Square Planar Complexes of the First Row Transition Metals

31 Square Planar Complexes of the First Row Transition Metals W. K E N N E T H M U S K E R

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Department of Chemistry, University of California, Davis. Calif. Numerous complexing agents are available for preparing planar complexes of the first row transition metals. Por phins, phthalocyanines, quinolines, and pyridines form planar complexes with certain metal ions under con trolled conditions. Appropriate sulfur-containing ligands can also be used, and the resulting complexes often can be further oxidized or reduced. Planar, octahedral, or tetra hedral complexes of the salicylaldimines, β-ketoimines, and related compounds can be prepared by choosing the appropriate ligand substitution. In β-ketoaminate and tertiary phosphine complexes an equilibrium between planar and tetrahedral forms can be observed. Com plexing agents without extensive πsystems are capable of holding the metal ion in a planar configuration if certain steric requirements are fulfilled. Carbon substituted ethylenediamines and several cyclic ring systems have been used successfully.

T ' phe

origin of inorganic complexes possessing the planar configuration is generally placed at the preparation of the a- and 0- forms of dichlorodiammineplatinum(II) by Peyrone (76) and Reiset (78). However, it was not until 50 years later that the structures of these compounds were adequately explained by Werner (96) as the cis- and trans- isomers of square planar platinum (Figure 1). Werner did not employ the expression "square planar," but his drawings are those of the spatial orientation which is now implied by that expression. "Square coordination," as a term, came into usage with Pauling's (78, 74, 75) development of the directed valence bond. Historically, the study of the square planar configuration has de veloped primarily through the preparation and property studies of platinum 469 In Werner Centennial; Kauffman, G.; Advances in Chemistry; American Chemical Society: Washington, DC, 1967.

470

WERNER CENTENNIAL

H

3

N

N

Pt

Pt

H N

CI

3


Figure

NH3

1. Planar platinum(II) complexes

and palladium compounds. Extensive reviews (5, 7, 64, 77) with this emphasis are available; it is the almost tdtally neglected area of first row transition elements having square planar geometry which will be summarized here. Experimental evidence supporting the square planar structure of first row transition metal ions was presented in 1931: [ N i ( C N ) ] " complexes were found to be diamagnetic, and K N i ( C N ) H 0 and K P d ( C N ) • H 0 were found to be isomorphous (78, 74). Pauling's prediction (75) that bivalent nickel complexes could be either square planar (dsp hybridization) or tetrahedral (sp hybridization) stimulated interest in the square con4

2

4

2

2

2

4

2

2

z

3d

® (ID ® (S>

Ni * +

square planar Ni+

dD dD dD (D CD

2

4p

4s

O O O

O O

O

O O O

tetrahedral figuration which, with the magnetic measurement indicating bond type, resulted in the discovery of many square planar complexes. Pauling (75) also considered the problem of configuration from the standpoint of properties of the complexing agent which stabilize the square planar configuration. In general, ligands containing an atom with a strong tendency to form covalent bonds favor the square planar geometry. Pauling (75) thus enumerated a series of nickel compounds as follows: Square Planar

Tetrahedral or Octahedral

cyano glyoximate dithiooxalate diacetyldioxime ethylxanthogenate ethyldithiocarbamate stilbenediamine (yellow form) phenylethylenediamine (yellow form) N i ( P ( C H ) 3 ) X X = CI, B r , I (red) disalicylaldimine (orange form)

aquo ammine hydrazine ethylenediamine acetylacetonate

6

5

2

2

stilbenediamine (blue form) phenylethylenediamine (blue form) N i ( P ( C H ) 3 ) 2 ( N 0 ) (green) disalicylaldimine (green form) 6

5

3

2

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

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Square Planar Complexes

471

In this discussion, the square planar geometry of first row transition metal complexes will be treated, emphasizing the use of structural modifica tions of the ligand in order to influence the geometry of the complex. De tailed descriptions of the properties of these complexes are found in the accompanying references.


