FIVE-COORDINATION IN 3d METAL COMPLEXESpublications.iupac.org/pac/pdf/1968/pdf/1701x0095.pdfFIVE-COORDINATION IN 3d METAL COMPLEXES the next lowest is larger than the spin-pairing

FIVE-COORDINATION IN 3d METALCOMPLEXES

LuIGI SAccoNI

Istituto di Chimica Generale ed Inorganica, Università di Firenze,Florence, Italy

INTRODUCTIONIn recent years a good number of five-coordinate complexes of metals

of the first transition row have been described, so that now this type ofcoordination is no longer exceptional'. We are going to discuss the propertiesof these complexes, paying particular attention to their thermodynamic,magnetic and spectroscopic aspects. In this contribution five-coordinatecomplexes of 3d metals with zero or negative oxidation states and compoundswhich have been known for a long time are not considered.

GEOMETRYAll the observed five-coordinate structures can be referred back to the two

most symmetrical configurations, square pyramidal (C4 symmetry) andtrigonal bipyramidal (D3h symmetry). These idealized structures are rarelymet with in practice (Figure 1). They can be interconverted by means ofsimple angular distortions, such as increasing the angle B,—M—B2 untilA1, B,, B2 and A2 are coplanar and vice versa. The metal atom need not liein the square pyramids basal plane, and if it does not, the bonds A,—M—A2are also involved in the interconversion.

A1

B1 II, ___

B3

A2

Figure 1. Interconversion between the trigonal bipyramid and square pyramid

The principal factors that determine the configuration are: electrostaticrepulsion between ligands; the nature of the metal—ligand bonds; thecrystal field stabilization energy; the shape of the ligand molecules and crystalpacking effects.

With regard to the ligand—ligand repulsion factor, it has been shown thatthe trigonal bipyramid is the more stable regular structure2 (Figure 2).However, a distorted square pyramidal structure may be only slightly lessstable, as for example when the apical angle is a little more than 1000. (Theapical angle is the angle between apical bond and the four equatorial bonds.)

95

C4 C2V

Figure 2. Behaviour of the respulsive energies of the type F cc R-. Coulomb repulsions,n = 2; Born repulsions, n = 10. Five ligands arranged according to the C4, point group(left side) and C2 point group (right side). The trigonal bipyramid occurs for cc = 120° and= 9004

ft

LUIGI SACCONI

Gillespie's analysis in terms of valence shell electron pair repulsions leadstowards the same conclusion3. In this treatment, a square pyramid distortedin the manner just described is only 8 per cent less stable than the trigonalbipyramid. In covalent complexes where presumably the interactionsbetween bonded pairs of electrons are more important than bonded—non-bonded electron pair interactions, the trigonal bipyramidal configuration is

ci)

U

0)>

nz2

ci0)

ci)

ci)>C)'

3aci)

n=1O

- -ep

120° 100° 9cJ°

1080 128°

1000 120°

(ci $) ci

140°

ND3h

(ft =90°)

favoured3. On the other hand, it is essentially the ionic compounds in whichthe interaction between bonding electron pairs and the d-shell predominatesand in which a square pyramidal configuration is to be expected3. But, weshall see that the other factors such as ligand geometries and lattice forcesoften determine the geometry of the complex so that a direct correlationbetween bond type and stereochemistry would not appear to exist.

Table 1 shows the CFSE for various electronic configurations4. One seesthat the square pyramid configuration is always more stabilized by crystalfield splittings than the bipyramidal when the apical angle is 900, but whenthe apical angle ranges from 100—105° the energy difference between the twoconfigurations is very small and the bipyramidal may even be the morestable, as with d4, d9 and low-spin d8. If the ligand is polydentate, the steric

96

FIVE-COORDINATION IN 3d METAL COMPLEXES

restrictions imposed by its geometry determine, in the final analysis, thestereochemistry adopted by the complex.

Structural data for five-coordinate complexes of the 3d elements havebecome fairly numerous. In square pyramidal structures the apical anglefound is usually between 1000 and 106°, as shown in Table 2. The only

Table 1. CFSE for five-coordinated 3d metal complexes4

d S Square Pyr.=90° =1O5°Trig. Bipyr.

d5d6d'd8

d', d6d2, d7d3, d8d4, d9

LOW-SPIN3/2 0894 0628

1 1192 O8l21/2 1•131 09200 1•192 0888

HIGH-SPIN0298 0•1840•596 03680635 O4760596 0444

06240•792085209l2

0168033703970456

Table 2. Apical angles of square pyramidal structures

Complex f3 Ref.

[Co(C104)(OAsMePh2)4]C1O4[Cr(en)][Ni(CN)s].1•5 H20oc-Chlorohaeminiron(iii)Methoxyiron(iii)mesoporphyrinCo(S2CNMe2)s(NO)VO(acac)2

100°100°102°103°103°106°

56789

10

regular trigonal bipyramidal structure occurs in the complex [Cr (NH3)6][CuC15]" (Figure 3). However, the complexes [MBr (Me6 lien)] Br'2[Me6 tren = tris (2-dimethylaminoethyl)amine; M = Mn to Zn] retainthe three-fold symmetry axis, with the angle 82° ranging from 81° to84 °,13 and C3 symmetry is also retained in the cluster compoundsCu4 0C16 (OPPh3)414 (Figure 3) and Cu4 0C16 (py)'5. All other knownstructures have lower symmetry. There is quite a large group of compoundswhich deviate from regular D3h symmetry only in regard to the anglesbetween the equatorial bonds. For example in TiBr3 2NMe316 theBr—Ti—Br angles are l2125°, 12125° and 1175° while in [Co(NCMe)5]ClO41 the equatorial angles are 1l59°, 115'9° and 128'3° (Figure 4).

