Metal orbitals, vacant

Figure 19.18 Schematic representation of the orbital overlaps leading to M-CO bonding (a) a overlap and donation from the lone-pair on C into a vacant (hybrid) metal orbital to form a u M <—C bond, and (b) 7T overlap and the donation from a filled d or dj orbital on M into a vacant antibonding n orbital on CO to form a tt M—> C bond.

The two eflPects above constitute what is called central field covalency since they aflFect both the a and the tt orbitals on the metal to the same extent. There is also, of course, symmetry restricted covalency which acts difiFerently on metal orbitals of diflFerent symmetries. This type of covalency shows up in optical absorption spectra as differences in the values of Ps and p -, as compared with 35. The first two s refer to transitions within a given symmetry subshell while 635 refers to transitions between the two subshells. This evidence of covalency almost of necessity forces one to admit the existence of chemical bonds since it is difficult to explain on a solely electrostatic model. The expansion of the metal orbitals can be caused either by backbonding to vacant ligand orbitals, or it may be a result of more or less extensive overlap of ligand electron density in the bond region. Whether or not this overlap density can properly be assigned metal 3d character is what we questioned above. At any... 

The nonequivalence of the cap (q 2 positions) and the belt ( i positions) metal atoms in P2Wig062 is reflected by the composition of metal-centered vacant orbitals (Fig. 24). It is worth noting that the lowest unoccupied molecular orbital (LUMO) (aj) is almost entirely developed on the ay positions (96%), with equal weights on each of the twelve metal atoms. With the reasonable assumption that the initial electron transfer involves the LUMO of the starting species, the shape of... 

Buell, McEwen, and Kleinberg have observed that weak acids such as hydrogen azide and acetic acid add readily across the double bond of vinylferrocene (XLI, M = Fe) (8). They have postulated that the mechanism of addition proceeds via intermediate formation of the a -ferrocenylcarbonium ion (X-LII, M = Fe), followed by conversion to the acetate (XLIII, M = Fe). Stabilization of carbonium ions of this type can result from overlap of filled metal orbitals with the vacant p -orbital of the carbonium ion. 

Transition metal complexes act as templates that regulate organic reactions that occur in the coordination sphere (4). Ligands are often activated or stabilized by participation of metal d orbitals, where the central metals are electronically amphoteric in contrast to the main group elements, which normally act as Lewis acids. The bonding scheme of an olefin-transition metal complex is illustrated in Scheme 2. The olefin 7T electrons are donated to a vacant metal orbital to make a a-type bond the metal d elections are back-donated to olefin anti-bonding orbitals with the same symmetry to form a ir-type bond. In this way, the olefin is activated by formal electron promotion from the tt to tt orbital, as... 

A metal ion might play a number of roles in modifying the behaviour of co-ordinated electrophiles. Co-ordination of a donor atom X to the metal polarises the C-X bond and increases the electrophilic character of the carbon. In the case of an unsaturated centre this polarisation may be countered or even completely reversed by back-donation from filled (d) orbitals on the metal into vacant 7t -(or similar) orbitals of the ligand. The X- group generated after nucleophilic attack on the carbon atom may also be stabilised by co-ordi-... 

Dative covalent bonds, or coordinate covalent bonds, are those in which electrons are shared (as in all covalent bonds), but in which both electrons involved in each bond are contributed from the same atom. Such bonds occur in organometallic compounds of transition metals having vacant d orbitals. It is beyond the scope of this book to discuss such bonding in detail the reader needing additional information should refer to works on organometallic compounds.12 The most common organometallic compounds that have dative covalent bonds are carbonyl compounds, which are formed from a transition metal and carbon monoxide, where the metal is usually in the -1, 0, or +1 oxidation state. In these compounds the carbon atom on the carbon monoxide acts as an electron-pair donor ... 

In the side-on arrangement, the bonding is considered to arise from two interdependent components. In the first part, a overlap between the filled n orbital of N2 and a suitably directed vacant hybrid metal orbital forms a donor bond. In the second part, the M atom and N2 molecule are involved in two back-bonding interactions, one having it symmetry as shown in Fig. 15.1.7(a), and the other with S symmetry as shown in Fig. 15.1.7(b). These n and S-back bonds synergically reinforce the a bond. 

Figure 5.6 Modes of chemisorption of carbon monoxide. (A) Combination of donation from the 5a orbital to the metal s vacant d-orbital + back-donation from a filled d-orbital to the 2ty antibonding orbital.

B) Donation from the 5[Pg.140]

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