Werner Centennial

15

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on August 17, 2015 | http://pubs.acs.org Publication Date: January 1, 1967 | doi: 10.1021/ba-1967-0062.ch015

Coordination Number, Electronic Configuration, and Ionic Charge as Discrete Variables in Coordination Chemistry R. B. K I N G Mellon Institute, Pittsburgh, Pa.

1

A formal system for organizing coordination chemistry is developed centering around the recognition of coordina tion number, electronic configuration, and ionic charge as discrete, independent, and non-transmutative variables. Using these and other discrete variables, matrices may be developed which permit the orderly listing of coordination compounds of a given metal or type. Furthermore, changes in the values of these discrete variables during chemical reactions of coordination compounds may be used to classify such reactions into seven types: a) Isoelectric exchange, b) Positive-moving exchange, c) Negative-moving exchange, d) Reduction, e) Oxidation, f) Oxidative addi tion, and g) Reductive elimination.

C i n c e 1958 the author has been actively engaged i n synthesizing novel transition-metal organometallic compounds. Possible new types of compounds, as well as the relationship between known types of com pounds, have been considered. I t became increasingly apparent that the principles governing properties of coordination compounds, although individually understood, had not yet been clearly organized. Originally, this paper intended to summarize and relate the factors influencing the chemistry of cyclopentadienylmetal carbonyls, one of the principal sub jects of the author's researches. However, a formal structure to the chem istry of coordination compounds had to be developed before i t could be properly used to account for the chemistry of the cyclopentadienylmetal carbonyls or any other special class of compounds. This paper thus develops such a formal structure. 1

Present address: University of Georgia, Athens, Ga. 203 In Werner Centennial; Kauffman, G.; Advances in Chemistry; American Chemical Society: Washington, DC, 1967.

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WERNER


Discrete and Continuous

CENTENNIAL

Variables

Discrete variables refer to the numbers of items of types which can not be divided without undergoing qualitative as well as quantitative change. Thus, a metal atom i n a coordination compound can only be surrounded b y an integral number of ligands. The coordination number (designated as C) is therefore a discrete variable. Furthermore, the total number of electrons arising from the central metal atom and the coordi nated ligands must also be an integer. The electronic configuration (designated as E) is thus also a discrete variable. Finally, a coordination complex can either be neutral, anionic, or cationic. If i t is anionic, an integral number of cations must be present to preserve neutrality of the compound. Similarly, if i t is cationic, an integral number of anions must be present to preserve neutrality of the compound. Thus, ionic charge (designated as Q) is also a discrete variable. B y contrast, other variables such as charge distribution and bond strength are continuous variables because they can assume any value within given limits. The following basic principle underlies the formal structure developed in this paper: A l l coordination compounds are too complex for exact mathematical computations of their properties. E v e n calculations with extensive simpli fying approximations can be too complex. Thus, the values of continuous variables are always uncertain. However, by contrast, the values of discrete variables can be found exactly because each discrete variable may have only a limited number of separated specific values. If coordination compounds of a given metal in an n-dimensional matrix are arranged where each of the n dimensions represents the value of one of n independent discrete variables, the effects of the continuous variables, as well as of the discrete variables, can be understood more readily because our studies can focus on a specific region of this matrix. The following terms are used i n considering the details of this matrix. Definitions Types of Ligands. Only ligands bonded to a metal b y means of only one electron-pair bond (i.e., monodentate) are considered i n this paper. Ligands bonded to a metal b y means of more than one electron-pair bond may be divided into two types: a) LOCALIZED C H E L A T I N G LIGANDS.

A n n-dentate chelating ligand

has non-adjacent atoms which are bonded to the central metal atom. F o r analysis here, an n-dentate chelating ligand is considered as n different monodentate ligands occupying a total of n coordination positions. b) DELOCALIZED 7T-COMPLEXING LIGANDS.