With the more distorted structures the designation as square pyramid ortrigonal bipyramid becomes almost a matter of personal taste, so far arethey from the limiting regular configurations.

ELECTRONIC STRUCTURES OF FIVE-COORDINATECOMPLEXES

The electronic configurations of five-coordinate complexes fall into twoclasses, high-spin and low-spin, but only the configurations d5, d6, d7 and d8can give both types of compounds. This can be understood by looking at an

97P.AC.—H

LUIGI SACCONI

CL

CL-i-

a = 1200= 900

Ph3

Ph3I0

Figure 3. Structural data for [Cr(NH3)e] [CuCI5]11,Cu4OC16(OPPh3)414

Br

Br Me-N-C

[MBr(Me6tren)]Br,'3 and

Me

N

N

Me

Figure 4. Structural data for TiBrs(NMe3)216 and [Co(CNMe)o]C10417

orbital splitting diagram for C4 and D3h fields (Figure 5). The highest orbitalis well separated in energy from the others, the 42 — ,,2 and the 42 orbital,respectively. When the energy separation between the highest orbital and

98

Co,46 =81°Ni,18 =84°Cu,8 =84°

:1

\0

PP h3

C

2°

C


the next lowest is larger than the spin-pairing energy, the total spin quantumnumber S is 3/2, 1, 1/2 and 0 for the d to d8 configurations. Ligands with P,As, and C as donor atoms generally give low-spin complexes, whereas 0and N ligands give high-spin complexes, usually. S, Se, Cl, Br and I may giveboth types of complexes.

(b1) dx2_y2

(a)

' I

(a1) d2 d_________ dx2y2,dxy (e)

(b2) ,"

__________,1(er')

Ce)

C4 0Th

Figure 5. Energies of 3d orbitals in C4., and D3 fields

FIVE-COORDINATE COMPLEXES WITH HIGH-SPINFor cønvenience we shall consider inner complexes later on. The type of

compounds in this section can be considered as adducts of metal salts withneutral ligands or halide ions; +2 and +3 oxidation states are usuallyfound.

Adducts with monodentate ligandsData on this type of compounds are summarized in Table 3. The series of

Ti, V, and Cr tribromide and trichioride (bis) adducts with trimethylamineand thioethers were largely prepared by Fowles and his coworkers, workingunder rigorously anhydrous conditions. The adduct of TiBr3(NMe3)216 wasfound to have a distorted trigonal bipyramidal structure, point group C2,

Table 3. Five-coordinate adducts of Ti, V and Cr halides

Ti TiX3(NMe3)218. 19, TiX4(NMe3)'8' 20, TiX4(SR2)2V VX3(NP3)22' 22, VX3(SR2)219. 22, VX3(PR3)221,

VX3(OPR3)221, VX4(NHMe2)24, VX4(NMe3)24,VOCI2(OPPh3)225, M21V0C14(M = pyH26, quinH+, iso-quinH+)

Cr CrX3(NMe3)227

and the other analogues with vanadium and chromium were found to beisomorphous. The lowest complex here reported CrCI3NMe3 is the uniqueauthentic compound of chromium(m) which is not octahedral. It is possiblethat suiphide adducts of vanadium are octahedral dimers in the solid state19.

Adducts of the titanium and vanadium trihalides with trialkyl phosphinesand trialkyl phosphine oxides are also known. Titanium and vanadiumtetrahalides give mono-adducts with di- and trimethylamine and withthioethers that are monomeric in benzene. Five-coordinate complexes of

99

LUIGI SACCONI

oxovanadium(iv) are also known, such as VOC12 (OPPh3)2, M21 [VOCI4}where M is pyridinium, quinolinium and isoquinolinium.

Examples of high-spin compounds of the type [MX5]- in which fivehalides are coordinated around one metal ion, are scarce. The only certainexamples occur in [Cr(NH3)6] [Cu015]1' in which the ion [CuCJ5]3 hasfull D3h symmetry and in [NH2 Me2] [CuC13] which consists of dimericCu2C16 units (mean Cu—Cl distance 23 A) joined in infinite chains bymeans of bridging chlorine atoms at a distance of 273 A. Other probablefive-coordinate complexes are the penta-chioro and penta-fluoro manganateions: (Et4 N)2 [MnCl5]29 and (K, Rb, Cs)2 [MnF5]3° which have magneticmoments corresponding to four unpaired electrons.

A set of five donor oxygen atoms is found in the compound[Co(OAsMePh2)4 C1O4] (dO4) which is formed from the bivalent metalperchiorates and the arsine oxide (Figure 6). The derivatives of the bivalent3d metals have the square pyramidal structure with a perchlorate oxygenatom apical5. The compounds are high-spin and the cobalt(Ir) moment of5.5 B.M. is unusually high.

0

/00

Kiooo(Ph2Me)As =---__ _ As(MePh2)

(Ph2Me)As ''As(MePh2)

Figure 6. Structural data for the ion [Co(C104)(OAsMePh2)4]+ (ref. 5)

The spectra of these Co(ii) and Ni(u) square pyramidal compounds3are given in Figures 7 and 8 together with the energy levels calculated for acrystal field of C4 symmetry4. The spectrum of the cobalt compound showsfour bands between 5000 and 25 000 cm-1 at c. 7000, 12 000—17 000 and23 000 cm-1 which are assigned to transitions from 4A2(F) to 4E(F),4B1(F), 4E(P) and 4A2(P). For the nickel complex (Figure 8) the bands atc. 12 000, 19 000 and 23 000 cm1 are assigned as transitions from B1 (F)to 3E(F), 3A2(P) and 3E(P). The very broad shoulder around 9000 cm1 isassociated with the excited states A2 (F) and B2 (F).

