Coordination entities

If the coordination entity is negatively charged, the cations paired with the complex anion (with -ate ending) are listed first. If the entity is positively charged, the anions paired with the complex cation are listed immediately afterward. 

Nitric oxide coordinated to iron modifies, in a striking manner, the properties and reactivity of free NO (Sec. 6.2). Probably the most famous such coordinated entity is the nitroprusside ion, Fe(CN)5NO . An incisive review of its reactions particularly related to its hypertensive action (it reduces blood pressure of severely hypertensive patients) is available. Nitroprusside ion reacts with a variety of bases... 

Pyrrole units form part of the coordinating entities of Schiff base ligands derived from pyrrole 2-carboxaldehyde, for example in the iron(II) complex of the ligand derived from pyrrole 2-carboxaldehyde and trien, which is low-spin despite the feeble coordinating properties of the pyrrole—CH=N— units. The synthesis, structure, and spectroscopic and electrochemical properties of tris-ligand iron(III) complexes of phenyldipyrromethenate (dipyrrin, (129)), and its... 

The replacement of one or more water ligands within a coordination complex (or coordination entity) by an anion. 

The centrally located atom in a coordination entity to which other atoms or group of atoms (generally referred to as ligands) binds. 

The number of central atoms joined in a single coordination entity or cluster by bridging ligands or by metal-metal bonds. Such complexes are referred to as being dinuclear, trinuclear, tetranuclear, polynuclear, etc. 

We have already mentioned the formulae for groups, such as S04 , without discussing the principles by which such formulae are assembled. These may (or may not) involve some reference to structure. The general approach is to select one or more atom(s) as the central or characteristic atom(s). This is so whether the ion or group is a coordination entity or not. Thus, I in ICl4 , V in V02 and Si and W in [SiW,204o] are all central atoms and are cited first. The subsidiary atoms then follow, in alphabetical order of symbols (this rule is slightly modified for coordination compounds). 

This last proviso is exemplified as follows. Ethylenediaminetetraacetic acid should be rendered H4edta. The ions derived from it, which are often ligands in coordination entities, are then (H3edta) , (H2edta) , (Hedta) " and (edta) ". This avoids monstrosities such as edta-H2 and edtaH 2 which arise if the parent acid is represented as edta. A list of recommended abbreviations is presented in Table 4.5. 

Oxidutlon number. Tlie oxidation number of a central atom in a coordination entity is defined as the charge K would bear if all the ligands were removed along with the electron pairs that were shared with the central atom. It is represented by a roman numeral. 

Terminations for names of coordilation entities. All anionic coordination entities take the ending -ate. whereas no distinguishing termination is used for cationic or neutral coordination entities. 

Alteraatively. the charge on a coordination entity may be indicated. The net charge is written m artibic numbers on the line, with the number preceding the charge sign, and enclosed in parentheses. It follows the name of the central atom without the intervention of a space.311... 

Coordination compounds. [In the formula of a coordination entity.] the symbol of the central atom(s) is placed first, followed by the ionic and then the neutru] ligands. Square brackets are used to enclose the whole coordination entity whether charged or not. This practice need not be used for simple species such us the common oxoanions (NO, NOT, SO -, OH-, etc.). Enclosing marks are nested within the square brackets as follows [()]. U<) , [()]>]. [( [()] ]]. etc. 

Coordination entity. A coordination entity is composed of a central atom, usually that of a metal, to which is attached a surrounding array of other atoms or groups of atoms, each of which is called a ligand. In formulae, (he coordination entity is enclosed in square brackets whether it is charged or uncharged. 

Central atom. The central atom is the atom in a coordination entity winch binds other atoms or groups of atoms (ligands) to itself, thereby occupying a central position in the coordination entity. The central atoms in [NiCL(H20)J, [Co(NH,)J3+. and [PlCTJ2- are nickel, cobalt, and platinum, respectively. 

Uses of cnclii,sinj> murks. The formula for the entire coordination enlily. whether charged or not. is enclosed in square brackets. When ligands are polyatomic, their formulae are enclosed in parentheses. Ligand abbreviations are also enclosed in parentheses. In the special case of coordination entities, the nesting order of enclosures is as given [on page A-5I. There should be no space between representations of ionic species within a coordination formula. 

Ionic charges and oxidation numbers. If the formula of a charged coordination entity is to be written without lhat of the counterion, the churge is indicated outside the square bracket as a right superscript, wiih the number before the sign. The oxidation number of a central atom may be represented by a roman numeral used as a right superscript on the element symbol. 

Attempts to produce descriptors similar to cis and trims for stereochemicidly more complicated coordination entities have tailed to achieve generality, and labels such as foe and mer are no longer recommended. Nevertheless, a diastereoisomeric structure may be indicated for any polyhedron using a configuration index as an affix to the name or formula. Finally, the chiralities of enantiomeric structures can be indicated using chirality symbols. 

A coordination entity results when a metal ion or atom accepts a share in an electron pair from some donor atom. In this sense, then, any... 

In its oldest sense, the term coordination entity generally refers to molecules or ions in which there is an atom (A) to which are attached other atoms (B) or groups (C) to a number in excess of that corresponding to the classical or stoichiometric valency (23). 

For coordination entities and the ocean, we should consider (a) the metal, whether a transition metal or not, the charge, and the size (b) the ligands, whether uni- or multidentate, perhaps whether organic or inorganic (c) the coordination number—i.e., the number of donor atoms attached to the metal. The metal and the ligands surrounding it comprise the coordination sphere, which is represented by the bracket-enclosed formula. 

More Metals. The apparent simplicity of the problem is misleading because although the concentration of transition metal ions is small, the ocean assuredly contains trace quantities of all naturally occurring metals. We now recognize two results of coordination the properties of the metal are altered, and, equally important, the properties of the ligand are altered (coordinated ammonia is less basic, cyanide ion is less toxic) (5). Most of the catalytic activity of coordination entities recently summarized involved coordination entities of transition metal ions examples involving magnesium ion constitute the main exceptions (24). 

Labile Coordination Entities. Commonly, metal complexes are classified as to stability vs. instability or lability vs. inertness (33, 41). Solution stability is a thermodynamic property and describes an equilibrium of the type shown in Equation 1. 

Because the stability constant is the ratio of the specific rate constants for the forward and reverse reactions, respectively, both rate constants could be large, and the ratio could also be large. In short, a coordination entity might have a high solution stability and yet be very labile. 

For our purposes, the prominent coordination entities must involve the derivatives of the populous metal ions listed in Table I. All are labile, most are probably stable, though more information is needed for coordination entities involving phosphate and carbonate ions. 

First, one would not suggest that coordination compounds would be successful in controlling Red Tide outbreaks in the sense that ligands or coordination entities might be added to stop or prevent outbreaks. Consider the cost of adding an inexpensive (20 cents/lb.) but reasonably lethal (0.125 X 10"6 gram/liter) coordination compound to an extremely small outbreak area (perhaps 30 acres and a 10-meter depth). (Marvin and co-workers (cf., 39) used as a criterion of acute toxicity—100% mortality within a 24-hour period at a concentration level of 0.01 p.p.m.)... 

Studies of the stereochemical features in the reactions of metal ions with optically active ligands have aroused great interest during the last years but are still far from being complete. A systematic discussion is not yet possible. Despite this, the known examples furnish sufficient indications as to the stereochemical requirements of ligands with a definite geometry in the coordination entity. The parameters which have to be considered in this context are ... 

---