Porphyrins, Phthalocyanines, Chelating Agents

and Other Polycyclic

In principle, square planar configuration about a metal can be assured by appropriate choice of a single ligand. The ligand, in the most rigorous case, must contain four atoms in the same plane capable of coordinating to the metal, have sufficient space between them to accommodate the metal in the plane, and be sufficiently stable to be practical. The porphyrin (33) and phthalocyanine (57) (Figure 2) systems meet these requirements. Of the first row transition metals, the nickel(II), cobalt(II), and copper(II) porphyrins are the most thermodynamically stable (83). A crystal structure determination of nickel(II) etioporphyrin confirmed (24) (Figure 2(a)) the planarity of the true porphyrin nucleus. Moreover, most of the first row transition metal ions form planar porphyrin complexes, but certain of these must be regarded as square pyramidal or octahedral com plexes inasmuch as one or two additional ligands occupy positions perpen dicular to the ring system. The stability of these compounds as well as of the strictly square planar complexes is generally attributed to the high degree of conjugation which is transmitted through the entire system (38). The effect of substitution of the porphyrin thus is readily felt in all parts of the ring system and in the extra planar substituents, when present. A l l positions of the pyrrole rings can be substituted with alkyl groups, etc., and replacement of the = C H linkage by an aza linkage is common. Specific biological activity is frequently a function of the particular protein coor dinated to one of the axial positions, the other axial position accommodat ing the same protein, a water molecule, oxygen, carbon monoxide, etc. (83). Recent reviews of the phthalocyanines by Lever (57) and by Moser and Thomas (66) are available. Complexes of phthalocyanine with all of the first row transition elements, excepting scandium, have been prepared. However, titanium and vanadium phthalocyanines are known only as the oxy- or chloro-compounds and are not then strictly square planar. That phthalocyanine has considerable ability to promote the square planar con figuration is further demonstrated by the existence of the chromium (III) and manganese (II) complexes (57). Although oxidation states of the central atom may vary between 0 and + 6 , the + 2 oxidation states are usual. Most complexes are thermally stable and sublime unchanged at 400°C. Phthalocyanines often exist in two or more polymeric modifications which, in certain cases, have different


472

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(b)

Figure 2. Square planar geometry of (a) nickel(II)etioporphyrin and (b) phthalocyaninenickel(II)

structures. The a-form of phthalocyaninechromium(II) is polymerized through the crystal lattice (29), whereas the /3-form, prepared by Ercolani (56), is square planar. These complexes are generally intensely colored and insoluble in common solvents. Phthalocyanine complexes exhibit magnetic moments corresponding to low-spin, electron-paired species (57). One exception is lithium phthalo cyanine manganate(O), a spin-free case which may occur for metals in their lowest oxidation state (57). On the basis of the low spin observed for phthalocyaninechromium(II) and phthalocyaninechloroiron(III), a metalmetal interaction which extends throughout the lattice has been postulated (55). In order to increase the solubility of porphyrin and phthalocyanine complexes, several structural modifications have been made, a, ($, y, 5Tetra-(4-pyridyl)-porphin complexes of copper(II), nickel(II), and zinc(II) have been synthesized (85) and their ultraviolet spectra determined in chloroform and in acid solution. B y utilizing sulfonic acid groups to i n crease solubility, complexes of 4,4',4",4' '-tetrasulfophthalocyanine com plexes of many metals were prepared (94, 95). This chelating agent was found to have a ligand field strength comparable to cyanide (94, 95). ,

The great stability of these ring systems was known, and possible biological implications of compounds having similar structures were pos tulated. A s a result extensive research in macrocyclic chelating agents developed (13). Quadridentate ligands can be obtained by condensing suitably sub stituted aldehydes and ketones with bidentate primary amines, and the resulting Schiff bases closely resemble the porphin ring system. G e n erally, one of the structural u n i t s — N = C — C = N — , — N = C — N H C = N — , — N = C — N H — N = C — , — C = N — C H = C H — N = C — , or — C = N — C H — C H — N = C — i s incorporated into the structure for maximum sta bility. For example, if or^/io-aminobenzaldehyde is condensed with ortho2

2


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phenylenediamine the following chelate can be obtained with nickel(II) (Figure 3(a)). M a n y quadridentate and tridentate chelating agents have been reviewed by Goodwin (40). M e t a l ions have recently been used to direct the course of these con densation reactions. These "template syntheses" are the topic of a recent review by Busch (18). The compound shown in Figure 3(b), having square planar geometry, is one of the products formed by dissolving bis(ethylenediamine)-nickel(II) perchlorate in acetone.