Adducts with tridentate ligandsThese complexes have the general formula MX2L where X =Cl

Br, I, NCS and L is a polyamine, Schiff-base or pyridine derivative containingN, 0 or S donor atoms. They are generally made in scrupulously dry solventsof low solvating power, such as high boiling alcohols, are soluble in organicsolvents, and do not tend to dissociate.

100


Ligands of the type shown in Figure 9 (dien) have been used by Ciampolini33and by Gray34 to give many five-coordinate complexes. All the complexeswith R = Me are five-coordinate in the solid state and in solution33. Thecompounds MBr2 (Me5 dien) are all isomorphous. The NiC12 (Et4 dien)is a square-planar complex which should be written as [NiC1 (Et4 dien)]C1.

E00>L.

CLU

0CCe

0-o

20

10

0

4A2

4E4B2

4E2

0 04 08"Dq",103cm1

12 0•5 10 15("Dq")ax/("Dq") base

Figure 7. Energy levels diagram for Co2 in square pyramidal fields4. Black circles indicatethe frequencies of the band maxima in the reflectance spectrum of

[Co(C104)(OAsPh2Mó)4](C104) 32

In organic solvents such as acetone it is partially undissociated as a 5-coordin-ate species. The complete structures of two of these have been determined:

101

I I5 10 15 20 25

Frequency, i03 cm'

Q)U

I-0U).0

Figure 8. Energy levels diagram for Ni2 in square pyramidal fields4. Black circles indicatethe frequencies of the band maxima in the reflectance spectrum of

[Ni(C104) (OAsPh2Me)4] (dO4)

CH2—CH2 CH2—CH2/R2N Y

MX2L, X = Hal., NCS

Y=NMe;R=Me;M= MntoZn0; R= Me; M == Fe to Cu

Y=S;R=Me;M=Co,NiY = NH; R = Et; M = Go, Ni(sln)

Figure 9. Some tridentate ligands

102

20

LUIGI SACCONI

(Dq") base 09 i03 cm1

3E3A2

0>;0)a)

LU

3E

3E

B2A2

A2______________3E 3E

3B1

0 04 08 12

'Dq 1O3 cm-105 1'0

(Dq') ax/CDq) base15

5 10

I I I I I15

Frequency, i0 cm-120 25

NR2

Ref.3344

34


CoC12(Me5 dien)35 and CoC12(Et4 dien)36 (Figure 10). Both have inter-mediate structures, but the first is more distorted on account of the sterichindrance produced by the central N-methyl group.

Schiff bases of the general formula shown in Figure 11 are formed fromN,N-disubstituted ethylenediamine and o-substituted benzaldehyde. Accord-ing to the nature of Y donor atom, the set of donor atoms may be NNN,ONN, and SNN. When NR2 is a diethylamino group the ligands aresuitably disposed for the formation of five-coordinate adducts. The nickelhalide adducts, however, give a mixture of five-coordinate and tetrahedralspecies in non-dissociating solvents. Cobalt(ii) salts, on the other hand, formfive-coordinate adducts only when Y NH; the ligands with and S9as the third donor atom give tetrahedral complexes with this metal.

H2C—CH2

CH=N,N—R2 RnMe, Et

Ki"Me MX2L, XHa(

Figure 11. Some tridentate Scuff bases

Some pyridine derivatives such as those shown in Figure 12 contain a set ofthree nitrogen atoms which are potential donor atoms. The complexesthese ligands give differ from those of dien derivatives because in solutionthey maintain their five-coordinate structure. This may be equated with thepossibility of back-bonding to the pyridine nitrogen atoms, and with theformation of a more stable 5-membered chelate ring. The metals which givefive-coordinate complexes with these ligands are also shown in Figure 12.

The complexes of Mn, Co, and Zn with paphy are mutually isomorphous32.CoCl2-paphy complex has an intermediate structure42 (Figure 13).ZnC12.terpyridyl has a distorted trigonal bipyramidal structure43 and theCo, Ni, and Cu complexes are isostructural with jt32 (Figure 13). Thecomplexes [CuX(dipy)2]X and [CuX(phen)2]X (X = halogen) are well

103

Et

'Me

Figure JO. Structures of the compounds CoCls(Mesdien)35 and C0CI2(Et4dien) 36

LUIGI SACCONI

known44 and x-ray analyses give the distances in [CuI(dipy)2]I asCu—N 2O2 A, and Cu—I 271 A5 (Figure 13). The trigonal bipyramid isfairly distorted.

Adducts with tetradentate ligandsThese compounds are adducts of the simple salts only in terms of the

empirical formula, since the structural formula is [MLX] X. The structuredetermination on the copper(n) complex with tris (2-aminoethyl) amine

(CH2)2,3 40 ri 38

CH=N N—R2 Me Me\ /

Ni,Co Ni,Co

PyAenNR2 MX2L PyA(NR)2PyAtriNR2

32

Co,Ni,Cu Mn,CoZn

OPEA paphy

Figure 12. Five-coordinate complexes with N,N,N-tridentate ligands

(tren) [Cu((H2N—CH2CH2)3N)NCS]NCS, ([Cu(tren)NCS]NCS) revealeda trigonal bipyramidal structure46 (Figure 14). The equatorial anglesand bond lengths were unexpectedly found to be unequal, giving thestructure a slight distortion. Co(NCS)2 and Co12 also give five-coordinateadducts with tris-(2-aminoethyl) amine47. With nickel(iI) however tren givesoctahedral adducts48 which persist in solution49, and a similar situation is notunlikely with Mn(ii) and Fe(ir)50.

The hexamethyl derivative of tren, (Me6 tren), forms only five-coordinatespecies with the divalent 3d metal halides'2. Each set of halide complexesforms an isomorphous group. The structure of [CoBr(Me6tren)]Br isshown in Figure 15; it is a trigonal bipyramid with C3 symmetry'3. The metalatom is situated 032 A below the equatorial plane so that apical angle is 810.In non-coordinating solvents the compounds of this series behave as 1:1electrolytes, so maintain their five-coordinate structure12. Presumably thegreater steric hindrance provided by the equatorial methyl groups does notpermit the close approach of the sixth ligand that can occur with tren itself.

