4594
SHUPACK, BILLIG,CLARK,WILLIAMS, AND GRAY
T h e coefficient 2 in eq. 16 is necessary because both the forward and reverse reactions of eq. 10 contribute to the observed electron transfer. Since the system is a t equilibrium, the rates of these two reactions are equal. Consequently, the forward reaction of eq. 10 is only responsible for half the observed electron transf e r ~ . ' ~Substitution of k g = 12.1 f 1.6 set.-' and k6 = 57.6 f 2 1W-l set.-' gives kg = 33.4 f 2.6 M-l sec.-l a t 25.0' and ionic strength 3.0 M , X comparison of the magnitudes of these rate constants shows that the electron exchange between F e 2 + and FeC12+ proceeds mainly b y a chlorine atom transfer mechanism (provided, as seems reasonable, the third-order term involving Fez+, Fe3f, and chloride may be neglected). T h e chlorine atom transfer presumably occurs in a chloride-bridged, inner-sphere activated complex, while the electron transfer may occur in either an outersphere activated complex or in a water-bridged, innersphere activated complex. It is of interest t h a t reaction via the chloride-bridged path is only about three (19) J. Silverman, P h . D . Thesis, Columbia University, S e w York, N. Y., 1951.
[CONTRIBUTION FROM THE
DEPARTMENT OF
Vol. 86
times as rapid as reaction via the outer-sphere or waterbridged paths. This small difference, and indeed the relatively small effect of chloride on the iron(I1)-' iron(111) exchange, contrasts markedly with its effect on the iron(11)-chromium(II1) and chromium (11)-chromium(II1) reactions. In the latter systems, chloride bridging increases the rates of the reactions by factors of lo4 and more than 2 X lo6, respectively.ld Moreover, the chromium(I1)-catalyzed dissociation of CrC12+is slower by a factor of more than lo4than the Cr2+CrC12+ exchange.20 T h e difference in the chloride effects cannot be ascribed to any difference in the mechanisms of the reactions since these studies show t h a t i t is very likely t h a t the chloride-catalyzed iron (11)-iron(II1) exchange, like the chloride-catayzed chromium( 11)-iron( 111) and chromium(II)-chromium (111) reactions, proceeds via an inner-sphere activated complex. Acknowledgment.-The authors wish to acknowledge helpful discussions with R. W. Dodson. (20) H. T a u b e and E. L. King, J . A m . Chem. Soc., 76, 4053 (1954).
CHEMISTRY, COLUMBIA UNIVERSITY, N E W Y O R K , N E W YORK
100271
The Electronic Structures of Square-Planar Metal Complexes. V. Spectral Properties of the Maleonitriledithiolate Complexes of Nickel, Palladium, and Platinum BY S.I. S H U P A C KE. , ' ~BILLIG,R. J. H. CLARK,RAYMOND WILLIAMS,'^
AND
HARRYB. GRAY
RECEIVED JUNE 8, 1964 The results of molecular orbital calculations of the square-planar nickel complexes of maleonitriledithiolate are reported. Electronic spectra of the nickel, palladium, and platinum complexes are assigned on the basis of the derived energy levels for the dinegative nickel complex. The observed bands are due to d-d, ligand-tometal and metal-to-ligand charge transfer, and intraligand transitions. The spectral assignments are consistent with the known spectra of square-planar halides, cyanides, and simple derivatives of rnaleonitriledithiolate. The orbital parameter AI is reported for all the complexes, affording a comparison of the x2-y2 --* x y splitting for several complexes containing a nickel-group central metal. Evidence is presented for the assignment of the highest filled orbital a s 4a,(xz-y*) in D 2 h symmetry for the dinegative complexes of the nickel group.
Introduction For several years we have been investigating the syntheses, electronic structures, and reaction mechanisms of square-planar metal complexes. Until very recently, the square-planar geometry was restricted to the diamagnetic, d8 complexes of Ni(II), P d ( I I ) , P t ( I I ) , R h ( I ) , Ir(I), and h u ( I I I ) , except for a few paramagsquare-planar Co(I1) and Cu(I1) comnetic ( S = plexes containing fairly complicated ligands. It is clear that one of the important problems in inorganic chemistry is the preparation and study of square-planar complexes containing as many different transition metals and with as many different ground states as possible. From such research we can hope to help determine the relative energies of the molecular orbitals in planar metal complexes, a problem for which many different solutions have been suggested in the past few years.2 (1) (a) N I H Postdoctoral Fellow, 1963-1964; (b) N S F Graduate Fellow, 1963-1964. ( 2 ) J . C h a t t , G. A . Gamlen, and L. E. Orgel, J . Chem. Soc., 486 (1938). ( 3 ) G. M a k i . J . Chem. Phys.. !28, 651 (1958); 29, 162 (1958); 2 9 , 1129 (1 9.58) ( 4 ) C . J . Ballhausen and A . D. Liehr. J . A m . Chem. Soc., 71, 538 (1959). ( 5 ) LT. I. B a n , A c f a C h i m . A c a d . Sci. H u n g . , 19, 459 (1959).
