Coordination compounds individual metals

The chloride ions that appear outside the brackets represent chloride anions that balance the positive charge on the coordination compound. When a coordination compound dissolves in water, the ligands (inside the brackets) remain bound to the metal cation, but the nonligands (outside the brackets) exist as individual ions. These chloride ions precipitate in the presence of silver ions. The chloride ions inside the brackets, which are ligands bonded to the cobalt center, do not precipitate as AgCl. 

Although not all facets of the reactions in which complexes function as catalysts are fully understood, some of the processes are formulated in terms of a sequence of steps that represent well-known reactions. The actual process may not be identical with the collection of proposed steps, but the steps represent chemistry that is well understood. It is interesting to note that developing kinetic models for reactions of substances that are adsorbed on the surface of a solid catalyst leads to rate laws that have exactly the same form as those that describe reactions of substrates bound to enzymes. In a very general way, some of the catalytic processes involving coordination compounds require the reactant(s) to be bound to the metal by coordinate bonds, so there is some similarity in kinetic behavior of all of these processes. Before the catalytic processes are considered, we will describe some of the types of reactions that constitute the individual steps of the reaction sequences. 

An ideal model compound should correspond to the biological metal centre in terms of structure, composition as well as coordination and oxidation states of the individual metal ions, and also possess comparable spectroscopic and chemical properties. However, real model compounds rarely meet all these requirements at once. Usually, only special aspects of a metal centre are modelled, such as the structure, magnetic or electronic properties (spectroscopy), or the reactivity (function), and the... 

The sulfide group forms a large number of complexes where it is in chelation with a different heteroatom. Among the common heteroatoms are N, P and As. These complexes are too numerous to list here, but individual complexes can be found from Table 9 or from refs. 1224 and 1667. It is also possible to synthesize compounds which will form bi-, tri- and tetra-dentate complexes to platinum(II), where sulfur, selenium and tellurium.are the only atoms which coordinate to the metal. A review of complexes formed from ligands of the type RS(CH2) SR has been recently published.1734 This article outlines the synthesis, reactions and spectroscopy of these complexes, and allows the complexes of platinum to be placed in context with those of other transition metals. 

The separation of coordination compounds into four types, as described above (a) encompasses practically all types of complexes reported in modern coordination chemistry [1,2,10,16,34,124-127] (b) is clear and informative with respect to a given molecule in a coordination compound as a whole, and does not rely on its individual portions (c) concentrates its attention on the special features of some types of metal complexes (d) follows from the modern approach to study the structures of metal complexes [16,34,125,126], together with their systematic nomenclature [122,123]. 

Ligands coordinated to transition metal ions may also be arranged in order of increasing A. This order is called the spectrochemical series, reflecting colour variations in chemical compounds of individual cations with different ligands. Thus, for Cr3+ and Co3+ cations in octahedral coordination with different ligands, the order of increasing A0 is... 

Recently, the matrix co-condensation technique has been used to synthesize a number of unstable and transient coordination compounds. For example, a series of nickel carbonyls of the type Ni(CO), where x= 1, 2, 3, and 4, have been synthesized by allowing metal vapor to react with CO diluted in Ar on a cold window and warming the matrix carefully. Figure 1-21 shows the result obtained by DeKock. The structures of Ni(CO)2 and Ni(CO)3 were concluded to be linear and trigonal-planar, respectively, since these compounds exhibit only one CO stretching band in the infrared. Similar methods have been applied to the synthesis of a number of coordination compounds ML , where M is Pt, Pd, Ni, and so on, and L is CO, Nj, O2, PFj, and so on. More detailed discussions of individual compounds will be given in Part 111. ... 

Coordination compounds or complexes consist of one or more central atoms or central ions, usually metals, with a number of ions or molecules, called ligands, surrounding them and attached to them. The complex can be nonionic, cationic, or anionic, depending upon the charges of the central ions and the ligands. Usually, the central ions and ligands can exist individually as well as combined in complexes. The total possible number of attachments to a central atom or central ion or the total possible... 

A widespread regularity among the shorter M-X distances in virtually all reduced compounds of these elements, metallic or clustered or not, warrants a brief comment. This property really arises from the absence of localized, reduced states on individual metal atoms in these (and many other) phases. The too-familiar behavior of the 3d metals in simple compounds is otherwise six-coordinate radii increments of 0.14 to 0.19 A accompany oxidation state changes of H-3 - H-2 (high spin) and 0.06 to 0.11 A with -h4 H-3 reductions (Shannon, 1976). No such differences appear on reduction of normal-valent compounds in many other situations basically the differentiating (reduction) electrons in the latter are delocalized over a collection of metal cores and do not screen the metal - nonmetal interactions, but rather are in effect repelled by the latter. Accordingly, the shorter metal-nonmetal separations in all of these compounds can be well represented by the sum of crystal radii for the normal-valent components, Nb, Zr, Y ", etc. In other words, no distance increment is observed to accompany the onset of metallicity in a series like SrS, YS, ZrS. Some examples are collected in Table 3 to... 

The thermodynamic stabilities of coordination compounds are typically measured using stability or formation constants, as shown in Equations(l5.l)-(l5.4) for Cu(NH3)4+. The tetraaquacopper(ll) cation is used as the starting material in Equation (15.1) because the hydration enthalpy Is so negative that most metal ions cannot exist as naked cations in aqueous solution. It is not always possible for the stepwise constants to be measured individually, so typically only the overall formation constant is reported, where n is the number of ligands attached to the metal ion. If the stepwise stability constants do happen to be known, then the overall constant can be determined from the product of each individual formation constant. The stepwise formation constants for coordination compounds usually decrease in magnitude as the value of n increases. This is an entropic effect that has to do with the number of available substitutions. Thus, for example, addition of NH3 to [Cu(H20)4] " in Equation (15.1) has four possible positions available for substitution, whereas addition of NH3 to [Cu(NH3)3(H20)] in Equation (15.4) has only one possible position available for substitution. 

The electron configurations for the transition metals discussed here and in Appendix B are for individual metal atoms in the gas phase. Most chemists work with the transition metals either in the metallic state or as coordination compounds (see Chapter 25). A solid transition metal has a band structure of overlapping d and s orbital levels (see Section 13-7). When transition metal atoms have other types of atoms or molecules bonded to them, however, the electronic configuration usually becomes simpler in that the d orbitals fill first, followed by the next higher s orbital. This is illustrated by Cr, which has a 4s 3d electronic configuration as a free atom in the gas phase. But in the compound Cr(CO)5, chromium hexacarbonyl, which contains a central Cr atom surrounded by six neutral carbon monoxide (or carbonyl) groups, the chromium atom has a 3d electronic configuration. 

The precious metals form a variety of salts, coordination complexes, and organometallic compounds. The metals are discussed individually to gain some understanding of how the compounds are formed and what ligands will stabilize a particular oxidation state. 

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