104


Figure 13. Structures of the compounds CoC12(paphy) 4, ZnC12(terpy) 3, and [CuI(dipy)2]I45

Cu—NCS 195X

Cu— N(av) 2O6

N1—Cu— N5177°

N2—Cu— N3N3— Cu—N4113—114°

N2—Cu —N4 1300

Figure 14. Structure of [Cu(NCS)trem] (SCN)46

Tris(2-methylthioethyl)amine, [(MeSCH2CH2)3N], (set NS3) forms acomplex with cupric bromide which is a 1:1 electrolyte in organic solvents4.This compound probably has a bipyramidal structure.

105

94.

(S

>,I-CUi

50

'300

100

LUIGI SACCONI

Figure 16. Energy levels diagramlSb for Cu2 and Cr2 in trigonal bipyramidal fields. Blackcircles indicate the frequencies of the band maxima in the spectra of

[CuBr(Meotren)JBr 12a and [CrBr(Meatren)]Br 12b

106

Figure 15. Structure of [CoBr(Meatren)jBr '

—1

Frequency, i03 cm1


Using the methods of crystal field theory, Ciampolini has calculated thesplitting of the terms of maximum multiplicity in fields of D3h and C3symmetry4' 12b, 51 for the configurations d, d6, d7, d8, and d9.51a. In thespectra of the complexes [MX(Me6tren)]X (M = Cr(ii), Fe(II), Cu(ii))which have true C3 symmetry, two bands attributable to d—d transitionsare observed. These are assigned to the two transitions between the threelevels (A1 + 2E) derived from the lowest D termSlb. The spectra and energylevel diagrams are given in Figures 16 and 17.

Figure 17. Energy levels diagram12" for Fe2 in trigonal bipyrarnidal fields. Black circlesindicate the frequencies of the band maxima in the spectrum of [FeBr(Me6tren)JBr

The spectra of the Co(ii) and Ni(ii) analogues are shown in Figures 18and 19 respectively. They are much richer in bands and crystal field splittingdiagrams show that many transitions are expected. The frequencies of theabsorption bands fit fairly well to the appropriate energy level schemes.

107

EU

CUi

—1

Frequency,103 cm1

High-spin inner complexesLUIGI SACCONI

Schiff bases formed from salicylaldehyde and mono-, di-, and triaminesmay be 2, 3, 4, and 5-dentate and give five-coordinate inner complexes withthe bivalent 3d metals.

The bidentate N-methylsalicylaldimine forms a five-coordinate zinccomplex in which the phenolic oxygen atom acts as a bridge between two

Frequency, i03 cmFigure 18. Energy levels diagram12 for Co2 in trigonal bipyramidal fields. Black circles

indicate the frequencies of the band maxima in the spectra of [CoBr(MeotrenflBr

metal atoms52 (Figure 20). The dimeric molecule can be described as beingmade up of two distorted bipyramids sharing an edge, though M—O bondswhich hold the dimer together are longer than the M—O bonds in the chelaterings. The analogous complexes of Mn and Co are isomorphous with thezinc complex, and so presumably are isostructural.

The and f3 forms of bis(N-methylsalicylaldiminato)nickel(ii), which areboth planar and low-spin, dissolve in the crystal lattice of the zinc complexand assume the molecular structure of the host complex53: the nickel becomesfive-coordinate and paramagnetic (Iteff = 31 B.M.) with the characteristic

108

E

1E 10U

CU0

05 10"Dq cm1

15 20 25


spectrum of high-spin five-coordinate nickel(ii) complexes. The coppercomplex also displays the same structural mimicry53.

The copper complex, besides having the two crystalline forms and 1 inwhich it is planar, has a third, y form, in which the copper becomes five-coordinate by means of dimerization through bridging oxygen atomsM(Figure 21).

The Schiff bases derived from salicylaldehyde and 2,6-dimethylanilineand 2,4,6-tribromoaniline give cobalt(ir) complexes which accept only onemolecule of pyridine because of the steric hindrance of the substitutedaniline moiety55. These high-spin five-coordinate species persist in solution.

The diethylaminoethylsalicylaldimines ([X—Sal—en----N(R)R']) arepotentially tridentate, but the diethylamino group is rather bulky so thatonce one molecule is attached to the central metal, there is sometimes roomonly for two other coordinating centres56. The two donor atoms may be:

109

EV

>,

a)

Iii

15

Frequency, iO crnFigure 19. Energy levels diagraml2a for Ni2 in trigonal bipyramidal fields. Black circles

indicate the frequencies of the band maxima in the spectra of [NiBr(Me6tren)]Br

(LUIGI SACCONI

N;:-O\ / ———

N1M = MnCoZn,Ni,Cu

Figure 20. Structure of [Zn(H—Sal—N-—-Me)2]52

(20)-N

,N -_____ p

Cl

0-H- N/(I \ Ni—

,,,/' /N- ' I

lcHoFigure 22. Structure of Ni[5C1—SaI—en-—N(Et)2]257 and

[Ni(H—Sal—en—N——Et2) (catec) 2]

110

Figure 21. Structures of [Cu(H—Sal—N—Me)2]54

Et2


(a) Oxygen and nitrogen from a second molecule of the Schiff base. Forexample, the complex [5-Cl----Sal---—enN(Et)2]2Ni has been shown to havea distorted square pyramidal structure57 (Figure 22). The bonded diethyl-amino group is 02 A farther from the nickel than the other donor groups,because of the steric interactions of the two ethyl groups. The other diethyl-.amino group is not bonded to nickel at all. The nickel atom is 036 A abovethe mean basal plane giving an apical angle of 101°. An isomorphous cobaltcomplex can be obtained58.