2+

2+ C

K

C = N

H

3 H / ~ A

CH

3

.N = C

W W N

CH

3

C H

\_J H

(a)

%

3

C H

3

(b)

Figure 3.

Macrocyclic chelates

If o-aminobenzaldehyde is treated with metal salts in absolute ethanol, the following structure (Figure 4(a)) can be obtained (18). Ligands con taining sulfur atoms have been alkylated to achieve planar coordination. If bis(mercaptoethylamine)nickel (II) is treated with a,a'-dibromo-oxylene, the complex in Figure 4(b) is obtained (18).

(a)

Figure

4*

Quinoline and Pyridine

(b)

Chelates resulting from syntheses

"template"

Heterocycles

Although square planar structures are generally assured by the use of cyclic tetradentate ligands, the use of simpler heterocyclic ring systems



474

WERNER

CENTENNIAL

often yields the square configuration with some first row transition metal ions. Square planar complexes can be prepared which contain quinoline, pyridine, and substituted pyridines as ligands, and solvents are known to influence the geometry of the complexes. It has been shown in particular (79) that the solvent used for dissolving tetrapyridinenickel(II) perchlorate (and tetrafluoroborate) can affect the geometry of the complex. The solid complexes are 6-coordinate, spin-free, tetragonal complexes having mono-coordinated anions. However, when the complexes are suspended in chloroform, they do not dissolve but change in color from pale blue to yellow. These yellow chloroform solvates have a reduced magnetic moment and react with moist air to regenerate a blue complex. The yellow species have the properties of diamagnetic, square planar nickel. Using substituted pyridines, a number of square planar complexes has been prepared with the empirical formula, ML4X2, for which structures in the solid and in noncoordinating solvents can be generalized in the following way (91). Solid

Complex Ni(3,5-lutidine)4X, Ni (4-picoline)4X2 Ni(3-picoline)4X»

X

Ni(3,4-lutidine)4X,

X X

= ClOr, B F r it

It

ti

it

= I= ClOr, B F r

oct. sq. pi. sq. pi. oct. sq. pi. sq. pi.

Solution CHtCh sq. pi. oct. oct. oct. sq. pi.

The solid chlorides are octahedral when pKa of the ligand < 9.0; the iodides are octahedral when pKa < 6.5; the bromides, however, change from octahedral to square planar at an intermediate pKa value. In solution all halide complexes are octahedral. Similarly, the perchlorates and tetrafluoroborates are octahedral when the ligand pKa < 5.5 and are square planar when the pKa > 6.2 (93). Quinoline and substituted pyridines also form complexes with nickel(II), with an empirical formula, ML2X2. Octahedral, tetrahedral, or planar species containing quinoline have been studied in this case as a function of the nature of X (88, 89). Ni (quinoline) C1 Ni (quinoline) 2CI2 Ni (quinoline) B r Ni(quinoline) I 2

2

2

2

2

2

yellow form blue blue " green

polymeric octahedral tetrahedral trans planar

The iodide complex is diamagnetic and gives a red solution in organic solvents with accompanying decomposition. A study (20) of the far infrared