From the results of our initial theoretical work, we believe t h a t one way to stabilize the square-planar geometry is to involve the metal d,,, d,,, and pz valence orbitals in an e:tensive n-orbital network spanning the entire complex, thus allowing as much delocalization of charge as possible. Two ligands which have good donor atoms and also a considerably delocalized a-orbital system are the dianions of maleonitriledithiol (MNT2-) and toluene-3,4-dithiol ( T D T 2 - ) , which are shown below. Whether or not for the reason given above,
( 6 ) S. K i d a , J. F u k i t a , K. Nakamoto, and R. Tsuchida, Bull. Chem. Soc. J a p a n , 31. 79 (19.58). (7) ( a ) R. F. Fenske, D. S. Martin. and K. Ruedenberg, I n o r g . Chem , 1, 441 (1962). ( b ) D. S . hlartin and C . S. Lenhardt, t o be published. (81 J. R. Perumareddi, A . D. Liehr, and A . W. Adamson. J . A m . C'hrm. So,.,86, 249 (1963). (9) J . Ferguson, J . Chem. Phys., 3 4 , 611 (1961). (10) C. K. J@rgensen,"Orbitals in Atoms and Molecules," Academic Press, Piew York, X. Y.. 1962, p. 114, and references therein.
Nov. 5 , 1964
ELECTRONIC STRUCTURES
O F SQUARE-PLAKAR I f E T A L
COMPLEXES
4595
LINT2- and T D T 2 - have proved to be excellent stabilizers for the square-planar geometry. From studies of the complexes formed by these ligands, to date the following new square-planar complexes have been characterized : a square-planar complex with S = 1 , C o ( T D T)2- l 2
a square-planar rhodium complex with S = 2 , Rh(lIr\’T)22- l 3 square-planar copper complexes with S = 0, Cu(TDT)g-, Cu(RfXT)2l4 square-planar nickel. palladium, and platinum complexes with S = ?, Ni(TDT)*-, Pd(TU)T)*-, Pt(TDT)*-, Ni(11NT)2-, P d ( h f S T ) 2 - Pt(h1NT)Zl 4 l5 1 square-planar iron complexes with S = and F e l T D T ) 2 - , FelSlT\;T)2- l 2 Independent of the LIST2- and T D T * - work, Schrauzer and co-workers16 have made a significant contribution to the solution of the general problem by characterizing many new planar complexes in the I f (S2C2(C6H6)2)2n and related systems Fig. 1.-Structure of the h 3 ( M S T ) 2 2 -complex and coordinate It is important to have a consistent theoretical framesystem for designation of molecular orbitals. work for the discussion of these new square-planar complexes Accordingly. we have carried out molecular orbital calculations on the Ni(51NT)22- and Xi(CHa)dN+ groups above and below the plane. The (LINT)?- complexes It is the purpose of this paper to structure, with bond lengths and bond angles, is shown present these calculations and to discuss the spectral in Fig. 1. properties of the Xi, Pd, and Pt complexes. SubseThe spectral and magnetic properties of r\‘i(IUNT)22quent papers will present the results of our work on are essentially t h e same in the solid and in a number of the LIPiT2- complexes of Co, Rh. Fe, Cu, and Au, and solvents, including good coordinating ones such as the T D T 2 - complexes of Fe, Co, Ni, Cu, Pd, Pt, and pyridine, and we conclude that Ni(MNT)22- is squarexU planar. with negligible axial interaction, in solution Since P d ( M X T ) 2 2 - and P t ( I l N T ) 2 ? - are diamagIt is our hope that these initial theoretical and experinetic, we conclude that these complexes are also squaremental studies will point the way to new ligand sysplanar The R [ N i ( M N T ) 2 ]and R [ P d ( 1 f N T ) 2 ]comtems which will afford even better stabilization of the plexes are also assumed to contain square-planar anions, square-planar structure Thus i t may be possible to since they are isomorphous with diamagnetic R [Cuprepare square-planar complexes of most of the transi(lZNT)2] in the R = (fl-C4Hg)dN+ series. The R [ P t tion metals exhibiting ground states from S = 0 up to (LINT),] complex is isomorphous with R [ A u ( I ~ P ~ T ) ~ ] and including S = 5 f 2 . with R = (C2Hj)&+, and thus P t ( M N T ) 2 - is undoubtedly square-planar.l 4 Experimental T h e complexes studied were prepared by published meth0ds.1~5’~ ;111 t h e solution electronic spectra were taken using a Cary 14 spectrophotometer and quartz cells of 0.01-, 1-, and 10-cm. path lengths. Acetonitrile was the solvent employed for the spectra reported in Tables 111, I\-,and V .