(b) Oxygen atoms from pyrocatechol. The isomorphous complexes[X—Sal—en-—-N(Et)2](C5H602)Ni and Co are dimeric and consist of twodistorted bipyramids sharing one edge59. One oxygen atom from eachcatechol moiety is bridging, the other in the form of a hydroxyl group ishydrogen bonded to the salicylaldehyde oxygen atom (Figure 22).

The tetradentate Schiff bases formed from two molecules of salicylaldehydewith one of ethylenediamine or C-methylated ethylenediamine form five-coordinate complexes with copper. The former is dimeric with bridgingoxygen atoms60 and the latter is a monoaquo adduct, in which the watermolecule lies in the axial position of a square pyramid61. The copper is02 A above the basal plane, giving an apical angle of 96°.

When two salicylaldehyde groups are linked by the ends of a dipropylene-triamine molecule, [HN(CH2CH2CH2NH2)2], or N-methyl derivative,[MeN(CH2CH2CH2NH2)2], pentadentate ligands result (Figure 23). Five-coordinate complexes are formed with all the metals from Mn to Zn62. Themethyl salicylaldehyde derivatives are all isomorphous. The nickel complex

R=H, Me

R

has a distorted bipyramidal structure in that the equatorial angles are not120°.63 In particular the O—Ni——O angle is 142°, a value that one finds in anumber of compounds with this stereochemistry. The n.m.r. spectra insolution, showing two peaks for each ring position, are in agreement withthe existence of two unequivalent aromatic fragments as shown by thestructural data (Figure 24)64. The nickel complex forms octahedral adductswith bases like pyridine, but the cobalt complex does not62, even though itcan function as an oxygen carrier.

111

Ficure 23. The anion (Sal)s—R—DPT and the structure of [Ni(H—sal—MeDPT)]63

LUIGI SACCONI

Vanadyl complexesOxovanadium(iv) cation may form five-coordinate complexes with

-diketones, particularly acetylacetone and their imino derivatives as wellas with Schiff bases65. Recently five-coordinate oxovanadium(iv) complexeswith Schiff bases of salicylaldehyde and N-substituted ethylenediamine have

6H

—1516 —1488 —890-872 —43 ÷35 +710 +794

—1445-1417 -932-920 ÷715+785

Figure 24. P.m.r. traces for ring protons of (A) {Ni(H—Sal-—DPT)],(B) [Ni(H—Sal—MeDPT)], and (C) [Ni(5C1—Sal—MeDPT)J in CDCI36

been described66. The spectrum of the complex deriving from N-phenyl-ethylenediamine is reported in Figure 25. According to treatment of Selbinand Morpurgo67' 68, the three low intensity bands between 10 000 and20 000 cm—1 are assigned as d—d transitions from the ground state levelto the excited state levels d) d2_2 and 4s. This scheme is known asa clustered level scheme since the first three levels are clustered togetherwith the next level, in a closely-knit group. Bands above 20000 cm-1 areassigned as charge transfer from the bonding (mainly an oxygen orbital) tothe half-filled vanadium orbital.

Complexes with sulphur chelatesThe sulphur containing ligands which form five-coordinate complexes

are all bidentate and are dialkyldithiocarbamates69, diethyldithiophosphate(DTP) 70, ethyixantate (EX) 70a and dithiolates71' 72 (Figure 26). Five-coordination is attained by means of dimerization of the ML2 componentsor the addition of a monodentate ligand. The magnetic moment of thecomplexes so far isolated is not known. Cr, Mn, Fe, Cu, and Zn formisomorphous dimeric, five-coordinate complexes with diethyldithiocarbam-ate69. The dipropyithiocarbamate of copper is also square pyramidal withapical angle 100°.

112

FIVE-COORDiNATION IN 3d METAL COMPLEXES

Five-coordinate adducts are formed by ferric dialkyldithiocarbamateswith halide ions, [FeX(S2CNR2)]74 and by cobalt (I') dimethyldithio-carbamates with nitric oxide, [Co(S2CNMe2)2N0J75. The configuration ofthis complex (Figure 27) is square pyramidal with the cobalt 054 Aabove

- d2

______ -- d22

12 34__flx dxy

—x———_ -UU)

>___ —-—XX Filled•xx

fbonding

________ ________jlevels

Weak a-and Strong a-andir-bonding iT -bonding

5

Figure 25. Energy levels diagram for oxovanadium(iv) complexes67. Spectrum of[VO(H—Sai—en—N(H)Ph)2]66

the basal plane. The NO group is inclined at 139° to the pyramid axis andis bound to the cobalt by an unsymmetrical r bond9. The nickel(ii) diethyl-dithiophosphate and ethylxanthate complexes form five-coordinate adductswith pyridine and quinoline and other organic bases, in solution70.

The cobalt(ii) dithiolate with R = CF3 is dimeric in carbon tetrachiorideand in the solid state. The structure is square pyramidal and is shown in

113P.AC.—I

10 15 20Frequency,103cm1

5-\ /N—C/ \

Alk S

EtO 5-P/\

EtO S

R RN // N-s 5-(R CF3, CN, etc.)

Figure 26. Some sulphur chelating anions

F3C -JJSCCF

F3C srccC9rts'.___C1

CF3

F3CF

_':'—'"CF3

Figure 27. Structures of [Co(NO)(S2CNMe2)2] and Co(dithiolate)272

Figure 27. The apical angle is 1000.72 With the dinitrilo dithiolate (R =CN)the cobalt becomes five-coordinate only by the addition of pyridine, NPh3,PPh3.71 These dithiolate complexes are the first examples of diamagneticcobalt(ui) though it is well known that metal oxidation states in dithiolatecomplexes are somewhat difficult to define. The electronic configurationmay more consistently be regarded as d8, corresponding to cobalt(i).

