Back-bonding

FIGURE 1.5 Molecular orbital, or ligand field picture, of metal ligand bonding in an octahedral ML complex. The box contains the d orbitals. 

Ligands are generally nucleophilic because they have available (high-lying) electron lone pairs. The metal ion is electrophilic because it has available (low-lying) empty d orbitals. The nucleophilic ligands, which are lone-pair donors, attack the electrophilic metal, an acceptor for lone pairs, to give the metal complex. Metal ions can accept multiple lone pairs so that the complex formed is not just ML but ML (/i = 2-9). 

FIGURE 1.6 Ovo lap between a filled metal d orbital and an empty CO nr orbital to give the n component of the M—CO bond. The shading refers to occupancy of the orbitals and the + and — signs, to the symmetry. The M—CO r bond is formed by the donation of a lone pair on C into an empty da orbital on the metal (not shown). 

FIGURE 1.7 Effect of tuniing on the n interaction between a ar-accefitor ligand and the metal. The unoccupied, and relatively unstable n orbitals of the ligand are shown on the right. Their effect is to stabilize the filled d, orbitals of the complex and so increase A. In W(CO)j, the lowest three orbitals are filled. 

Series of compounds such as V(CO)j , Cr(CO)6, and Mn(CO)6 are said to be isoelectronic complexes because they have the same number of electrons distributed in very similar structures. Isoelectronic ligands are CO and NO or CO and CN , for example. Strictly speaking, CO and CS are not isoelectronic, but as the difference between O and S lies in the number of core levels, while the valence shell is the same, the term isoelectronic is often extended to cover such pairs. A comparison of isoelectronic complexes or ligands can be useful in making analogies and pointing out contrasts.  

6 shows how overlap takes place to form the M—C it bond. It may seem paradoxical that an antibonding orbital like the it (CO) can form a bond, but this orbital is antibonding only with respect to C and O, and can still be bonding with respect to M and C. 

We can make the ligand field diagram of Fig. 1.5 appropriate for the case of W(CO)e by including the -it levels of CO (Fig. 1.7). The set of levels still find no match with the six CO((t) orbitals, which are lone pairs on C. 

They do interact strongly with the empty CO tt levels. Since the Md set are filled in this complex, the result is that electrons that were metal-centered now spend some of their time on the ligands this means that the metal has donated some electron density to the ligands. This is called back bonding and is a key feature of M—L bonds where L is an unsaturated molecule (i.e., has double bonds). Note that this can only happen in d or higher configurations a d ion like Ti cannot back bond and does not form stable carbonyl complexes. 

The fate of this model depends on whether it finds favor in the scientific community, and we will not use it extensively in what follows. Textbooks can give the impression that everything is settled and agreed upon, but that agreement is only achieved after much argument, leading to an evolution of the community s understanding. Ideas that come to dominate often start out as a minority view. The sd model may therefore either fade, flower, or be modified in future. 

The hgand field diagram of Fig. 1.6 has to be modified when the ligands are ir acceptors, such as CO, because we now need to include the CO -K levels (Fig. 1.9). The Md, set now interact strongly with the empty CO ir levels to form M-C tt bonds. For (f complexes, such as W(CO)6, where the Md set is filled, the electrons now spend some of their time on the ligands by back bonding. 

Their structures show that tx back donation is a big contributor to the M=C bond in metal carbonyls, making the M=C bond much shorter than an M-C single bond. For example, in CpMo(CO)3Me, M-CH3 is 2.38 A but M=CO is 1.99 A. A true M-CO single bond would be shorter than 2.38 A by about 0.07 A, to allow for the higher s character of sp CO versus sp CH3, leaving a substantial shortening of 0.32 A that can be ascribed to back bonding. 

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Th e ability to perform m oleciilar orbital (MO ) calculation s on m et-als is extremely useliil because molecular mechanics methods are gen erally unable to treat m etals. This is becau se m etals h ave a wide range of valences, oxidation states, spin multiplicities, and have 1111 usual bonding situations (e.g.. d%-p% back bonding). In addition. the 11 on direction al n at are o ( m etallic hon din g is less am en a-ble to a ball and spring interpretation. 

Electron correlation is often very important as well. The presence of multiple bonding interactions, such as pi back bonding, makes coordination compounds more sensitive to correlation than organic compounds. In some cases, the HF wave function does not provide even a qualitatively correct description of the compound. If the weight of the reference determinant in a single-reference CISD calculation is less than about 0.9, then the HF wave function is not qualitatively correct. In such cases, multiple-determinant, MSCSF, CASPT2, or MRCI calculations tend to be the most efficient methods. The alternative is... 

Perhaps because of inadequate or non-existent back-bonding (p. 923), the only neutral, binary carbonyl so far reported is Ti(CO)g which has been produced by condensation of titanium metal vapour with CO in a matrix of inert gases at 10-15 K, and identified spectroscopically. By contrast, if MCI4 (M = Ti, Zr) in dimethoxy-ethane is reduced with potassium naphthalenide in the presence of a crown ether (to complex the K+) under an atmosphere of CO, [M(CO)g] salts are produced. These not only involve the metals in the exceptionally low formal oxidation state of —2 but are thermally stable up to 200 and 130°C respectively. However, the majority of their carbonyl compounds are stabilized by n-bonded ligands, usually cyclopentadienyl, as in [M(/j5-C5H5)2(CO)2] (Fig. 21.8). 

Structures have been determined for a number of these compounds, showing that the Rh-P bonds are little affected by the m-ligands (Figure 2.22). The shorter Rh-C distance in the thiocarbonyl is probably a result of greater Rh=C back-bonding. Addition of S02 results in the formation of a 5-coordinate (sp) adduct with the expected lengthening in all bonds. 

Calculations for trigonal bipyramidal ML4(NO) systems with axial NO-like [Ir(NO)(PPh3)3H+] give a d orbital sequence of xz,yz < x2 — y2, xy < z2 so that in such an IrNO 8 system, the z2 orbital is unoccupied not only does bending not produce any stabilization but in fact dxz, dyz — 7r back-bonding is lost, favouring a linear Ir—N—O bond. 

There is (a) cr-donation from a filled oxygen orbital to an empty platinum orbital and (b) 7r back-bonding from a filled metal d orbital into an empty oxygen 7r -anti-bonding orbital. 

Back-bonding, with formation of a 7r-bond, from a filled metal d orbital to an anti-bonding it -ethene orbital. 

Multiple Bonds and Back-bonding (see also Tables 3.23. 6.10 and 21.2)... 

Figure 6-13. Synergic back-bonding in a platinum alkene complex. In (a), the interaction of a (filled) platinum 5d orbital with the tf molecular orbital of the alkene is shown, whilst in (b), the interaction of a dsp hybrid orbital with the n molecular orbital of the alkene is shown. Note that the two interactions result in electron density moving in opposite directions.

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