Carbonyls, metal bonding

Figure 4.101 displays the subtle variations in metal-carbonyl bond lengths in the group 6 M(CO) complexes. In each case one can clearly distinguish the coordinate omc bonds (solid lines) from the hypervalent toMc prebonds (dashed lines). The latter are about 0.1A longer, but exhibit a similar vertical variation within the group. 

The general order of metal-carbonyl bond lengths is found to be Cr <bond lengths, except that W and Mo bond lengths are usually similar, with bonds at W being slightly longer (a feature... 

Further crystallographic evidence for metal-carbonyl ir bonding is found in phosphine and phosphite derivatives of hexacarfconylchromium- Substitution of R3P for CO in Cr(CO)ft creates a complex of C4 symmetry in which one CO group lies trans to the phosphorus ligand (Fig. 11.29). The two trans ligands will compete for the same ir orbital, but carbon monoxide is a better v acid (ir acceptor) than the phosphine (Fig. 11.30). As a result, the Cr—CO r bond should be shorter relative to Cr—CO and to Cr—CO in CrtCO). The data in Table 11.12 show that these predictions are borne out. in keeping with the substantial ir character in the metal-carbonyl bond. 

The metal-H bond may be profitably compared with a metal-carbonyl bond since both involve a donation to the metal by the ligand and both ligands can accept 77 electnin density into antibonding orbitals. The accepting orbitals for CO are empty 71 orbitals, whereas for H, they are a orbitals (Fig. 11.23). Like the C—O bond, the H—H bond is weakened as a result of this metal-ligand tt interaction. A strong d-o interaction can sever the H—H bond and lead lo formation of a classical complex. 

Reductive decarboxylation of (20) yields C02, H+, and a Co(I) species at a measurable rate (94). In the presence of CO, the starting cobalt complex is regenerated, and a catalytic system for the oxidation of CO by ferricyanide is established. It is significant that in this system the metal-carbonyl bond is formed when the cobalt is in a reduced state. It is the subsequent oxidation of the cobalt by electron transfer that activated the carbonyl to attack by water or hydroxide. That this activation results in a weaker metal-carbonyl bond is evident since the Co(III)-carbonyl may be hydrolyzed in acidic solution with loss of the carbon monoxide ligand (94). 

The success of the synergetic (a + n) approach to the solution of metal carbonyl bonding led to its application to transition metal-phosphorus bonding. Mainly because it is symmetry allowed, chemists for the last two decades have drawn orbital diagrams such as those shown in Figure 19. Howevet, the experimental facts that indicated (M—P)ji bonding gave no indication as to the nature of the phosphorus orbital. 

The relative reactivity of cationic metal carbonyls has been predicted in the case of Mn(CO)6+ relative to Cr(CO)6 and V(CO)6 (55). The substitution behavior of Re(CO)6+ relative to W(CO)6 indicates that the cation is not more readily substituted by neutral ligands (3, 6), which correlates with the spectroscopic investigations (3, 4) of the metal-carbonyl bond. 

The M-C a bond is formed by donating the lone electrons on C to the empty d,2 orbital on M (upper portion of Fig. 7.3.10). The it bond is formed by back donation of the metal d7r electrons to the it orbital (introduced in Chapter 3) of CO. Populating the it orbital of CO tends to decrease the CO bond order, thus lowering the CO stretch frequency (lower portion of Fig. 7.3.10). These two components of metal-carbonyl bonding may be expressed by the two resonance structures... 

In transition metal complexes, the metal-carbonyl bond is also easily cleaved by M14 anions (Scheme 27)4a. 

Replacing the alkoxy carbene substituent by a better electron-donating amino group stabilizes the metal carbonyl bond. As a result, CO insertion in vinyl carbene D is hampered instead, cyclopentannulation via the chromacyclohexadiene I leads to aminoindenes K, which are readily hydrolyzed to indanones L (Scheme 6) [20]. 

It is interesting to note that, at the experimental bond lengths, the London dispersion type electron correlation contributes about 10 kcal/mol to each metal-carbonyl bond energy. This is approximately... 

The work on cobalt compounds raised two questions which could best be answered by studying a smaller model compound. One was the role of dispersion forces in metal ligand binding. The other was the elusive role of pi back bonding which is supposed to account for a large fraction of the metal carbonyl bond strength. 

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