114

LUIGI SACCONI

Alk

Dialkyldithiocarbamates

Diethyldithiophosphate

Ethylxanthate

5-./EtO=C

S

Dithiolate

:0ft

104 1/20


FIVE-COORDINATE COMPLEXES WITH LOW-SPINLow-spin complexes are only possible with d5, d6, d7, and d8 configurations.

The ligands which give low-spin complexes of this type usually have donoratoms of low electronegativity such as P, As, or C capable of forming ir bonds.

Complexes with unidentate ligandsThe types of complexes found in this category are summarized in the

Table 4. Diphenyiphosphine appears to have the optimum size for theformation of five-coordinate complexes with nickel and cobalt halides.

Table 4. Five-coordinate complexes of nickel and cobalt withunidentate ligands

MX3(PR3)276; X = Cl, Br; R Alkyl, etc.M = Co, 1eff = 3B.M.; M = Ni; Jheff 2 B.M.non-polar, probably trigonal bipyramidal complexesM"X2(PHPh2)37' 78, X = Br U M = Co Ni[Co'(CNCHs)s]'7; [Co"(CNPh)5]2 79;[Co"Xs(CO) (PEt3)280; {Co"X2(NO) (PEt3)2]8°;[Co'(CO)3(PEt3)s]+ 81; [Co'(CO)s(PPh3)X]82

Co(PHPh2)sBr2 and Ni(PHPh2)312 have been shown by x-ray methods tobe distorted bipyramids with phosphorus atoms in the apical positions(Figure 28)83. These complexes have normal low-spin magnetic momentsexcept the compound Ni(PHPh2)aI2.77 78 This compound has a momentwell below the value required for two unpaired electrons which has beenattributed to a thermal population effect involving the singlet ground stateand a low-lying triplet excited state. Cobalt(i) and cobalt(u) give isocyanidecomplexes [Co(CNCH3)5]+'7 and [Co(CNPh)s]2+ .

The ion [Ni(CN)5]3— which has long been known to exist in solution84has recently been isolated by Raymond and Basolo85 in the form of thecompounds [Cr(NH3)6] {Ni(CN)5].nH2O and [Cr(en)3] {Ni(CN)5].nH2Oand a potassium salt has also been obtained by low temperature crystalliza-tion. The unit cell of[Cr(en)3] [Ni(CN)5] 15 H20 contains two [Ni(CN)5]3—units: one is a tetragonal pyramid and the other is a distorted trigonalbipyramid (Figure 28).86 This bipyr amid is flattened because the apicalbonds are shorter than the equatorial bonds, and in the equatorial plane theangles are 1410, (l095° and 109.5°) similar to those noted before. In thesquare pyramid the mean apical angle is 1000 and the apical bond is longerthan the others.

Complexes with bidentate ligandsThe greatest number of low-spin 3d five-coordinate complexes are formed

by polyfunctional ligands. The following bidentate ligands have been knownto give five-coordinate complexes. Some of these are shown in Figure 29.The DAS complexes described by Nyholm were the first low-spin five-coordinate complexes of nickel (ii) to be discovered88.

115

LUIGI SACCONI

jBri

P1—Co —P2 =176°

P1—Co __P3 = 900

P3—Co —Br2=136°

P3—Co—Br= 98°

Br1—Co —Br2=126°

Figure 28. Structures of CoBr2(PHPh2)383 and [Cr(en)3][Ni(CN)5].15H2O 86

Set ofdonors Ref.

PPh2

J__-AsPhz

PPh2

e

AP PAs 89

Complexes with tridentate ligandsThe following tridentate ligands can give five-coordinate complexes

(Figure 30). They form low-spin monomeric complexes with nickel halides

116

N

IIIC

N

N

N

C

N

N C

IllN

Ph2P—CH2-—CH2-—PPh2 OPPE

AsMe2

KI_AsMezDAS

PP 87

AsAs 88

PS

Figure 29. Some bidentate ligands

PS 89


Set ofDonors Ref.

CH2—CH2—CH2—AsMe2

PhP DAP PAs2 89

CH2—CH2-—CH2-—AsMc2

CH2—CH2—CH2—AsMe2

MeAs" TAS As3 90

CH2—CH2----CH2---AsMe2

CH2—CH2—CH2—PPh2

CH2—CH2—CH2-—PPh2

CH2—CH2—CH2—AsPh2

s" AsSAs SAs2 38

CH2—CH2—CH2—AsPh2

o-C6H4—SMe

PhP" DSP PS2 89

o-C6H4—SMeFigure 30. Some tridentate ligands

of the general formula NiLX2. These complexes are generally more stablethan those involving only bidentate or monodentate ligands. They arenon-electrolytes though in some instances a slight dissociation occurs withthe production of some square-planar species, as for example [Ni(PSP) '2].The structure of [Ni(TAS)Br2] was one of the first five-coordinate structuresdescribed by x-ray analysis92.

Complexes with quadridentate ligandsFive-coordinate complexes containing quadridentate ligands are the

most stable and numerous. They are of the type [MLX]Y where X = Cl,Br, I, NO3, Cl04, NCS and Y = Br, I, Cl04, BPh4, etc. The ligands so farused are given in the Table 5.

All these ligands give five-coordinate complexes with nickel(u). QP96 andPTAs93 with cobalt(ii) and QP98 with iron (Ii). These complexes are intenselycoloured, 1:1 electrolytes in organic media and stable five-coordinate. Thegeometry of the ligands should give the complexes a configuration which isnot far from trigonal. Indeed a recent x-ray study has shown the trigonalbipyramid to be regular in the compound [NiTAP(CN)](Cl04)'°' (Figure31). Tris(o-methylthiophenyl)arsine (TSA, set AsS3) does not react with thenickel halides89.