Molecular Orbitals for Ni(MNT),“- Complexes
The coordinate system adopted for the calculation is given in Fig. 1. The metal valence orbitals are 3d, .Is, and 4p. The wave functions for the ligand x-orbitals perpendicuhr to the molecular plane (a,,orbitals) were Structure of the M ( M N T ) 2 n - Complexes obtained from a separate calculation. l 9 The in-plane T h e structure of the diamagnetic [(CH3)4N]2[Ni- ligand functions ( u and a h ) were taken as hybrids of the 3s and 3p sulfur orbitals, with one-third s- and two(LINT)*] complex has been determined by X-ray thirds p-character. diffraction techniques. l8 T h e I\;i(lfKT)22- anion is The basis functions and group-overlap integrals in truly planar in the solid, with only the noncoordinating the D 2 h symmetry are given in Table I. The calcula(11) H. B . G r a y a n d C . J . Ballhausen, J . A m . Chem. Soc., 8 5 , 260 (1963). tion of the energies and valence-orbital coefficients of (12) H. B. G r a y a n d E . Billig, i b i d . , 86, 2019 (1963). the molecular orbitals was carried out by a method (18) E Billig, S I . Shupack. J. H \ V a t e x , R . Williams, a n d H B . Gray, t b r d . , 86, 926 (1964). which has been described in detail elsewhere.2n For (11) A . Davison, N. Edelstein, R. H. H o l m , and 4 . H . M a k i , Inovg. both Ni(Mh’T)22- and h’i(lfNT)2-, an iterative proC h r m . 2 , 1227 (1963). (1.5) R . Williams, unpublished results. cedure was used until the coulomb integrals used in the (16) ( a ) G N Schrauzer a n d V. Rlayweg. J . A m . Chem. Soc., 84, 3221 calculation corresponded to the extrapolated values (1962). ( b ) G K. Schrauzer. V. Mayweg, H . W. F i n c k , U. Rluller-Westerappropriate for the final calculated charge distribution h o f f , and W . Heinrich, A n g r w . C h e m . , 76, 345 (1964), ( c ) G. S . Schrauzer and V. hlayweg, Z .Vat?ti,fwsch.,19b, 192 11964). and orbital populations. ( 1 7 ) E. Billig, K XVilliams, I Bernal, J. H . Waters, and H . B . G r a y , Cheltl , 3 , 663 (1964) (18) R . Eisenherg, J . A . Ibers, R. J . H. C l a r k , and H . B. G r a y , J . A m C h e w Soc.. 86, 113 (1964).
ZIlOVg.
(19) J. Halper, \V.D . Closson, a n d H B. G r a y , t o be published. ( 2 0 ) ( a ) A . Viste and H . B . G r a y , I n o v g . C h i n . , 3 , 1113 (1964) C. J. Ballhausen and H B. G r a y , ibid., 1, 111 (1962).
fb)
SHUPACK, BILLIG,CLARK,WILLIAMS, AND GRAY
4596 TABLE I
TABLE I1 CALCULATED ORBITAL ESERGIES"
BASISFUSCTIOSS ASD GROUP-OVERLAP IXTEGRALS FOR Si(MNT),2 Irl-educi ble representation in Dxh
Vol.
> 0I
hletal function
G,,b
a,
3d2 4s 3dz9_y 2
0 128 0 857 0 078
a,
...
