Transition metal coordinate bonding

Transition metal (TM) species offer spectacular opportunities for coordinative bonding, due to the presence of both donor (LP) and acceptor (LP ) functionality in the metal valence shell. The unique shapes and symmetries of sd -based TM hybrids also offer highly unusual covalent geometries and delocalization patterns (cf. V B, Chapter 4), quite unlike those of common organic species. The present section only hints at the richness of TM covalent and coordinate bonding phenomena that offer one of the most exciting frontiers of modem chemical research. 

As a simple example, let us consider nickel (Ni, Z= 28), a common constituent of metallic alloys. Although the nominal configuration of an isolated Ni atom is (3d) (4s), nickel easily achieves the promoted (3d) °(4s)° configuration, which is its primary identity in the molecular coordination complexes to be described below. 

Nickel is found to make sticky complexes with virtually any small lone-pair-bearing molecular species ( ligand, Lig) one might choose, such as 

Indeed, nickel generally forms robust Ni(Lig) complexes with all these ligands in multiple coordination stoichiometries, 

The geometries of successive coordination complexes tend to adopt VSEPR-like structures of high symmetry. This is illustrated for Ni(CO) complexes in Fig. 8.15, where the successive aggregates form linear (Coov = 1) slightly bent digonal 2, n = 2), trigonal D h, n = 3), and tetrahedral (T n = 4) symmetry species as complexation proceeds. 

---

B. Divalent Organotin Compounds Containing a Tin-Transition Metal Coordination Bond... 

A self-assembly reaction that involves the connection of individual building blocks via noncovalent interactions permits the rational integration of desired functional groups into the resulting molecules. Transition metal coordination bonds have been exploited in the synthesis of numerous metal-based supramolecular architectures in recent years. Complexation of metal ions to multidentate ligands generates equilibrium mixtures of various structures based on numerous possible combinations of metals and ligands.In the situation of thermodynamic control (see Thermodynamics Laws), the... 

MOMEC is a force field for describing transition metal coordination compounds. It was originally parameterized to use four valence terms, but not an electrostatic term. The metal-ligand interactions consist of a bond-stretch term only. The coordination sphere is maintained by nonbond interactions between ligands. MOMEC generally works reasonably well for octahedrally coordinated compounds. 

A very significant recent development in the field of catalytic hydrogenation has been the discovery that certain transition metal coordination complexes catalyze the hydrogenation of olefinic and acetylenic bonds in homogeneous solution.Of these catalysts tris-(triphenylphosphine)-chloror-hodium (131) has been studied most extensively.The mechanism of the deuteration of olefins with this catalyst is indicated by the following scheme (131 -> 135) ... 

The specific behavior of surface compounds, being the propagation centers of polymerization catalysts, are mainly determined by two of their features the coordinative insufficiency of the transition metal ion and the presence of the transition metal-carbon bond. 

Two possible reasons may be noted by which just the coordinatively insufficient ions of the low oxidation state are necessary to provide the catalytic activity in olefin polymerization. First, the formation of the transition metal-carbon bond in the case of one-component catalysts seems to be realized through the oxidative addition of olefin to the transition metal ion that should possess the ability for a concurrent increase of degree of oxidation and coordination number (177). Second, a strong enough interaction of the monomer with the propagation center resulting in monomer activation is possible by 7r-back-donation of electrons into the antibonding orbitals of olefin that may take place only with the participation of low-valency ions of the transition metal in the formation of intermediate 71-complexes. 

However, these reactions remain hypothetical, and the mechanism of alkylation of low-valent coordinatively insufficient ions during their interaction with hydrocarbons calls for a detailed study. When the activation by some additives is performed the formation of the active transition metal-carbon bond by oxidative addition is also possible, e.g. in the case of such additives as alkylhalogenides or diazocompounds according to the schemes ... 

The processes of reversible adsorption of the coordination" inhibitors (including the adsorption of organometallic compounds) result in an increase in the lifetime of the transition metal-carbon bond. It is possible that due to this, in the case of propylene polymerization by two-component catalysts based on TiCU, at low temperatures a long-term increase of molecular weight with time was observed (192,193). 

The mutual influence of ligands in transition metal coordination compounds with multiple metal-ligand bonds. E. M. Shustorovich, M. A. Porai-Koshits and Y. A. Buslaev, Coord. Chem. Rev., 1975,17,1-98 (345). 

Indium and gallium coordination compounds containing phosphine ligands have recently aroused interest for their widespread application as intermediates in the preparation of the Group 13 - Group 14 semiconductors [4], Since the early reports about compounds with transition metal-indium bonds [51, relatively little research has been reported in this field. However there is a growing interest in the coordination chemistry and structural features of heterometallic indium [6] and gallium complexes [7] which are also attractive as potential precursors of new materials with particular properties. 

Transition metal centered bond activation reactions for obvious reasons require metal complexes ML, with an electron count below 18 ("electronic unsaturation") and with at least one open coordination site. Reactive 16-electron intermediates are often formed in situ by some form of (thermal, photochemical, electrochemical, etc.) ligand dissociation process, allowing a potential substrate to enter the coordination sphere and to become subject to a metal mediated transformation. The term "bond activation" as often here simply refers to an oxidative addition of a C-X bond to the metal atom as displayed for I and 2 in Scheme 1. 

Polymerization occurs by repeated migratory insertion of olefin into the (Tv-oriented metal-carbon bond by the generally accepted Cossee mechanism [5, 60]. This mechanism is believed to be shared by all transition metal coordination polymerization... 

Whereas 3c/4e hypervalent interactions (4.77) tend to be relatively uncommon and fragile in main-group compounds (often leading to transition states for nucleophilic displacement reactions, rather than stable equilibrium species), the corresponding interactions in transition-metal coordination compounds are ubiquitous and robust. The far higher prevalence of hypervalent co-bonding in transition-metal chemistry may be attributed to three major factors. 

In 1937 Jahn and Teller applied group-theoretical methods to derive a remarkable theorem nonlinear molecules in orbitally degenerate states are intrinsically unstable with respect to distortions that lower the symmetry and remove the orbital degeneracy.37 Although Jahn-Teller theory can predict neither the degree of distortion nor the final symmetry, it is widely applied in transition-metal chemistry to rationalize observed distortions from an expected high-symmetry structure.38 In this section we briefly illustrate the application of Jahn-Teller theory and describe how a localized-bond viewpoint can provide a complementary alternative picture of transition-metal coordination geometries. 

The basic assumptions common to most mechanism studies relative to transition metal catalyzed polymerizations are as follows (i) The mechanism is essentially monometallic and the active center is a transition metal-carbon bond.13-15,18,19 (ii) The mechanism is in two stages coordination of the olefin to the catalytic site followed by insertion into the metal-carbon bond through a cis opening of the olefin double bond.13,20,21... 

In all the above reactions the hydrido phosphorus centre acts as a pen-tavalent phosphorus. Interesting reactions are known in which the tautomer of the hydridophosphazene containing a P(III) centre acts as a trivalent phosphine towards transition metals. Thus N3P3Ph4(Me)H on treatment with MCI2 (M = Pd, Pt) [246] or AUCI3 [247] results in complexes [N3P3Ph4(Me)H]2 MC1 (M = Pd or Pt n = 2, M = Au n = 3). These complexes have been shown to possess phosphorus-metal coordinate bonds (Eq. 49). 

Table 5-10 Metal-Ligand Bond Distances in Transition-Metal Coordination Compounds... 

---