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spectra of these complexes confirmed the suggested structures. For N i (quinoline) 2I2, the N i - I vibration occurs at 218 c m . " (similar to the tetrahedral vibration), but the Ni-quinoline vibration occurs at 299 cm." , about 80 c m . higher than tetrahedrally-coordinated quinoline. This observation suggests that v ( M - L ) vibrations occur with higher frequencies in planar situations than in other configurations. A tabulation of x-ray structures of M ( p y ) X complexes, where X = CI and B r and M = Co, N i , C u , and Zn, is presented by Clark and Williams (20). These complexes are mainly distorted octahedral or tetrahedral with no square planar struc tures reported for unsubstituted pyridines (87). Chromium (II) complexes (87, 47) of stoichiometry C r ( p y ) X have distorted octahedral structures with powder patterns similar to the corresponding copper(II) complexes. Here again, using substituted pyridines, trans planar structures can be obtained. Ni(2,5-lutidine)2X is reported to have a planar structure in the solid but, in solution in noncoordinating solvents, these compounds are distorted tetrahedral (12). Complexes of the general formula, N i L X , where X = B r or I and L = 2,5- or 2,6-dimethylpyrazine, have planar geometry (59). The planarity of these structures was confirmed by an x-ray study (3) of Ni(2,5-dimethylpyrazine)Br . The intense purple color of this complex was attributed (3) to conjugation throughout the aromatic system in the solid state shifting the normal blue absorption into the green and therefore changing the observed color of the complex from yellow to purple. A spectroscopic study of numerous pyrazine nickel(II) complexes has been reported by Lever, Lewis, and N y h o l m (59). 2,2'2"-Terpyridine was reported (65) to give square complexes with zinc (II), however, the complex has been found to be a trigonal bipyramid with the anions occupying two positions (21). Neither 2,2'-dipyridine nor o-phenanthroline complexes has been shown to form square planar struc tures with any of the first row transition metals (2, 10), although in the presence of weakly coordinating solvents and anions this geometry may be achieved. 1

-1

- 1

2

2


2

2

2

2

2

Phosphorus-Containing Ligands. A class of compounds analogous to the nitrogen donors and having a similar formula are the phosphine com plexes, M ( P R ) X , where R may be alkyl or aryl and X is CI, B r , or I. These complexes have been studied recently by Venanzi and Sutton (11, 28) and by Hayter (44, 4$)- Phosphine complexes are especially interest ing in that tetrahedral or planar isomers can be isolated in the pure state, depending on the nature of R and X . When R is diphenylalkylphosphine, both isomers can be isolated. I n fact, both isomers exist in the same unit cell in a similar complex, N i ( P ( C H C H ) ( C 6 H 5 ) ) B r (52). Differentia tion between the planar and tetrahedral isomers was accomplished by con sidering their spectral and magnetic properties (45). Diamagnetic dark 3

2

2

2

6

5

2

2

2


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WERNER

CENTENNIAL

red or brown compounds which show no absorption in the 800-1000 ma region and the powder patterns of which resemble palladium complexes are planar. Paramagnetic compounds (chlorides are blue-purple, bromides are green, and the iodides are brown) with an absorption in the 800-1000 m/x region and with powder patterns resembling zinc or cobalt compounds are distorted tetrahedral (45). -1

For the complexes, N i ( P R ) X , the tetrahedral form is favored over the square planar form as the substitution on phosphorus varies in the series (28, 45) E t P < E t P C H < E t P ( C H ) < P ( C H ) , and as X varies i n the series S C N ~ < C l ~ < B r ~ < I~. The tetrahedral form is favored over the square planar form as the ligand field is decreased. A steric factor also favors the tetrahedral configuration because steric inter actions between ligands are reduced in this conformation (23). It also appears that i n the alkyldiphenylphosphines, as the size of the alkyl group increases from methyl to n-amyl, the tetrahedral form is favored. The diphenylphosphine complex, N i ( P H $ ) I , was prepared and found to be planar, but the corresponding chloro- and bromo-complexes could not be isolated (44) • In all of these planar compounds the trans-isomer is formed. It should be noted that the cobalt(II) complex, C o ( P E t ) ( S C N ) , is square planar in the solid state (92). 3


3

2

6

2

2

5

6

2

2

5

2

6

5

3

2

3

2

2

Chatt and Shaw (17) alkylated the tetrahedral molecule, N i ( P R ) B r , 3

2

2

with sodium acetylide in liquid ammonia and obtained planar N i ( P R ) 3

(C = C C H ) . 6

5

2

2

Arylation of other bis(triarylphosphino)dihalonickel(II),

cobalt(II), and iron(II) complexes with or^/io-substituted aryl Grignard reagents has also been accomplished (18).

Simple alkyl and aryl Grignard

reagents, however, gave impure products.

The first planar complex of

iron(II) containing only monodentate ligands was F e ( E t P C 6 H ) ( C C l 5 ) 2 5

2

(18).