;jas 3b3u 3b2" 5biu 4au 5ba~ 5bPK 4b1u
0 239
0 000 0 159 0 224
4b3, 3% 3b1~ 4% 4b2g 2bi, 3b1~ 2b2, 2b3" 3bag
0 049
3% 3bg 2% 1bau 1b,u 1b1, 1%
0 058
2au 2b3g
0 267 0 044
0 035
0 010
2b1" 2b2, 1bag la, lb,, 1b1u
Energy (103cm. -1) for N i ( > I S T ) 2 2 -
518 3 145.2 145 2 -14 0 -36 2 -36 2 -48 3 -48 5 - 70 0 -70 4 -75 0 -94 5 -96 2 -100 0 - 100. 1 -100 0 - 100 0 -106.2 -107.9 -110 5 -113 2 -110 3 -110.3 -130.6 -111.6 -1.51 6 -152 9 -163 5 -163 8 -177 4 -177 4 -186 4 -186 5
509 0 133 4 133 4 -12 8 -36.2 -36 5 -48.3 -48.4 -70.0 -70.4 -73.2 -93 1 -91.8 -100.0 -100.0 -100.0 -100.0 -101 7 -103 2 -107 1 -109 7 -110.0 -110 0 -127 9 -I39 7 -151.6 -152.7 -163.6 -163.7 -177.4 -177 4 -186 4 -186.6 Ni charge
Input S i ( M S T ) r 2 Output S i ( M S T ) a 2 Input S i ( M S T ) , Output S i ( h l S T ) a -
E n e r g y (103 cm. -1) for Xi'SISTjn-
0 25 0.27 0.25 0 26
k7i configuration c18.85s0.86
d8. asso.
P0' 04
8ipn.
dR.77sil.81p0.
n4 14
dS.fiuso.a6pn.zo
Coulomb integrals for the valeiice orbitals and eigenvectors for the MO's are given in Tables 1.111 and I S in the ;\ppendiz The absurdly high energies of 3h?,, 3bn,, and *5aEare of course Radial wave functions used are given in Table 1-11 (-1~-due to the neglect of higher orbitals on Si and S. pendis). T h e atl orbitals are in the plane of the molecule. The eight ar orbitals ( 1 T ~2aV, , 3aV,e t c . )are the approsiniate a-molecular orbitals perpendicular to the plane of the molecule, obtained intense and are very likely due to allowed transitions. by separate calculation. In order of increasing energy, they are The band a t 11,690 c m - ' is very weak and therefore is 1 r v , %a,-,37r, , ? notation ~ for the most , ST,.. T h u s l ~ ~is the assigned as a parity forbidden d-d type transition. stable ligand T,. orbital which in the metal coniples includes sul'The allowed bands are due to charge-transfer transitions fur atoms 1 and 2 , Eigenvectors of the sulfur p n orbitals in the (11+ L or L --t 51) or to transitions localized o n the rV orbitals are given in Table 1.11 (.ippendis). Bond distances used are given in Fig. 1. ligands (L + L*).
Table I1 gives the calculated energies of the molecular orbitals, and the effective charge and orbital population on Ni, for both Ni(lIXT)2?- and Ni(hINT)2-. The levels of greatest spectroscopic interest in this study are shown in Fig. 2, The ground state of Ni(hINT)22- is, according to our diagram, , , .(4bzg)?(4ag)?= 'Ag. The separation of the highest filled level (4a,) and the lowest empty level (3bl,) is called A I ; since these levels have the symmetries x*-y? and xy. 41 here is analogous to the A1 (xy H x 2 - y y ) splitting in planar tetrahalides a n d tetracyanides.' I
First, a general assignment of the spectrum in terms of d-d, L + 1[,11 + L, and L + L* transitions will be attempted, after a consideration of the spectra of square-planar halide and cyanide complexes, and a suitable derivative of LINT", bis(methy1thio)maleonitrile. Second, the detailed band assignments will be made, using the molecular orbital calculation as a guide. d-d Type Band.-Following the assignment'" I ' of the first spin-allowed band in simple square-planar complexes, we assign the ll,GSO-cm-' band as the inplane d-d type transition, 'A, lB1, [4ag :3blK(x2-y ? + xy)]. -4ssuming F 4 = 8 0 c m - ' (F2 = l(l/:, = SO0 c n - I ) . the experimental AI value of N i ( l I X T ) ? ? - -is 14,490 a n . The calculated separation of ;3blKalld l a , is 19,900 cm.-', in fair agreement with experiment. + .
Electronic Spectrum of Ni(MNT)22T h e electronic spectrum of N i ( l l N T ) ? ? - is shown in Fig. 3 . There are five maxima in the region between 10,000 and 50,000 cm.-'; four of the bands are quite
+ .
