D8 complexes

A simple symmetry-based analysis of the bonding orbitals for the carbene and metal fragments of a trigonal bipyramidal d8 complex suggests that the vertical orientation (i) should be preferred (100) ... 

In most palladium-catalyzed oxidations of unsaturated hydrocarbons the reaction begins with a coordination of the double bond to palladium(II). In such palladium(II) olefin complexes (1), which are square planar d8 complexes, the double bond is activated towards further reactions, in particular towards nucleophilic attack. A fairly strong interaction between a vacant orbital on palladium and the filled --orbital on the alkene, together with only a weak interaction between a filled metal d-orbital and the olefin ji -orbital (back donation), leads to an electrophilic activation of the alkene9. 

Any computational treatment of TM systems must account for the LFSE. QM methods achieve this implicitly but d-electron effects must be explicitly added to MM (4). Some effects can be modeled within conventional MM. For example, low-spin d8 complexes are planar by virtue of the LFSE (21,22), but a planar structure can also be enforced using a normal out-of-plane term (22). However, the simplest general model for describing d-orbital energies is ligand field theory (LFT) (23) which was itself derived from the earlier electrostatic crystal field theory (CFT) (24) approach. 

A quick inspection of Table II soon reveals that octahedral d1 and d2 complexes should be labile (negative CFAE for associative pathways) while d3 species should be inert. High-spin d6 and d7 species should be labile while high-spin d8 complexes should be relatively inert. In the case where there is no stabilization energy contribution, the reactivity is expected to follow that of... 

Four-coordinate d8 complexes can display a closely related electronic and geometric equilibrium between paramagnetic tetrahedral and diamagnetic planar isomers. Numerous examples are known in nickel(II) chemistry (80). In this case, as well as with the octahedral complexes described above, there is no change in the coordination number of the metal ion. 

With the exception of the d -d8 complexes, the A-values in Table III all are approximately 35,000 cm.-1. It is somewhat surprising to find similar A-values for complexes with widely varying charges on the central metal. One of the good examples for comparison purposes is the negligible change in A in going from Fe(CN)64 to Fe(CN)68. Recall that in halide, ammine, and aquo metal complexes, an increase in the... 

A detailed study of the electronic spectra of the d8 complexes [M(dto)2]2 (M = Ni, Pd, Pt) and [Au(dto)2] has been reported. The spectra are virtually identical in a variety of solvents, indicating that axial perturbations due to the solvent are minimal. Dithiooxalate has a high position among square-planar NiS4 chromophores in the spectrochemical series for dithio ligands the series is maleonitriledithiolate < (CF3)2C2S2 < diethyl dithiophosphate < ethyl xanthate < diethyl dithio-carbamate < 2,3-dimercaptopropanol anion < dithiomalonate dithiooxalate.145... 

In the case of square coplanar d8 complexes - Pd(II) and Pt(II) systems have been most studied - it is clear that an associative mechanism operates. This is not surprising, since the steric constraints which might discourage formation of a seven-coordinate Cr or Co complex will be less important for a five-coordinate complex of the larger Pd or Pt. 

The central atom in a square planar d8 complex is sometimes described as coordinatively unsaturated. This term is best used to denote a tendency to take up additional ligands without change in oxidation state it does not necessarily follow that a coordinatively-unsaturated species will be susceptible to oxidative addition. However, square d8 complexes do have some tendency to take up a ligand to become trigonal bipyramidal, 18-electron species. Trigonal bipyramidal d8 complexes formed by Fe(0),... 

Reactions such as this are analogous to the oxidative cleavage reactions of other d8 complexes such as Ir1 and Ptn (59). 

Fig. 2 a Illustrating the displacement of the potential energy surface for the d-d excited state in a square planar d8 complex, formed by population of the dxz-y2 orbital, compared to the ground state, b Even though other excited states (e.g., d-7r or n-n ) may lie at lower energies, the d-d excited state can provide a thermally activated non-radiative decay pathway. Thick arrows represent absorption of light thin ones indicate vibrational relaxation and non-radiative decay... 

Most other work with d8 complexes concerns 5-coordinate organometallic complexes. Some examples are given in Table 14.73 74,76"80 The electronic spectrum of Fe(CO)s has been measured and the lowest absorption has been identified as LF dxy, dx2 y2 -> dz2 according to the one-electron level scheme in Scheme 11.81 Such a transition should yield labilization principally along the z-axis due to the... 

These oxidative—addition reactions have been treated extensively by Su et al. (29-31), using the VBSCD model. In all cases, a good correlation was obtained between the computed barriers of the reaction and the respective AEst quantities (which enter into the expression of G), including the relative reactivity of carbenoids, and of PtL2 versus PdL2 (29-31). Another treatment led to the same reactivity patterns for C—F bond activation reactions by Rh(PR3)2X and Ir(PR3)2X d8 complexes, which are isolobal to carbenoids (30). A similar extended correlation was found recently for C—Cl activation by d10-PdL2 (32), and is dealt with in Exercise 6.9. 

Orgel diagram for d2, d3, as well as high-spin d7 and d8 complexes. Note that, in the figure on the left, the 1 states are more bent (than those on the right) because they are closer in energy. 

These are d8 complexes and we may once again employ the Orgel diagram shown in Fig. 8.5.5 to interpret their spectra, which are displayed in Fig. 8.5.15. Three spin-allowed transitions are expected and observed ... 

Fig. 2. Tanabe-Sugano diagram for octahedral d8-complexes and tetrahedral d2-complexes. Notation as in Fig. 1.

The spin-forbidden transitions in octahedral d8 complexes are less influenced by sub-shell configuration intermixing and are for instance ... 

The occurrence of the fourth-power terms of the Stevens delocalization coefficients in 833 and (B55 is easily understood. All the J- and K-inte-grals of eq. (22) are essentially proportional to the product of one electron density with another and each electron density is proportional to the square of the delocalization coefficients. It is seen from eq. (24) that if one knows both p35 and P55 in an octahedral d3-complex (cf. Table 1) or both p33 and p3s in an octahedral d8-complex (cf. Table 2), then the ratio between these two quantities represent... 

---