2

6

A n aryl substituted cobalt(II) complex, C o ( E t P C H ) ( m e s i t y l ) , 2

6

5

2

2

was proved to be planar by an x-ray study of its crystal structure (72). Ditertiary phosphines also form planar complexes with nickel(II). N i ( ( C 6 H ) P C H C H P ( C H 5 ) ) B r (8) and N i ( E t P - C H - C H - P E t ) B r (99) have been prepared and are diamagnetic and apparently planar. 5

2

2

2

6

2

2

2

2

2

2

2

Some planar complexes containing arsenic can be prepared, oPhenylenebisdimethylarsine (diarsine) reacts with nickel carbonyl to form Ni(CO) (diarsine) (70). Oxidation of this complex with iodine gives a red, diamagnetic, planar complex, Ni(diarsine)I . The corresponding bromides and chlorides could not be purified and appear to be much less stable (70). 2

2

Ketones and Derivatives. Probably the most extensive series of ligands which form planar complexes studied to date are the derivatives of carbonyl compounds, primarily the Schiff bases. T o include here all of the


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477


available data on these compounds would be a review in itself. only a general outline will be presented.

Therefore,


In brief, the recent studies have sought to prepare various Schiff bases to distinguish between the square planar and tetrahedral configura tion (15) and to determine how the ligand might be modified to favor one stereoisomer over the other. It was hoped that the correct ligand would produce an observable equilibrium in noncoord mating solvents between the planar and pseudo-tetrahedral forms. The following general type of complexes has been studied extensively (Figure 5). These bidentate ligands, which form bis-complexes of nickel(II), copper(II), and cobalt(II), are nonelectrolytes and therefore have moder ate solubility in nonpolar, noncoordinating solvents. Furthermore, using these solvents reduces additional coordination. The equilibrium between the planar and pseudo-tetrahedral conformations may be altered by changes in solvent, temperature, and substituents. R'

(d)

(e)

Figure 5. fi-Ketoaminates and related complexes

B y altering the structure of R in type a compounds of nickel(II), three classes of compounds in the aromatic series can be formed (49): (1) When R is an orf/w-substituted aryl group, the compounds are either diamagnetic or weakly paramagnetic. (2) When R is phenyl or p-substituted phenyl, either diamagnetic (phenyl, p-fluorophenyl) or strongly paramagnetic solids (p-tolyl, p-chlorophenyl) are obtained, but these solids are always paramagnetic in solution. (3) When R is a meta-substituted phenyl, the compound is always paramagnetic. Species in Class 1 are planar, and those in Class 2 are considered to be an equilibrium mixture of diamagnetic and paramagnetic dimers. Species in Class 3 are tetrahedral. In the aliphatic series, as R varies from methyl to sec- and tert-alkyl groups, a change from essentially planar diamagnetic behavior to paramagnetic, pseudo-tetra hedral behavior was realized (50, 71, 80, 85). When R is sec-alkyl, a con-