Nov. 5 , 1964
ELECTRONIC STRUCTURES
OF
L +- L* Band.-The first band in the spectrum of bis(methy1thio)maleonitrile occurs a t about 30,000 c1n.-l.l9 Thus we assign the band that appears a t 3 1 , 8 0 0 cm.-' in Ni(l\lNT)**-to a L + L* transition. T h e detailed assignment is 'Ag lBqu (3bl, 4bag), with a calculated one-electron energy of 30,000 cm.-l, in good agreement with the experimental result. Charge-Transfer Bands.-The charge-transfer transitions in square-planar halide complexes are known to be hf type." Typically the complexes exhibit of the L two such bands, separated by approximately 10,000 cm.-', with the higher energy band being considerably more intense. Thus, we assign the bands a t 26,400 and ;37,OOO cm.-' as the L + h1 charge-transfer transitions. The detailed assignments suggested from the calculation are as follows: the 26,40O-cm.-' band is 'A, IBz,, 'B3, (2bau, 2b2, + 3bl,), which is a transition from inplane "ligand" n-orbitals to the 3blg(u*) "metal" molecular orbital; the 3,000 cm.-' band is lX, ]Bo,. lBQu(IbQu,lbzu 3bl,), which is the strongly allowed transition from "ligand" u-bonding orbitals to the 3bl,( u * ) molecular orbital. The calculated oneelectron separations are 26,SOO and 36,500 em.-' for the two transitions, in good agreement with the observed band positions. T h e band system with a maximum at 21,000 cm.-' and two shoulders, a t 17,500 and 19,250 cm.-', remains to be assigned. Lye recall that the 51 + L chargetransfer transitions in ?uTi(CX)i2-form a closely spaced, three-band system, and thus assign the band maximum a t 2l,(JO(Jcm.-' as the first allowed 11 + L transition in Si(AlNT)?*-, lX, --t 'BaU (Abap 3au). T h e transitions responsible for the shoulders on the 21,000-cm.-' band cannot be assigned unambiguously, since both dd and orbitally forbidden 11 + L transitions should appear in this region. The detailed assignments for the three-band system suggested by the calculation are as follows: 17,500 cm.-', I X g + lBa, (4bPg --t 3bIg); 19,250 cm.-', 'A, lAI,(dag +- ?,a,); 21,000 em.-', 'A, lB?u (4b?, 3a,).
SQUARE-PLANAR METAL
4,197
T
+-
+
COMPLEXES
I'
-+
\1
3
4a
2 2 (x -Y )
+-
-
-
h M
m
5
-
-
+-
+-
TABLE I11
Fig. 2.--,4
diagram of the most important energy levels in N i ( M S T j o n -complexes.
its simple derivatives. The comparison of experimental and theoretical results is given in Table 111. Electronic Spectra of Pd(MNT)22- and P t ( M N T ) 2 ? -
THEELECTROSIC SPECTRUM OF S i(M S T ) 2 2 -
The spectra of P d ( M N T ) ? * - and Pt(SfIKT)22- are given in Table 11'. These spectra are similar to the spectrum of Xi(l\lNT)22-,and the assignments given in Assignments cm. cm. Table IV were made by using the N i ( M N T ) ? ? - spectrum as a guide. The first spin-allowed d-d type band 11,690 30 'A, 'Big (4a, 3hlg) 17,O0Ob l i , 3 0 0 (sh) ,570 1.4, lBge (4h2, Zbl,) 17,600* is assigned as the x2-y2 --t xy transition. Each comIlr,2.50 ( s h ) 1,250 LA, 'An ( l a , 3a,,) 1 7 ,60OC plex exhibits two intense bands spaced by cn. 1 1 , 0 0 0 21,000 .'3 ,800 1.4, 'BgU ( 4 b q + 3 a u ) 18.400b 26,400 6,800 'A, 'Bzu, 'B3u (Zhau, 2bru 3big) 26,800 cm.-'. assigned as the allowed L + 11 transitions, an 31 ,300 30.000 'A, IRzu, lBgU (Shiu 4bag) 30,000 intense band in the neighborhood of 30,000 cm. - I , as37,000 50.000 'A, lBztl. 'BaU (Ibau, lbz,, 3hiK) 3 6 . 8 0 0 signed L -+ L*, and a slightly less intense band between Measured in acetonitrile solutions of [ ( ~ Z - C ~ H ~ ~ ~ S20,000 ] ~ [ S and ~ - 30,000 cm.-l. assigned as the first allowed ( l I S T ) 2 ] . Corrected for interelectronic-repulsion energy, 51 L transition. The results will be presented acusing F? = 10Fa = 800 cm.-'; see ref. 11. Correct relative cording to the transition type. energy of orbital is ca. -888.000 cm.-' from t h e position of d-d Type Bands.-The IXg IB1, band in Pdthe l.XK + 'BI, hand. (-MNT)2*- is observed a t l5,iOO cm.-'. Assuming E', = 60 em.-' (Fz= 10F4 = GOO c m . ~ - I )A, , for P d ( l f N T ) ? ? ~ In summary then. the observed electronic spectrum of is 17.800 cm.-I. This represents a 23% increase in AI r\;i(XlST)22-is composed of the x2-y2 + xy band, the in going from N i ( l l N T ) 2 2 - to P d ( l l S T ) 2 ? - - ,a very two L + l f charge-transfer bands shown by squarereasonable result. planar halides. the 11 L band system shown by Both the singlet- and the triplet-excited states arising square-planar cyanides, and, in addition, the L + L* from the -lag + 3blg (x*-y* --t xyj transition are observed transition which is known to be present in M X T * - and band is a t 14,41(J in Pt(AINT)2-. The 'Ag Band
-1
Calculated energies,
--
-1
--
-
-
-+
-
-+
,
-+
+
+
-+
-
+
+-
+-
4.598
SHCPACK, BILLIG,CLARK,~VILLIAMS, AND GRAY
Vol S(i
Frequency, ern.-'
Fig. 3.-The
electronic spectrum of Si(S R . I T ) 2 2 - in acetonitrile solution.
