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Chapter 9 Coordination Compounds 2. Covalent bonding between the ligand and the central atom is not considered in this theory. 3. This theory ignores the attractive influence of the nuclear charge in the ligand atom on the metal ion d electrons.
Practice Problems 10. Ni(CO)4 is a diamagnetic complex, whereas NiCl4 is paramagnetic. Explain. 11. Both the complexes Ni(CO)4 and [Ni(CN)4]2- have coordination number 4; however, Ni(CO)4 is tetrahedral in shape whereas [Ni(CN)4]2- is square planar. Explain the difference in geometry in terms of the difference in hybridization.
12. According to crystal field theory, how do five degenerate d orbitals split in the presence of ligands in octahedral complexes?
9.6 Bonding in Metal Carbonyls The first metal carbonyls, Ni(CO)4 and Fe(CO)5 were discovered in 1890 and 1891, respectively, and now carbonyl derivatives of at least one type are known for all the transition metals. The simplest carbonyls are of the type M(CO)x which have simple well-defined structures. For example, V(CO)6 and Cr(CO)6 are octahedral, Fe(CO)5 is trigonal bipyramidal and Ni(CO)4 is tetrahedral. Of the d-block elements, the ones that form stable mononuclear carbonyls are those that require an integral number of carbonyl ligands to attain the number of electrons of the succeeding noble gas atom. OC Ni OC
CO
Fe CO
OC
OC
OC OC
CO
CO
OC OC
CO
Cr
CO
CO
There are numerous polynuclear carbonyls that are homonuclear, such as Fe3(CO)12 or heteronuclear, such as MnRe(CO)10. In these compounds, there are not only linear M−C−M groups but also either M−M bonds or M−M bonds with bridging carbonyls. For example, Mn2(CO)10 is made up of two square pyramidal Mn(CO)5 units joined by an Mn−Mn bond and Co2(CO)8 has a Co−Co bond bridged by two carbonyl groups. OC
OC OC OC
Fe
OC CO Fe
CO
CO CO
Fe2(CO)8
OC CO OC OC Mn OC CO
OC
CO Mn CO CO
OC
O C
OC
O C
Co
Mn2(CO)10
C O
Co C O
CO CO
Co2 (CO)8
The bonding in linear M−C−O groups is depicted in Fig. 9.9
C O σ bond
C O π back-bond
Figure 9.9 Bonding in linear M−C−O groups. First, there is a dative overlap of the filled carbonyl carbon s-orbital with the vacant metal orbital. The second dative overlap is of the filled dp metal orbital with an empty antibonding pp orbital of the carbonyl group. This bonding mechanism is synergic, that is, the effects of s-bond formation strengthen the p bonding and vice versa.
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