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WERNER CENTENNIAL

formational equilibrium between planar and tetrahedral forms was proposed to exist in solution (50, 80). Copper(II) complexes of some of these ligands have also been prepared (81, 100). The red diamagnetic nickel complex of acetylacetoneimine (type b) was interpreted (1) to have the trans-planar configuration as a result of analyzing the ligand field absorption bands. The presence of cis-isomer cannot be ruled out entirely but, since two isomers could not be detected by chromatographic methods, its existence is doubtful. B y a comparison of the crystal structures of the acetylacetoneimines of nickel(II) and copper(II) with palladium (II), the square planar nature of these chelates was ascertained (48). The trans-arrangement of ligands and the planarity of the metal atom and its four nearest neighbors were also confirmed (48). A n extensive series of /3-ketoimine complexes of type b with nickel(II) (31) and cobalt(II) (82) have recently been studied. Nickel(II) complexes, where R = H , have been found to be 100% planar up to 80°C. in chloroform solution. Similar nickel (II) complexes, where R = C H , are < 5 % tetra hedral at room temperature, whereas the corresponding cobalt(II) com plexes are totally tetrahedral (32). H o l m (32) proposed that the most important conclusion from these results is that ligands which stabilize a measurable amount of tetrahedral nickel(II) induce ~ 1 0 0 % tetrahedral cobalt(II), and ligands which stabilize a measurable amount of planar cobalt(II) induce ^ 1 0 0 % planar nickel(II). In the nickel(II) complexes, when R is sec-alkyl, the complexes are 100% tetrahedral, whereas with nalkyl groups the planar form dominates. This behavior is similar to the salicylaldimines. In all these complexes there is little doubt that there is much 7r bonding between the metal ion and the ligand system. Acetylacetonates with bulky substitution on the 1 and 5 positions may form square planar complexes (22). Bis(2,2,6,6-tetramethyl-3,5-heptanedione)nickel(II) is square planar, whereas bis(2,6-dimethyl-3,5-heptanedione) nickel (II) exhibits an equilibrium between the square planar and tetrahedral configuration (22). Compounds of type c have been studied for cobalt(II), nickel(II), and copper(II) (48). Copper(II) complexes, with R smaller than 2-butyl, are planar. Nickel (II) complexes, where R is sec-butyl, are involved in a planar pseudo-tetrahedral equilibrium. When R is smaller than sec-butyl, planar complexes are obtained. N o planar complexes of cobalt(II) with this ligand were reported (48). Nickel complexes of type d have been studied by Sacconi (82-84,91) and similar compounds by H o l m (16). If N R is an ort/io-substituted anilino group, the complexes are planar and diamagnetic. If N R is anilino or para-substituted anilino, the substances are octahedral or planar, depending on X . In inert solvents, compounds with a substituted anilino group exist as an equilibrium mixture of octahedral and planar forms, with the planar form predominating at higher temperatures (88). If the N R 3

2

2

2


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group is a piperidino group, the complexes are square planar, whereas, if N R is a dimethylamino group or pyrrolidino group, a distorted octahedral structure results. When N R is a diethylamino group, the compound may be planar or octahedral depenling on the substituent X (84)* Tetracoordinated cobalt(II) complexes with similar ligands are tetrahedral rather than planar (82). 2

2


In summary it can be stated that, in general, as the size of the group R increases, the steric interference between ligands causes the planar con figuration to be destabilized with respect to the tetrahedral or pseudotetrahedral configuration. Numerous aminotroponeimineate complexes of type e, with nickel(II), have been prepared and are found to undergo square planar-tetrahedral interconversion (27). Extensive nuclear magnetic resonance data have been accumulated on this system along with magnetic moment and elec tronic spectral measurements (27). Certainly a discussion of ketone derivatives would not be complete without mentioning the planarity of bis (dimethylglyoxime) nickel (I I) in noncoordinating solvents. Other dimethylglyoxal derivatives such as the pyridylhydrazide form planar complexes with nickel(II) and copper(II) (19). Sulfur-Containing Chelating Agents. Perhaps the greatest interest in square planar complexes has arisen in the area of sulfur donors and the resulting discussions concerning multiple oxidation states of nickel (25, 68, 86, 87,

88).

It was reasoned that the way to stabilize planar geometry was to i n volve the nickel atom in an extensive T system containing sulfur atoms; therefore numerous complexing agents were synthesized. The resulting complexes are very stable, and the crystal structures of several species have been determined (28, 80, 36, 42). A listing of some of the sulfur-containing ligands is given in Figure 6 (41, 54). Although Ni(II) has been the metal ion generally used in these complexes, [Co(mnt) ]~ and [Cu(ran2) ]~ have been prepared and are essentially planar (36). 2

2

2

2

B y analogy to the spectrochemical series which Chatt suggested for the ligand field splitting of platinum(II) complexes, an approximate spec trochemical series for planar Ni(II) complexes was suggested by Gray (41): rant- = I~ < B r ~ < C l ~ < R T e < dpt~ < R Se < dto~ < R S < R A s < piperidine < R P < (RO) P. Using polarography and compar ing the half-wave potentials, the stability of the first row metal ions was determined with respect to different ligands (41). In the mnt~ series C u > N i > Co » Fe, and in the tdtr* series N i > C u » Co > Fe. Comparing the first, second, and third row transition metals with the same anion, N i < Pd > Pt. 2

3

2

3

2

2

3

2


2

480

WERNER CENTENNIAL

\ r

2 -

s

v

C N

/A\ *

II

S

C N

A

(b)

(a)

s

" \

^C-OEt

/ £t


S

N=C

\

S

(d)

fc)

2\ H

/ "

-=4

2

/ v C !