If cm - I , and the ]Ag- lB1, band is a t 18,500 cm --I t h e one-electron assignment is correct, the bands should be separated by iOF4 ' I The singlet-triplet system is fitted with the reasonable value F4 = 60 cm - I , which is good evidence for the correctness of the assignment None of the other possible d-d type transitions is as consistent with the observed singlet-triplet system in Pt (LINT) L 2 " The weak band a t 15,650 cm is tentatively assigned as the transition to the second triplet-excited state, IX, 3B3,
-
Charge-Transfer Bands.-The two alIowed L + .If bands in Pd(hlNT)22--are located a t 22,iOO and 33,900 cm --I The less intense 51 L band is found a t 25,SOO cm - I The higher energy of 51 L charge transfer in Pd(?.lNT)L?- as compared with T\;i(hINT)22-is consistent with the known M L energies in XiiC?'rT)d2and P C ~ ( C N ) ~ ~ The two L 51 bands in Pt(51NT)22- are found a t The L 51 charge transfer 32.300 and 43,SOO cm - I . in Pt(MNT)L2- occurs a t much higher energy than in -+
-
-+
-
T.4BLE
E L E C T R OSPECTRA ~IC OF M(hfNT)22-C O V P L E X E S
Ni (hlN T ) 2 2 -
Transitions
WITH A
(4a,)'
=
'-4, GROCSDSTATE'
F'd(MST)??
Pt ( M NT)$ 2
d-d type
'X,-+ 3B1, 'A, 'A, 'A,
L
+
' B a a( a+ r y )
b 11,690 630) b 17,500 (570)'
b 15,700 (64) b b
14,110 (49y 18,500 (1 , 220)',d 15,650 (56)' b
lBpu
31,300(30,000)
30,800 (20,200)'
29,700 (15,600)
26,400 ( 6 , 6 0 0 ) 37,000 (50%000)
2 2 , i o 0 (5,700) 33,900 (47,000)
32,300 ( 13,400)
19,250 (1,250)' 21,000 ( 3 , 8 0 0 ) Sone
25,800 (2,840)" b 37,800(45,000) 42,800 (34,500)
18,500 ( 1 , 2 2 0 ) ' d 21,100 ( 3 , 4 7 0 )
(.X-Y~
-+
'BI, (r2-y2
-+
3B,
-+
(SZ
-f
+
XY)
-+
XY)
sy)
L'
'.I,
+
L + M charge transfer
-
-+ 'B?,, 'Bau ( L ( T ) '.la+ 'Bn,,, 'Bat3 ( L ( u ) hf + L charge transfer 'Au + 1.1" 'A, -+ 'B?,, L'nassigned bands
-+
M) M)
" Maxima in c i n -' for spectra in acetonitrile solutions; band e's in parentheses. probabll- contains two electronic transitions.
L* Band.-The L -+ L* band is observed a t L about :jO 000 cm - I as follows Pt(51NT)22 30 SO0 cm --I Pt(;ZINT),L- 29 i 0 0 cm --I
Not observed.
