Transition elements complexes coordination

The X-ray crystal structures of many of these complexes have now been determined representative examples are. shown in Fig. 4.11 from which it is clear that, at least for the larger cations, coordinative saturation and bond rhrectionality are far less significant factors than in many transition element complexes. Further interest in these ligands stems from their use in biochemical modelling since they sometimes mimic the behaviour of naturally occurring, neutral, macrocydic antibiotics such as valinomycin, monactin, nonactin, nigericin... 

Octahedral six-coordination is the most common chemical environment for the transition elements (see Coordination Numbers Geometries). Apart from the great number of octahedral complexes, the formally ionic oxides and halides usually adopt stmctmes in which the cation is in an octahedral... 

When considering the structures of coordination compounds it is worth noting that transition element complexes are usually formed from reactions between their salts and Bronsted bases in solution. However, the structures of the compounds formed are usually determined in the solid state using samples crystallized from solution. While it may usually be assumed that the solid state structures are similar to the solution structures, this may not always be so. and some complexes may adopt different structures in solution and the solid state. 

The force constant analysis of 1 indicates that the H2 ligand here is further activated toward OA than may have been previously thought. It has been a paradox that the dHH in 1 or any of the group 6 complexes in Table 8.1 (0.85-0.89 A, solid state NMR) are not as stretched as some of those found in later transition element complexes (1.0-1.5 A), yet the H-H bond in 1 undergoes equilibrium cleavage in solution. Thus the observed dm may not always reflect the degree of readiness to break, i.e., a very late transition state may exist. At the other end of the spectrum, for W(CO)5(H2) and other complexes with extremely weakly bound H2, the T-shaped entity pictured above with one internal coordinate, the H-H stretch, may be a more appropriate model for vibrational analysis. However, it is difficult to determine which analysis should be applied because some minimal BD and incipient... 

Luminescence has been observed from a large number of later transition element complexes and a rich array of excited states have been observed. Related sections from CCC (1987) include Chapter 16.5 on Pt, Rh, and Ir complexes, 36.3 on Mo halide clusters, 43 on Re complexes, 45.4 on Ru polypyridyls, 46.4 on Os polypyridyls, 48.6 on Rh complexes, and sections of Chapter 52 on Pt complexes. Several recent reviews have been published on polynuclear complexes, the photophysics of gold complexes,and platinum diimine complexes. Many other more narrowly focused review articles have been published on transition metal complex luminescence a significant number are published in the journal Coordination Chemistry Reviews and some of these reviews are cited in this chapter. 

Transition metal complex Coordination compound containing the d- and f-block elements. 

Carbene is a liable intermediate in organic reactions however, if it coordinates to a transition element, the coordination bond is stable and it is able to be isolated as a complex. Singlet carbene ( CX2) forms an sp hybrid orbital, has six electrons with two p-orbitals, and a third orbital remains as vacant (p orbital). When the carbene forms a complex, lone pair electrons of one of the sp hybrid orbitals a-donates to the metal atom and 7T-electrons of the metal back-donate to the vacant orbital p, of the carbon atom (a-donation rr-back donation). Therefore, this MC bond should thus exhibit partial double bond character. However, as the p, vacant orbital which is used for back donation didn t have enough acceptor orbitals available for the back-donation process, the double bond character is thought to be small [53]. 

Many transition elements form coordination compounds, which consist of a complex ion and counter ions. A complex ion has a central metal ion and surrounding molecular or anionic ligands. The number of ligands bonded to the metal ion determines the shape of the complex ion. Different positions and bonding arrangements of ligands lead to various types of isomerism. (Section 22.2)... 

Many metal ions, especially those of the transition elements, form coordinate covalent bonds with molecules or anions having lone pairs of electrons. For example, the silver ion, Ag, can react with NH3 to form the Ag(NH3)2 ion. The lone pair of electrons on the N atom of NH3 forms a coordinate covalent bond to the silver ion to form what is called a complex ion. (See Figure 18.7.)... 

Some of the oxidation states given above, especially the higher oxidation states (7, 6) and oxidation state 0, are found only when the metal atom or ion has attached to it certain groups or ligands. Indeed the chemistry of the transition elements is so dominated by their tendency to form coordination complexes that this aspect of their behaviour must be considered in some detail. 

This is the most common coordination number for complexes of transition elements. It can be seen by inspection that, for compounds of the type (Ma4b2), the three symmetrical structures (Fig. 19.6) can give rise to 3, 3 and 2 isomers respectively. Exactly the same is true for compounds of the type [Mayby]. In order to determine the stereochemistry of 6-coordinate complexes very many examples of such compounds were prepared, particularly with M = Cr and Co , and in no case was more than 2 isomers found. This, of course, was only negative evidence for the octahedral structure, though the... 

As with other transition elements, the lanthanides can be induced to form complexes with exceptionally low coordination numbers by use of the very bulky ligand, N(SiMe3)2 ... 

Borides, in contrast to carbides and nitrides, are characterized by an unusual structural complexity for both metal-rich and B-rich compositions. This complexity has its origin in the tendency of B atoms to form one- two-, or three-dimensional covalent arrangements and to show uncommon coordination numbers because of their large size (rg = 0.88 10 pm) and their electronic structure (deficiency in valence electrons). The structures of the transition-element borides are well established " . 

With the exception of a brief report of a dimethylaluminum complex [5], the coordination chemistry of the monomeric anion in (4) has not been investigated. By contrast, Stahl and co-workers have carried out extensive studies of both main group element and transition-metal complexes of the chelating dianion in the cube (7), which have been summarized in a recent review [9]. A noteworthy feature of the ligand behaviour of this N,N chelating dianion is the additional in-... 

Organometallic porphyrin complexes containing the late transition elements (from the nickel, copper, or zinc triads) are exceedingly few. In all of the known examples, either the porphyrin has been modified in some way or the metal is coordinated to fewer than four of the pyrrole nitrogens. For nickel, copper, and zinc the 4-2 oxidation state predominates, and the simple M"(Por) complexes are stable and resist oxidation or modification, thus on valence grounds alone it is easy to understand why there are few organometallic examples. The exceptions, which exist for nickel, palladium, and possibly zinc, are outlined below. Little evidence has been reported for stable organometallic porphyrin complexes of the other late transision elements. 

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