CH 0H

S

1

2

—

n U

(«) S - C * °

S

"

2-

.CH

2 3

\ _

Figure 6. Sulfur-containing chelating agents which promote square planar geometry

If the cyano groups i n mnt~ are replaced by phenyl groups, a nonelectrolyte, diamagnetic complex is obtained (Figure 7) (86, 87). This complex can be reduced to give [Ni(S C I 2)2]~ and [ N i ( S C $ ) ] . The oxidation of [Ni(mnt) ]~ to [Ni(mnt) ]~ to give N i ( I I I ) complexes has been described (25, 63). However, alternative formulations of these complexes as species containing nickel(II) and radicals have also been proposed (86, 87, 88, 90). The best description of complexes of nickel, where the metal atom has a formal oxidation state different from N i ( I I ) , is still being discussed. The extensive review of these sulfur donors and a 2

2

2

2

2

2

2

2

2

-2

2

Ii 0^ Figure 7. Planar complex having multiple oxidation states for nickel


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discussion of the electronic properties of square planar complexes should be consulted for additional data (41). Complexes containing sulfur donors but which lack the extensive ir system have been prepared. A cobalt complex of 2,5-dithiahexane, [Co(CH S-CH CH -S-CH3)2](C104)2, having a low magnetic moment, 2.46 B . M . , indicates that this complex has square planar geometry (14)- In addition, a series of complexes which are related to both the imines and sulfur donors are the iV-alkylthiopicolinamides (58). The copper(II) and nickel(II) complexes have square planar geometry. Other complexes hav ing nitrogen and sulfur donors are also reported (34, 90, 98). Downloaded by COLUMBIA UNIV on May 5, 2015 | http://pubs.acs.org Publication Date: January 1, 1967 | doi: 10.1021/ba-1967-0062.ch031

3

2

2

Saturated Nitrogen Ligands. Square planar complexes of the first row transition metals, where there are no p or w electrons on the donor atom (sp nitrogen), are extremely interesting. The raeso-stilbenediamine (raeso-stien) (Figure 8(a)) complexes of nickel(II) (Lifschitz complexes) (60-62) exist in two forms, a blue paramagnetic form and a yellow diamag netic or sometimes weakly paramagnetic form. B o t h blue and yellow forms can be obtained containing the same anion, and in certain cases they are interconvertible. 3

Explanations of this behavior have been the subject of much specula tion (4, 48, 51, 75). Pauling (75) and K a t z i n (51) considered them to be tetrahedral (blue) and square planar (yellow) configurations. However, it appears from an examination of the electronic spectra that the blue form has octahedral coordination rather than tetrahedral. The yellow complex has but a single absorption maximum near 450 mu. Complexes of the type Ni(raeso-stien) (RCOO) can be interconverted easily by gentle heating and cooling, by removing solvent or crystallization, or by dissolving in suitable organic reagents (46). In Ni(raeso-stien) (Cl CHCOO) , both blue and yellow crystals can be obtained simultaneously from 9 5 % ethanol (46). A n x-ray study of these crystals showed that the blue crystals were indeed octahedral, and the yellow crystals contained both octahedral 6-coordinate nickel and planar 4-coordinate nickel (69). The absorption spectrum of this complex in aqueous acetone indicated the presence of both forms. 2

2

2

Ni(meso-stien) (Cl CHCOO) + H 0 ^± Ni(meso-stien) 2

2

2

2

2

2

2

+2

+ 2Cl CHCOO" 2

It was suggested that the geometric structure of the raeso-stien complex caused the axial positions to become blocked and octahedral coordination was prevented. This was conditioned by the fact that in d,Z-stien (Figure 8(b)), where the phenyl groups do not block the axis, square planar com plexes cannot be isolated (46). Utilizing the concept that appropriate carbon-substitution of ethylenediamine gives square planar complexes, Basolo (6) prepared a yellow dia-


482

WERNER CENTENNIAL

H

.7

P^.

x N

m

f

H

2

2+

2t

H

H2N

Jr

TT"'

2 ~~~ N

—

H

(b)

H


Figure 8.

