electrons in what is defined as the transition-metal valence shell, i.e. the group of nj, n- 1) d, np sublayers. It contains the electrons that are initially in the metal valence shell and the electrons brought by the ligands. 

This bond requires non-bonding d electrons in the metal valence shell. Thus, d° complexes such as those of Sc , Ti or V cannot give stable olefin complexes. In the classic metal-olefin complexes, the backbonding is not taken into account in... 

On the descriptive side, previously known binary carbonyl cations are usually of the [M(C0)6] type with M = Mn, Tc or Re (82), The oxidation state of the metal in these or other ternary cations is 0 or +1, and the ionic charge of the complex does not exceed +1. In addition, far more basic anions are used as counter ions. The effective atomic number rule, which plays an important role in judging stability, structure and reactivity of transition-metal carbonyls, is not valid for the noble-metal carbonyl compounds reported so far. The silver(I) and gold(I) carbonyl derivatives have 14, and the Pt(II) carbonyls have 16 electrons in the metal valence shell. 

Transition metal (TM) species offer spectacular opportunities for coordinative bonding, due to the presence of both donor (LP) and acceptor (LP ) functionality in the metal valence shell. The unique shapes and symmetries of sd -based TM hybrids also offer highly unusual covalent geometries and delocalization patterns (cf. V B, Chapter 4), quite unlike those of common organic species. The present section only hints at the richness of TM covalent and coordinate bonding phenomena that offer one of the most exciting frontiers of modem chemical research. 

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