13,800(43,500)
3 8 , 5 0 0 (17,000)'
Shoulder.
ii
Broad shoulder
Pd(5fNT)A2- a result in accord with the fact t h a t L -+ 11 charge transfer in PtC1d2- occurs a t much higher energy than in P d C & - The allowed 51 L band in
-
Nov. 5 , 1964
ELECTRONIC STRUCTURES OF SQUARE-PLANAR METALCOMPLEXES
4599
TABLE V THEELECTROSIC SPECTRUM OF h-i(iMNT)2Band maxima,' cm. -'
8,330 11,790 16,700 18,350 20,800 27,300 31,800 36,600 43,500 a
General classification
Assignments
f
Parity forbidden L ( T )-.+ M Parity forbidden Parity forbidden L(u) -L M L ( T )-.+ M L+L* L(u) M
329 8,000 500 690 2,500 13,650 42,350 42,300 23,900
-+
Not assigned
Spectrum of [(n-C4H9)4S][ S i ( M S T ) z ] in acetonitrile solution
( M N T ) 2 2 - contains one extra band, a shoulder a t 38,500 cm.-I, for which we suggest a spin-forbidden L + M assignment.
bands a t 16,700 and 18,350 cm.-'. Table V gives the assignments suggested for these bands by the calculation and summarizes all the band assignments discussed above.
Electronic Spectrum of Ni(MNT)2The ground state of Ni(MNT)2- is . . .(4bzg)*(4ag)' = ZA,. The spectrum of Ni(MNT)%- is given in Table V. The most striking feature of the spectrum of Ni(klNT)z- is the intense band a t 11,i90 cm.-l, a very low energy. If the L + M assignments are correct for Ni(MNT)Z2-, we expect two additional L + M bands in Ni(-MNT)2-, since in the latter complex, transitions are possible to the 4ag orbital. Although the 4ag orbital in Ni(MNT)2- is calculated to have more ligand than metal character, the transitions from almost purely ligand levels to 4ag are still effectively "L + M." The position of the first L --t M band in Ni(MNT)2- can be estimated by subtracting A1 from the first L + l!f band in Ni(MiYT)z2-. This estimate is 26,400 - 14,650 = 11,750 ern.-', in excellent agreement with the observed L + M band in Ni(MNT)2-. Thus we confidently assign the intense bands a t 11,790 and 20,800 cm.-' as L + M transitions. The two "regular" L + hI bands are located a t 27,300 and 36,600 cm.-l, only very little shifted from their positions in Ni(MNT)22-. The separation of the centers of the two L + M systems in Ni(hlNT)z- is ca. 15,700 ern.-', which we shall take as a reasonable estimate of A1 for this complex. The L + L* band in Ni(hSNT)e- appears a t 31,800 cm.-l, which is approximately the same energy as the L + L* band in Ni(MNT)22-. There are three weak bands in the Ni(MNT)aspectrum, a t 8330, 16,700, and 18,350 cm.-l. Since the observed one-electron separation of 4a, and 4b2, in Ni(MNT)Z2- is ca. i 0 0 0 c m - ' , it is reasonable to assign the band a t 8330 cm.-' as 4bzg + 4ag, the first parity-forbidden transition. According to the calculation, both 4ag and 4bZg are mainly "ligand" orbitals delocalized over the four sulfur atoms in Ni(MXT)Z(see Table IX) and thus the difference in the interelectronic-repulsion energies of the 2Xg and 2R2, states is probably small. Indeeed, this is very likely the case, is diamagnetic with the since the C O ( M N T ) ~complex probable ground state . . .(4b2p)2= l k ., There is a spin-allowed band in C O ( M N T ) ~a- t 7430 cm.-', which must be interpreted as the transition 4b2, + 4ag.22 There are several possible assignments for the weak I
( 2 2 ) C. H. Langfard a n d H. B. Gray, unpublished results
Discussion According to the molecular orbital level scheme presented in this paper, the highest occupied MO in the d8 diamagnetic square-planar complexes is the in-plane H* orbital, 4ag. The available experimental information definitely supports this view. First, the observed singlet-triplet band system in Pt(MNT)Z2- is consistent only with the assignments 'A, --t 3B1g, lB1,. Second, the 11,690-cm.-' band of Ni(MNT)22-is not shifted in solvents with sharply different coordinating properties, such as acetone and pyridine. l7 The 11,690-cm.- I band is not even affected in an acetone solution 0.1 -21 in the powerful ligand 1,10-phenanthroline.22 These facts strongly suggest t h a t the 11,690-cm.-' band is due to an electronic transition localized in the plane of the molecule, or x2-y2+ xy. From the spectral assignments given in Tables 111, IV, and V, experimental values of the orbital parameter A1 (3blg-4ag separation) were calculated. These values are presented in Table VI, along with A1 values for other representative planar complexes. As exTABLE \'I THEVALUEOF AI Complex
Ni( M S T ) 2 2Ni(Mh-T)2Ni(dtp)* Ni( DTC)2 Ni( D@G)* Xi( C S ) a 2 Pd( M S T ) p 2 PdBr42PdClr*Pd( CS)42P t( MKT)z2PtBr42PtC142-
P t ( CN)42-
IN
SOMEPLAXAR COMPLEXES~ AI, cm
-l
14,490 15,700 17,300' 18,80OC 25,000' 25, 300d 17,800 18,100d 18 ,good > 3 0 , OOOd
20,600 21,800d 23, l O O d
> 3 0 , mod
F2 = 10Fj = 800 cm.-l for Ki; FZ = 1 O F d = 600 cm.-' for Pd and Pt. h Spectrum quoted in ref. 10; d t p = SnP(OC2Hi)s. Spectrum quoted in W . Manch and W . C. Fernelius, J . Cheni. Ednc., 38, 192 (1961); DTC = S 2 K C ( C * H ~DQG ) ~ ; = diphenylglyoxime. From ref. 11.
