Metal atoms with vacant coordination site

Of course, valence electron concentration is not only related to the metal atoms but also to the number and valence of the ligands. Ligand deficiency creates vacant coordination sites at metal atoms and results in cluster condensation, which is the fusion of clusters via short M-M contacts into larger units ranging from zero- to three-dimensional. The chemistry of metal-rich halides of rare earth metals comprises both principles, incorporation of interstitial atoms and cluster condensation, with a vast number of examples [22, 23]. 

The two-coordinate phosphorus atom of a phosphonio-phospholide moiety is generally a weaker Lewis-base than a tertiary phosphine as a consequence, the complexes are thermally less stable (this holds in particular for metal fragments with low back donation capability, e.g. Mn(CO)4Cl) [35, 43], benzophospholide ligands are easily displaced by tertiary phosphines such as PhsP [27, 44], and bidentate ligands comprising a phosphonio-ben-zophospholide and a phosphine site react with transition metal fragments preferentially at the phosphine site coordination of both phosphorus atoms is only observed if the substrate offers two vacant coordination sites [27, 47]. 

In the process of olefin insertion, also known as carbometalation, the 1,2 migratory insertion of the coordinated carbon-carbon multiple bond into the metal-carbon bond results in the formation of a metal-alkyl or metal-alkenyl complex. The reaction, in which the bond order of the inserted C-C bond is decreased by one unit, proceeds stereoselectively ( -addition) and usually also regioselectively (the more bulky metal is preferentially attached to the less substituted carbon atom. The willingness of alkenes and alkynes to undergo carbometalation is usually in correlation with the ease of their coordination to the metal centre. In the process of insertion a vacant coordination site is also produced on the metal, where further reagents might be attached. Of the metals covered in this book palladium is by far the most frequently utilized in such transformations. 

Dinuclear elimination processes are possible only when at least one of the ligands to be eliminated is a hydride. This is supported by our observations (dinuclear elimination from Os(CO)4(H)CH3 ana Os(CO)4H2 but not from Os(CO)4(CH3)2) and is explained reasonably by the unique ability of a hydride to bridge a pair of transition metal atoms. Tne interaction of Os-H with a vacant coordination site on another Os to form a dinuclear species appears to be an essential part of the dinuclear elimination process. 

With regard to the valence electron count, this number determines whether the transition metal ion is using its full complement of valence shell orbitals— i.e., the five nd s, the (n + l)s, and the three (n + l)p s. If the valence electron count is eighteen, all of the orbitals are fully utilized in bond formation and electron pair storage, the effective atomic number (EAN) rule is fulfilled and the metal ion is said to be saturated. If it is seventeen, the metal ion is covalently unsaturated, and if it is sixteen or less, the metal ion possesses at least one vacant coordination site and is said to be coordinatively unsaturated. The importance of the valence electron count in homogeneously catalyzed reactions has been discussed by Tolman (7). 

The occurrence of this reaction has been deduced [81] from the effects of the concentration of transition metal compound on polymer molecular weight, and the mechanism is not understood. It is conceivable that the chains are transferred to transition metal atoms without vacant sites for coordination of monomer and which do not then propagate or that the chain is eliminated with deactivation but exposes a fresh transition metal atom for alkylation and subsequent propagation. 

Rhodium(Il) acetate (Structure 3), a dinuclear molecule of D4 symmetry and vacant coordination sites (—>) at each metal atom, is the most commonly employed catalyst for this reaction. Copper catalysts are no longer used because they are inferior in terms of both activity and selectivity. The diazoalkane constitution in eq. (1) includes compounds with... 

In summary, Py, Q, and their alkylated derivative.s tend to bind through the nitrogen atom -T] (N)- to electrophilic metal centers with a single vacant coordination site. If three coordination sites are available and the metal center has filled dn orbitals, the preferred bonding mode is the T]. In the intermediate situation offering two free coordination sites, the electronic nature of the metal ion determines whether the substrate binds to the metal using a C=C bond or the C=N bond. 

The most important reactions in catal5nic cycles are those involving ligands located in the coordination sphere of the same metal center. The molecular transformations generally require a loose coordination of the reactants to the central atom and facile release of the products from the coordination sphere. Both processes must proceed with an activation energy that is as low as possible, and thus extremely labile metal complexes are required. Such complexes have a vacant coordination site or at least one weakly bound hgand. 

The reverse of reaction 24.42 is a 13-elimination step. It involves the transfer of a P-H atom (structure 24.47) from the alkyl group to the metal and the conversion of the a-alkyl group to a 7r-bonded alkene, i.e. a C-H bond is activated. For P-elimination to occur, the metal centre must be unsaturated, with a vacant coordination site cis to the alkyl group (equation 24.43). The first step is thought to involve a cyclic intermediate 24.48 with an agostic M—H—C interaction. 

The existence of the coordinatively unsaturated cluster 3 leads one to consider whether the synthesis of clusters with still larger vacant coordination sites might be possible. 8ince these vacant sites arise as a direct consequence of the steric screening of the metal atoms by peripheral ligands, several possible synthetic routes immediately suggest themselves ... 

Atom abstraction reactions have been discussed in Chapter 4 where it was noted that the excited state is a diradical that can abstract an atom from another molecule. For such reactions to be efficient, it is advantageous for the reactant molecule to be closely associated with a vacant coordination site of the metal complex. An idealized cartoon of such a process is... 

---