These ligands have more

than two adjacent atoms donating more than one electron pair to the cen tral atom, generally by delocalized t bonding. The 7r-cyclopentadienyl ligand is the most common example. For simplicity, this paper does not discuss i n detail complexes with these ligands.

In Werner Centennial; Kauffman, G.; Advances in Chemistry; American Chemical Society: Washington, DC, 1967.

15.

Variables in

KING

205

Coordination

The calculation of a) electrons donated by a ligand, and b) the oxida tion state of the central metal atom are defined below.


a)

N U M B E R OF ELECTRONS D O N A T E D BY A M O N O D E N T A T E LIGAND.

Consider the neutral two-electron M - L bond as being formed by neutral M (metal) and L (ligand) fragments. If both electrons arise from the ligand, the ligand is a two-electron donor. If both electrons arise from the metal, the ligand is a zero-electron donor. If one electron arises from the metal and one from the ligand, the ligand is a one-electron donor. Further examples are discussed below. b)

OXIDATION STATE OP T H E C E N T R A L M E T A L

ATOM.

With

the

central metal atom i n an oxidation state with a) value of zero, consider the various two-electron bonds between the central metal atom and the ligands surrounding it as being formed from neutral metal and ligand fragments. For each electron in these bonds arising from the neutral metal atom, add + 1 to the oxidation state. For the oxidation state thus found add (algebraically) the ionic charge of the coordination complex. W i t h these definitions, it is possible to classify all monodentate ligands into one of the following five types: a)

T H R E E - E L E C T R O N DONOR (DESIGNATED AS T ) .

I n forming a bond

with the metal atom, the neutral ligand T first transfers one electron com pletely to the metal atom forming the cation T+. This cation T then acts as a two-electron donor to form the M - T bond. Thus, the neutral ligand T donates a total of three electrons to the metal atom and contrib utes — 1 to its oxidation state. The only two examples of "localized" monodentate three-electron donor ligands are the nitrosyl and arylazo ligands. Characteristic of both of these ligands is the stability of the species T . +

+

b)

T W O - E L E C T R O N DONOR (LEWIS B A S E ) ) DESIGNATED AS L ) .

In

forming the M - L bond, the ligand L contributes both electrons and zero to the oxidation state of the metal atom. Lewis bases, including trialkylamines, tertiary phosphines, carbon monoxide, alkylisocyanides, and dialkylsulfides, fall into this very common category. c)

ONE-ELECTRON

DONOR (DESIGNATED AS X ) .

In

forming

the

M - X bond, the ligand X contributes one electron and + 1 to the oxidation state of the metal atom. Halides, pseudohalides, and alkyl groups fall into this very common category. d)

Z E R O - E L E C T R O N DONOR (LEWIS ACID) (DESIGNATED AS Z ) .

The

ligand Z contributes neither electron to the two electron M - Z bond but + 2 to the oxidation state of the central metal atom. Lewis acids, es pecially derivatives of tricovalent boron, such as B H and the oxide ion 0~ , fall into this category. 3

2

e)

O N E - E L E C T R O N ACCEPTOR (DESIGNATED AS N ) .

The nitride ligand

as present i n [Os0 N]~ and related compounds is the sole representative 3



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WERNER

CENTENNIAL

of this category. This ligand accepts one electron from the metal atom and contributes neither electron to the two electron M - N bond but adds + 3 to the oxidation state of the central metal atom. M o s t of the ligands i n the complexes under consideration will be either two-electron donors L or one-electron donors X . The ability to classify all ligands into these very few types simplifies the fitting of known coordination compounds of a given metal into a very simple matrix. Types of Discrete Variables. Discrete variables can be either transmutative or non-transmutative. Changing of a transmutative discrete variable, while holding all others constant, changes (transmutes) the central metal atom. However, changing of a non-transmutative discrete variable while holding all others constant does not change the central metal atom. The most important transmutative discrete variable is the atomic number of the central metal atom, designated as A . The three, non-transmutative variables of importance i n this paper are coordination number (C), elec tronic configuration (E), and ionic charge (Q). The three independent, non-transmutative variables are defined as follows: a) COORDINATION N U M B E R (C).

