D sp orbitals

The six coordination sites are generated by hybridization of two 3[Pg.522]

Structural studies have been made of several of these compounds. Ru (CO)4l2 forms octahedral molecules with the I atoms in cis positions (Ru—C, 2-01 A, Ru—I, 2-72 A), and a similar configuration has been assigned to Fe(CO)4l2 on the basis of i.r. evidence. The structure of [Rh(CO)2Cl]2 is more complex. This compound is dimeric in solution and also in the crystal, in which the dimers are bonVed into infinite chains by Rh-Rh bonds as shown in Fig. 22.10(a). To account for the diamagnetism it has been suggested that bent metal-metal bonds are formed by overlap of d sp orbitals at an angle of 56°. 

The six lone pairs of electrons enter the unfilled Sd, 4s and 4p orbitals and all the d sp orbitals are hybridized, i.e. made equal and spread evenly about the central metal ion as shown in Figure 3.3. 

An octahedral set of d sp orbitals on a sulfur atom. The small lobe of each hybrid orbital has been omitted for clarity. 

This requires an octahedral arrangement of pairs and in turn an octahedral set of six hybrid orbitals, or d sp hybridization, in which two d orbitals, one 5 orbital, and three p orbitals are combined (see Fig. 9.23). Six electron pairs around an atom are always arranged octahedrally and require d sp hybridization of the atom. Each of the d sp orbitals on the sulfur atom is used to bond to a fluorine atom. Since there are four pairs on each fluorine atom, the fluorine atoms are assumed to be sp hybridized. 

Six electron pairs (octahedral arrangement) requires d sp orbitals... 

The hybrid orbitals used by the metal ion depend on the number and arrangement of the ligands. For example, accommodating the lone pairs from the six ammonia molecules in the octahedral Co(NH3)e ion requires a set of six empty hybrid atomic orbitals in an octahedral arrangement. As we discussed in Section 9.1, an octahedral set of orbitals is formed by the hybridization of two d, one s, and threep orbitals to give a set of six d sp orbitals (see Fig. 21.18). 

However, the covalent bond assiunption seems to be more convincing. The octahedral distributions of the bonds are possible only for the d sp and d sf configurations [39, 40], The bonding in the LnS-iype sulfides can be represented as consisting of hybrid d sp orbitals of the rare-earth and sulfur atoms. 

Group VI elements like S, Se and Te also form hexahalides (A Sj and molecules (Figure 2.11) with strictly octahedral symmetry about the A atom. Some d hybridization must be postulated in order to form he 6 necessary symmetrically oriented and only half-filled highly assymetrical d sp orbitals. The corresponding crystals are trigonal Se and Te, and a- and jS-monoclinic Se. 

The all-terminal Fe3(CO)i2 is easily built from three Fe(CO)4 fragments [66]. Its HOMO and HOMO-1 orbitals represent the symmetry-adapted form of three equivalent Fe-Fe bonds as expected by the 18-e rule (Fig. 16). The construction of the bridging Fe3(CO)i2 needs two Fe(CO)3 units, and a Fe(CO)4 unit and a (p-CO)2 unit will parallel the previous work [65, 67]. As mentioned above, we will concentrate our descriptimi on the formation of the cluster bonding using the frontier orbitals (Fig. 17). Each Fe(CO)4 unit and Fe(CO)3 has two or three d sp orbitals not used by o-donation from terminal COs. To assemble Fe3(CO)io fragment in the absence of the two bridging carbonyls, Fe(CO)4 unit contributes two... 

Fig. 15 (a) The molecular orbital diagram of Fe(CO)4 fragment, (b) The two hybrid d sp orbitals in the o-acceptance orbital cluster I. (c) The molecular orbitals reorganized from the two hybrid orbitals... 

The recombination of Co(CO)4 to Co2(CO)8 can also be understood within the octahedral d sp hybridization scheme. Let us put six of the nine Co d-electrons into the three nonbonding Co d-orbitals. Four of the six d sp orbitals form bonding orbitals with the four 5(7 CO orbitals, that each donate two electrons. Two lone-pair type d sp orbitals are left. One orbital can be doubly occupied whereas the other contains the final electron that is left. The latter orbital can combine with an equivalent orbital of another Co(CO)4 radical. A Co(CO)3 fragment contains three dangling d sp orbitals each of which is occupied with one electron. Hence it can combine with three other such fragments to form Co4(CO)i2 (see also Chapter 5,pages 226, 227). 

These six localized d sp orbitals are used to form electron-pair bonds with the six water molecules. The valence-bond description of the ground state of Ti(HsO)e + is shown in Fig. 9-7. The unpaired elec-... 

Metal ions in solution are truly oxy-acids. Many of these have large kernels and use d sp orbitals for six-coordinate bonding. They can hold HjO as well as OH or 0 of the smaller oxy-acids. Ions like Fe strongly hold six water molecules and withdraw electron cloud from the oxygen. This produces acid behavior in no way different from that of the other acids we have considered,... 

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