The spectra of [M(QP)X]+ with M = Fe, Co, Ni have been studiedrecently by Norgett, Thornley and Venanzi102. They have derived crystalfield energy level diagrams for D3h symmetry and on the basis of a crystalfield model the bands in the region 5000—26 000 cm1 were assigned asd—-d transitions. Their high intensities (emax 500—5000) are probably due tomixing of odd configurations of higher energy.

117

(zQ)

probable stereochemistry

118

LUIGI SACCONI

Table 5. Some tetradentate ligands

CH2—CH,----CH2—-ASPh,/Z—C112—CHS—GH,—ASPh,

CH2—CH,---CH2—ASPh,Z = P (TAP, set PAss)9' = As (TTA, set As4)99

Z = P; Y = PPh, (QP, set P4)94. ' '; = SMe(TSP, set PS,) SeMe (TSeP, set PSe,) 100;= AsPh, (PTAs, set PAss)Z = Sb; Y = AsPh, (SbTAs, set SbAs,)97;Z = As; Y = AsPh, (QAs, set As4) 94,96(MXL)+; M = Ni" (all of the ligands);= Go" (with QP and PTAs); =Fe" (with QP)

Me

Me

Me

Me

AsTNi—As2=1196°As1— Ni—As3=11B1°

As2— Ni As312O3°As1— Ni—P = 94.70

AsrNiP = 94.40

As3—Ni—P = 9500P — N—C1=1784°

Figure 31. Structure of [Ni(CN) (TAP)J (dO4) 101


The spectrum of [Fe(QP)NOs]+ is given in Figure 32 with the pertinentenergy level diagram. The assignments made are: (e")4 (e')2 —- (e")3 (e')3at 9000 cm—1; -÷ (e")4 (e')' (a,') at c. 18 000 cm—'; -÷ (e")3 (e')2 (ai') at25 000 cm—1 (shoulder).

Figure 32. Energy levels diagram for Fe2 in trigonal bipyramidal fields and absorptionspectrum of {Fe(NO3)QPj 102

[Co(QP)NO3] + shows at least four bands at c. 10 000, 15 000, 20 000 and24 000 cm—1 (Figure 33). The strong-field model leads to the followingassignments: the first band is due to the (e")4 (e')3 —- (e")3 (e')4; the set ofbands between 15 000 and 20 000 cm1 being associated with the transition(e")4 (e')3 -÷ (e")4 (e')2 (a'i). The other one electron transition is probablythe shoulder at 24 000 cm.

The spectrum of [Ni(QP)C1] + shown in Figure 34 has two bands whichhave been assigned to (e")4 (e')4 — (e")4 (e')3 (al') at 17 000 cm' and

119

Q

10 20 30 40

Frequency, lO3cm_1

LUIGI SACCONI

(e")3 (e')4 (al')2. The second band is expected to be weaker on the basisof selectin rules. It appears that the d—d transitions move to lower frequencyfollowing the spectrochemical series: P > As > S > Se > N.

0

4F

-20

ct-40

-60

-80

06

04

02

0><

Figure 33. Energy levels diagram for Co2 in trigonal bipyramidal fields and absorptionspectrum of [Co(NOs)QPJ+ 102

Complexes with hybrid ligandsWe have seen that the ligands which produce high-spin complexes usually

donate through nitrogen and/or oxygen atoms whereas ligands which givelow-spin complexes usually contain P, As, and C, that is bonding donoratoms of low electronegativity. It seemed then interesting to investigate thecomplexing capacities of ligands containing both types of donor atoms. So

120

Frequency,103cm'


03F 40

30

C20

10

—20

—60

-80

)( 10

20 30Frequency, 103cm1

Figure 34. Energy levels diagram for Ni2 in trigonal bipyramidal fields and absorptionspectrum of [Ni(Cl)QP] (ref. 102)

far a lot of complexes formed by this kind of ligands which can be named"hybrid ligands" have been reported in Table 6.

The first type of ligands reported in Table 6 contain N and P or As with setsNP2 or NAs2. The complexes with nickel iodide are low-spin and monomeric.

The hybrid ligands of the second type are Schiff bases containing twodonor atoms of high-spin type and only one of low-spin character38. Thefive-coordinate complexes they form with nickel bromide are all high-spin38.Those with nickel iodide are low-spin only when the set of donor atomscontains NNP and SNP. This seems to confirm that the electronegativity ofthe donor atoms plays an important role in determining the electronic groundstate adopted by the metal.

The third type of ligands is tetradentate with a central nitrogen atom andthree phosphorus or arsenic atoms in peripheric positions. The complexesthey form with nickel are low-spin and show spectra diagnostic of trigonalbipyramidal structure. The cobalt bromide complexes are high-spin38.This seems to indicate that the change from a P4 to a NP3 set of donors caninfluence the spin multiplicity of the ground state only for cobalt. The

121

LUIGI SACCONI

Table 6 Some 'hybrid ligands

CH2—CH, CH,—CH, 36 Y= P, As;YPh2 NiI2L low-spin

R sets: NP2, NAs2

Z=P,As?H2 H, Y= 0, S, NH

CHN ZPh2 NiX2L

I— X= Br high-spin/ X= I high- and low-spin

sets: ONP, NNP, SNP, ONAs NNAs, SNAs

103

NH—CH—- Y= P, TPN, set NP3CH,—CH2—YPh2 Y= As, TAN, set NAs3

X=Hal.M = Co": high- and low-spin

V M=Ni": low-spin

absorption maxima of trigonal bipyramidal low-spin complexes with C,symmetry are shown in Table 7. Substitution in QP of the apical P by Ngives a greater lowering of the frequencies than substitution of the threeequatorial P, for example, by three S. This is presumably due to the apicalposition of the substituted atom (in QP) which causes the highest perturba-tion on the ground state 'A1 corresponding to two electrons in the d2inC3 symmetry.