The variable C corresponds to the

number of monodentate ligands surrounding the central metal atom. Hexacoordinate complexes (C = 6) are often favored. b) ELECTRONIC CONFIGURATION (E).

The variable E is the sum of

the outer metal nonbonding electrons and the electron pairs of the metalligand bonds. F o r compounds with the favored "rare gas" electronic configuration, E is 18. For square planar complexes of the late transition metals, the electronic configuration E is 16. c) IONIC C H A R G E (Q).

The variable Q is the net charge on the co

ordination complex. The variable "oxidation state" (designated as 0) is also a discrete variable, but i t is a dependent one rather than an independent one because 0 depends on C (coordination number), A (number of outer electrons of the neutral free central metal atom which is closely related to its atomic number), and E (electronic configuration as defined above) as follows: 0 = 2C + A -

E.

Thus, the oxidation state is defined exactly once A , C, and E are specified. The

CEQ-Matrix

Consider the compounds of a given metal such as rhenium. Then consider a three-dimensional matrix with the three independent, discrete variables: electronic configuration (E), coordination number (C), and ionic charge (Q) corresponding to the three dimensions. The known rhenium complexes can be placed i n this matrix i n the appropriate cells corre-


207

Variables in Coordination


Coordination Number, C

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sponding to the values of the variables C, E, and Q. I n order to cover a large number of rhenium complexes with a given number of entries the generalized ligand designations T , L , X , and Z as defined previously are used. Table I shows a simplified version of such a CEQ matrix for rhenium. In order to obtain a more convenient two-dimensional table, the axis corre sponding to the ionic charge Q has been eliminated. Instead, i n each box corresponding to definite values of C and E, the extremes of different values of Q are given. For example, in the box E = 18, C = 6, the two species [Re Le] (Q = +1) and [Re X ]~ (Q = —5) are shown. In addi tion, each box contains specific examples of the species illustrated by the general formulas. Hexacoordinate species (C = 6) with the rare gas configuration (E = 18) are particularly favored. Therefore, in the CE/Q-matrix the row corresponding to C = 6 and the column corresponding to E = 18 are out lined i n double lines. In the rhenium CEJQ-matrix (Table I), as well as the other Cj&Q-matrices given i n this paper (Tables II to V I I ) , the complex types are concentrated along the C = 6 row and the E = 18 column. To further illustrate CEQ matrices, those for the six first-row transition metals vanadium through nickel, inclusive, are given in Tables I I to V I I . T

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Types of Reactions Depending upon the effects on the discrete variables C, E, and Q, reactions of coordination complexes may be of seven basic types falling into four main categories. A) Isoelectric Exchange. In this type of reaction, the values of all three discrete variables C, E, and Q are the same i n both the starting mate rial and the product. This is thus the case where a ligand is replaced by another ligand with the same charge. A n example is the reaction of hexacarbonylmolybdenum with triphenylphosphine to give (C H6)3PMo(CO)5, i.e., 6

Mo(CO) + (C H ) P -> (C H )3PMo(CO) + CO. 6

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B ) Positive-moving Exchange / Negative-moving Exchange. I n these types of reactions, the values of C and E are the same i n both the starting material and the product, but the value of Q is different i n the product than in the starting material. Depending on the direction of change of Q, two types of reactions are possible. 1. POSITIVE-MOVING E X C H A N G E .

Q is greater algebraically i n the

product than i n the starting material. A n example of this rather rare type is the reaction of ferrocyanide ion (Q = —4) with carbon monoxide to give the [ F e ( C N ) C O ] - ion (Q = - 3 ) , i.e., 3

6

[Fe(CN) ]-* + CO - » [Fe(CN) CO]- + C N " . 6

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Werner Centennial

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