Dsp2, hybrid orbitals

Formation of a <7-bond by donation from the 7r-orbital of ethene into a vacant metal dsp2 hybrid orbital... 

FIGURE 16.10 The structure and bonding in the anion of Zeise s salt. A cr-bond results from the overlap of a dsp2 hybrid orbital on the metal and the 7T orbital on ethylene. Back donation from a d orbital on the metal to the -k orbital on ethylene gives some 7r bonding (shown in (c)). 

If a complex having a coordination number of 4 is produced, only two of the 4p orbitals are used and the hybrid orbital type is dsp2, which is characteristic of a square planar complex. For Ni2+, this is illustrated by the following scheme ... 

A persistent feature of qualitative models of transition-metal bonding is the supposed importance of p orbitals in the skeletal hybridization.76 Pauling originally envisioned dsp2 hybrids for square-planar or d2sp3 hybrids for octahedral bonding, both of 50% p character. Moreover, the 18-electron rule for transition-metal complexes seems to require participation of nine metal orbitals, presumably the five d, one s, and three p orbitals of the outermost [( — l)d]5[ s]1[ p]3 quantum shell. 

AB49 planar. We require A [g + B+ E atomic orbitals. The two possible combinations are dz2 + 4rJ-v2 + px + py and s + d 2- 2 + px + /V The latter is usually chosen and designated briefly as dsp2 hybridization. However, not just any choice of d or p orbitals is implied, but rather only the specific one just mentioned. 

In square planar complexes, the metal uses a set of four hybrid orbitals called dsp2 hybrids, which point toward the four corners of a square. By pairing up the two unpaired d electrons in one d orbital, we obtain a vacant 3d orbital that can be hybridized with the 4s orbital and two of the 4p orbitals to give the square planar dsp2 hybrids. These hybrids form bonds to the ligands by accepting a share in the four pairs of ligand electrons ... 

Valence bond theory describes the bonding in complexes in terms of two-electron, coordinate covalent bonds resulting from the overlap of filled ligand orbitals with vacant metal hybrid orbitals that point in the direction of the ligands sp (linear), sp3 (tetrahedral), dsp2 (square planar), and d2sp3 or sp3d2 (octahedral). 

Cuprous ion complexes with four ligands are normally tetrahedral, involving spi hybrid orbitals (electronic distribution A). However, the cuprous hydrogen complex II, which is of the form (Cu X3H), is isoelec-tronie with four coordinate complexes of cupric ion, of the form (Cu11 ), which are known to be planar and to use dsp2 orbitals (distribution B). It seemed possible, therefore, that because of its unusual electronic structure, complex II was also planar. Construction of scale (Fischer-Taylor-Hirschfelder) models indicates that this is probably not the case. A planar model of I can be constructed but not of II insufficient space exists to accommodate the hydrogen atoms between the copper ions in II. If, however, tetrahedral coordination is permitted about the copper ions, no... 

If the two unpaired electrons in a d ion such as Ni were paired, one 3d orbital would be available that would permit the formation of a set of dsp2 hybrids, the orbital type for bonding in a square planar complex. Such a complex would have no unpaired electrons, and this is exactly the situation with [Ni(CN)4]2. For the d8 ions Pd2+ and Pt2+, only square planar complexes form when the coordination number is 4. The reasons for the difference in behavior between Ni2+ and these second- and third-row metal ions will be explained later in this chapter. 

In spite of all the theoretical evidence, accumulated over many years, it is still commonplace for students to be taught that the existence of hypercoordinate molecules such as SF and PFri relies on the utilization of d orbitals to expand the octet . Indeed, even models based on d sp , dsp2 and dsp3 hybrid orbitals or pn-dn back-bonding are still in use to describe hypercoordinate bonding to second-row elements. Of course, the consensus view that has emerged from most of the... 

Fig. 4.01. The bond configurations corresponding to some simple hybrid orbitals. (<2) sp—linear (b) sp2—to comers of an equilateral triangle (c) dsp2—to comers of a square (d) spz—to comers of a regular tetrahedron (e) d2spz and sp3d2—to comers of a regular octahedron (/) d sp—to comers of a trigonal prism.

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