Table 7

1A,(D) —÷ 'E(D) and 'Ai(D) —- 1E(D) Bands (Cs)for low-spin five-coordinate Ni(ir) complexes

Complex Set (cm') Ref.

[Ni(QP)C1]+ PP, 17500 26600 102[Ni(TSP)ClJ+ PS3 15400 20950 99[Ni(TSeP)Cl]+ PSe3 15150 20300 100[Ni(TPN)C1] NP3 13900 19000 103[Ni(TPN)Br}+ NP3 13600 18600[Ni(TAN)Br] NAs3 13000 17800[Ni(QAS)Br] AsAss 15800 21800

103103102

THERMODYNAMICS OF FIVE-COORDINATE COMPLEXESAn explanation for the scarcity of simple five-coordinate complexes has

been proposed by Nyhoim and Tobe'°4. In the particular case of bivalentand trivalent metals they showed, using an electrostatic model, that the twoprocesses of the type

[MmX5]2- + X— -÷ [M111X6]3-

[MmX5]2- -÷ [M111X4] + X-are exothermic and the disproportionation also occurs with a decrease inenergy in the system.

122


The stoichiometric formula of a compound is no indication of a possiblefive-coordinate structure. For example CoCl2dien is in reality octahedralCodien2 and tetrahedral CoCI4.33 The tridentate PNP ligands form five-coordinate species with nickel halides Ni(PNP)X2 which on warmingdissociate into components of the type [Ni(PNP)X]2[NIX4] in which allnickel atoms are planar or tetrahedral four-coordinate.38 Whereas thecompounds CoX2 (Et4dien) are five-coordinate, the nickel analogues areplanar with the formula [NiX(Et4dien)]X.34

The thermodynamics of the equilibria between five-coordinate complexesand four- or six-coordinate species have been studied in a few different systems.

The diethyithiophosphate and ethyixantate complexes of nickel(II) formfive- and six-coordinate donor acceptor adducts. The formation ofNi(DPT)py is accompanied by JH = —70 kcal/mole and JS = —17e.u. changes?Ob. Graddon has studied the formation of adducts of Cu(acac)2and Cu (etacac) 2 with amines and heterocyclic bases spectrophotometric-ally105. He correlates the formation constants with the steric requirementsof the base. Turco found that the complex Co(PEt3)2(NCS)2 dissolves inpolar solvents to give an equilibrium mixture of tetrahedral and dimericfive-coordinate species'°6. He also studied equilibria of the typeCo(PR3)2(NCS)2 + PR3 Co(PR3)3(NCS)2 and found that the stabilityof the five-cOordinate species followed the order PEt2Ph > PEt3 > PPr3 >PEtPh2 > PEt2C6H11. Yamada has shown that tetrahedral complexes ofcobalt with sterically hindered Schiff bases of the type give mono-pyridineadducts55. On the other hand the five-coordinate complexes (X—Sal—-DPT)Ni and (X—Sal-----MeDPT)Ni also give mono adducts with pyridine62.The equilibria between five- and six-coordinate species were studied inbenzene through the nuclear magnetic resonance64 and optical spectro-scopy62. The 4H values vary from 55 to 88 kcal/mole that is, about halfthe values of AH for the formation of bis adducts. Surprisingly the cobaltcomplexes can behave as oxygen carriers but do not form pyridine adducts62.

Five-coordinate . six-coordinate equilibria are also well known withvanadyl compounds'°7. Vanadyl complexes with Sal—en(R)R' form six-coordinate adducts with pyridine, giving equilibria between five-coordinateand six-coordinate species.

Carlin has measured the enthalpy of formation of adducts of vanadylacetylacetone with a number of nitrogen and oxygen donors in nitro-benzene'°8. The values found are in general correlated to the electron donat-ing power and steric properties of the ligands.

There are a few compounds of cobalt and nickel which may change fromfive-coordinate to four-coordinate when one atom of a mono or polydentateligand becomes detached. Examples are furnished by the compoundsNiX2MOB, NiX2MAB, NiX2PSP, MX2(daes), and NiX2(MSB).

Calorimetric measurements on the two series of compounds[M(Me5dien)Br2] and [M(Me6tren)Br]Br (M = Mn, Fe, Co, Ni, Cu, Zn)give the values of 4H of formation for both the gaseous compounds (Figure35), and for aqueous solutionsl09 (Figure 36). The enthalpy of the reactionMBr2(aq) + Ligand(aq) —- M Ligand Br2(5) increases to a maximum with Cusuch that the order of stability is Mn <Fe Ni <Co <Zn < Cu forfive-coordinate complexes. This order should be compared to the well known

123

LUIGI SACCONI

Figure 35. Relative bond dissociation enthalpies for two series of five-coordinate complexes'°9

0E

cOLi

-10

-20 - -10 -

0

[M(Me5tren}Br] {MBr4]2

-20 -

0 -

M (Me5dien) Br2 EM (dien)2 ]2I I I i I I I

Mn Fe Co Ni Cu Zn Mn Fe Co Ni Cu Zn Mn Fe Co Ni Cu Zn

Figure 36. Relative enthalpy changes for the formation of three series of complexes in aqueoussolution109

124

0 10


sequence for octahedral compounds"° Mn <Fe < Co <Ni <Cu> Znand for tetra-halo-complexes Ni <Fe < Co < Mn < Cu < Zn.111 Thiscan be attributed at least in part to CFSE differences. Maximum CFSEstabilization of exa-aquo ions occurs with Ni, but in the five-coordinate seriesit is expected to occur with Cu.O9 These results may explain why thenon-methylated ligand tren gives five-coordinate species with Cu(ii) andCo(ii) in aqueous solution whereas with Ni(ii) an octahedral complex isformed.

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