4600
SHUPACK, BILLIG,CLARK,\YILLI;\US,
pected, the A, values increase in the order Ni(hINT)22< Pd(Mn'T)22- < Pt(AlNT)?"-. The Xi(AINT)2complex has a slightly larger AI value than Ni(AlNT)?'-, which is reasonable. X comparison of values in Table VI gives the spectrochemical series order M I W < Br- < C1-- < d t p - < D T C - < D@G- < C N - for a planar situation. \Ye now address ourselves to an important question regarding the new square-planar complexes, which is the formulation of the ground state. The usual method of oxidation states gives XI(II1) and AI(I1) for the mononegative and tiinegative complexes, respectively. IVe recently proposed t h a t some of the AI(AfNT)?- and l l ( T D T ) ? - complexes be considered as composed of M ( I ) and radical-anion ligands. In this formulation a filled bonding orbital must be mainly localized on the metal, adding an extra pair of electrons to be associated with the metal. Schrauzer and Mayweg had earlier made the suggestion t h a t Xi (\SPC2(C6H5)2)2 be forniulated as S i ( 1 I ) and two radical-anion ligands.Ifia The following evidence supports the radical-anion formulation for certain of these complexes. 1. .kcording to the calculation of the N i ( l l N T ) 2 n system, the 4beg and 4a, orbitals are definitely more "ligand" than "metal," while the 3b2, and 2a, orbitals, which are bonding, are mainly localized on the metal. This means t h a t square-planar complexes containing LINTP-, in which the 4b2, and l a , levels are nol filled, are not best described in the usual oxidation state formalism. The diamagnetic complex Co( MNT)2-, with the probable electronic structure , , . (4b2,)2(iag empty). contains two h I N T radical anions; four "metal" orbitals are filled, ;3bz,,3b,lg,3a,, and 2a,. The paramagnetic complex Ni(SfNT)?-. with the electronic structure , . .(la,) contains one AIST radical anion. XI1 the complexes with the electronic structure. . . ( l a g ) ?contain AIKT2- ligands and can safely be forrriulated in the usual way. T h a t is, Ni(LlXT)2*- contains Ni(I1) and Cu(SlNT)?- contains Cu(II1). 2, Recently we have shown t h a t Co(SIN'T)?- reacts with certain ligands t o give stable five- and six-coordinated complexes.':' For the reactions with monodentate ligands, i t was observed that the five-coordinate adducts are substantially more sfublc~than the six-coordinate ones, and. in many cases, the six-coordinate state was not found a t all. In addition, the stabilities of the five-coordinate adducts indicate that Co(h1NT)Zfollows the Xhrlanci, Chatt, and Davies type-B beh a ~ i o r ,c.g.. ' ~ Co(llNT)2(P(C6H5):j)is more stable than C 0 ( 1 1 S T ) ~ ( p y-.) Thus the ciicmicnl evidence in this case points t o a complex in which the metal ion is effectively d'; certainly the behavior of C o ( l I N T ) ? - is completely different from any usual d 6Co(II1) complex. :i. I t has been pnssible to prepare neutral. sixcoordinate complexes of the type XIITDT)?,with 11 = Xi and C o . ' j These complexes would call for hl(\'I) in the standard formulation of oxidation state. 1T-e believe this is absolutely unreasonable for a complex containing sulfur-donor ligands, and it is very likely that in these cases (it i m s t one j i l l r d hondiizg orhiti11 is mniniy l o c n i i x d on t i i r ~nzc~ial. ~
1:ii
C . 1.1 I.angf