Lifschitz complexes

magnetic nickel(II) complex of 2,3-dimethyl-2,3-diaminobutane ( C , C tetraMeen). This yellow color and diamagnetism persists in aqueous solu tion indicating that aquation of the planar species does not occur. Wilkins (97) determined that the rate of dissociation in aqueous acid of nickel(II) and copper(II) decreased as substitution on carbon increased. Going from methyl to ethyl substitution decreased the rate further. Under certain conditions, ethylenediamine itself may form square planar complexes with nickel(II). Sone and K a t o (89) reported that yellow Ni(en) + is formed when [Ni(en)2(H 0)2] is heated i n alcohol. The bis-ethylenediaminenickel(II) complex, N i ( e n ) ( A g I B r ) , and the corresponding platinum (II) and palladium (II) derivatives are isomorphous, indicating square planar nickel in this complex (58). The anions may be changed to C u l " , I " , P b l ~ , and H g l " " without destroying the planarity of nickel. Cobalt(II) and copper(II) also form square planar complexes of this type. These square planar species appear to be stable only i n the solid state for they decompose when dissolved in water (58). A tridentate ligand, 1,1,7,7-tetraethyldiethylenetriamine (Et dien), forms square planar complexes with nickel (II) (26). The Ni(Et dien)Cl2 complex is diamagnetic i n the solid state and in ethanol solution but ex hibits some paramagnetism i n acetonitrile, D M F , and acetone. In these latter solvents a pentacoordinate species is postulated (26). The tetradentate ligand, 1,4,8,11-tetraazacyclotetradecane (cyclam), forms solid diamagnetic orange to brown N i ( I I ) complexes with iodide and perchlorate anions (9). When the anions are changed to chloride and bromide, the solid complexes are paramagnetic (9). However, all the com plexes are soluble i n water, methanol, and benzene, giving yellow solutions. The yellow color remains on treatment with 122V HC1 and excess chloride ion (9). It is interesting to note that this amine is a derivative of a reduc tion product of the "template syntheses" between bis(ethylenediamine)nickel(II) and acetone reported earlier. Using the knowledge that sterically blocking the axial positions i n square planar systems effectively prevents an octahedral configuration from forming, nickel(II) and copper(II) complexes of a secondary mediumring diamine, 1,5-diazacyclooctane (daco), were synthesized (Figure 9) (67). In the most stable configuration the axial positions are blocked by 2

2

2

+2

2

2

8

3

2

4

4

4



31.

MUSKER


Figure 9.

Idealized conformation for octane complexes

483

bis-1,5-diazacyclo-

the hydrocarbon system. The diamagnetic nickel(II) complex is yellow in the solid state and also in water, acetonitrile, and other solvents. N o 1:1 nickel(II) amine complex can be detected spectrophotometrically. This behavior is similar to the [Ni(C,C'-tetraMeen)]+ reported by Basolo (6). The bis-daco complex can be heated with concentrated HC1 or treated with excess E D T A with very slow decomposition. The bis-daco copper(II) complex is maroon in color, and its aqueous solution exhibits a single visible absorption maximum much higher in energy than other bis-diamine copper(II) complexes. The stability constant of the nickel(II) complex is higher than other bis-diamine complexes. We feel that this ligand may be a prototype for another series of square planar complexes with the first row transition metal ions. 2

This discussion has only presented an outline of the chelating agents which produce planar complexes. Extensive additional data on each topic can be obtained from the key references in the bibliography. Unfortu nately, a number of chelating agents have been omitted from this presenta tion, but most of these are similar to those reported. Hopefully, this com pilation has united the variety of ligands which have been used to form planar complexes of the first row transition metals. Literature (1) (2) (3) (4) (5) (6)

Cited

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484

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CENTENNIAL

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Square Planar Complexes of the First Row Transition Metals

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