5721
Ligand-Field Parameters (cp-l) for the Metal Hexacarbonyls
Table VII.
Complex
11
45
P = C
A
Bcpx
V(CO)sCr(C0)s Mn(CO)s+ Mo(CO)s W(C0)s Re(CO)&+
25,500 32,200 41,050 32,150 32,200 41,000
430 520
From ref 25.
* Estimated value. From ref 26.
.., 380 390 470
Biree iona
. ., 790 870 460 370 47@
40 -
BopxlBfree ion
1300* 17Wb 2600 1100 1300 1375
...
35
0.66
... 0.83 1.1 1.0
-
7 30-
E
0 250 20-
a
/
:: COC$
151
in the transition from Fe(I1) to Co(II1) the increased u bonding destabilizes 3e,u* to the same extent that 2tZgis destabilized by the decreased M - w * C N bonding; the result is a constant A. In the hexacarbonyls, as in O S ( C N ) ~ ~and - II-(CN)~~-,’ the evidence of increasing A values indicates that along an isoelectronic series as above, the u bonding energetics are affected to a considerably greater extent than the analogous 7r bonding quantities. Another effect of interest is the variation of A with ligand in an nd5 or nd6 series ( n = 3-5). Several comparisons are shown in Figure 5. The percentage increase in A in going from 3d to 5d valence orbitals again depends on the type of ligand involved. For C1-, a very large fractional increase in A is observed; a substantial percentage increase in A is observed for NH3, but A(C0) in one series actually shows a slight decrease. These observations are consistent with the above analysis, because interelectronic-repulsion effects should decrease in the order 3d > 4d > 5d. Thus, the 7r-donor ligand is able to move into a better d, overlap position as n increases with a resulting destabilization of the 3e,u* level. The C O ligands, presumably, have already fully exploited the do bonding for the lowest n value and have less to gain in the 4d
IO SI-
3d
4d
56
Figure 5 . Dependence of A on the n quantum number of the d valence orbitals for octahedral complexes.
and 5d series. Indeed, evidence from I P and chargetransfer spectral data (uide supra) indicates the 2tZg level remains stationary down a series such as M(C0)6 M = Cr, Mo, W). We infer from these data that the d, bonding actually decreases, and it is the increased H bonding in carbonyl complexes that holds A(4d) and A(5d) at values comparable to the A(3d) values. Acknowledgments. We thank the National Science Foundation for support of this research. Dr. Richard Marsh kindly communicated to us certain pertinent results from the Ph.D. thesis of G. M. Nazarian. We are grateful to Professor D. W. Turner and Dr. D. R. Lloyd for allowing us prepublication access to vertical ionization potential data for the metal hexacarbonyls. We thank Dr. E. Billig for help with the synthesis of [Mn(C0)6][BF4]and for several useful discussions.
Electronic Structures of Square-Planar Complexes W. Roy Mason, 111, and Harry B. Gray’
Contribution No. 3664 f r o m the Gates and Crellin Laboratories of Chemistry, California Institute of Technology, Pasadena, California 91 109. Receiced April 19, 1968 Abstract: Electronic spectra of several square-planar complexes of the type [MXJZ (M = Pd(II), Pt(II), Au(II1); X = C1-, Br-, CN-, NHJ are reported in nonaqueous media at room and liquid nitrogen temperatures. In most
cases the spectral resolution is considerably enhanced in the rigid, low-temperature glasses. Detailed assignments of spectra measured under carefully controlled conditions allow a comparison of electronic structures and energy levels in the various square-planar systems. The halide complexes show strong Iigand+metal charge-transfer absorptions, the cyanides show metakligand bands, whereas the ammine complexes exhibit allowed d + p transitions. Trends in d-d and charge-transfer energies are discussed. ecent investigations of the electronic structures of indicate that the square-p1anar tronic energy levels vary according to the nature of the The nature of the variation, however, is (1)
Author to whom correspondence should be addressed.
not presently clear.
In view of the several theoretical
(2) H. B. Gray and C. J. Ballhausen, J . A m . Chem. SOC.,85, 260 (1963). (3) C. J . Ballhausen, N. Bjerrum, R. Dingle, K. Eriks, and C. R . Hare, Inorg. Chem., 4, 514 (1965). (4) H . Basch and H. B. Gray, ibid., 6,365 (1967).
Mason, Gray
Electronic Structures of Square-Planar Complexes
5722
discussions of energy levels in square-planar complexe~,~-’it is appropriate at this point to emphasize the value of an experimental approach to the question and the need to formulate an experimentally based energy-level scheme for representative types of ligands. Unfortunately, the reliable, high-resolution electronic spectra for square-planar complexes are limited to a few single-crystal studies of low-intensity d-d bands. A number of aqueous solution spectra, particularly those of halide complexes, are subject t o question because of the susceptibility of square-planar systems to hydrolysis. loThe charge-transfer spectra of the cationic ammine complexes with halide counterions have not been properly measured because of the strong absorption of the counterions in the same region of the spectrum. Furthermore, careful remeasurement of the spectrum of the Au(CN)?- anion has revealedI4 results completely different from those previously reported.’j All of the bands reported in the previous spectrum were found to be absent, and it is suspected that impure compounds were used for the spectral measurements. In the present study, detailed spectral measurements are reported for a variety of complexes of palladium(I1) and platinum(I1) of the type [MX$, where X = halide, cyanide, and ammonia ligands; measurements were made on Ni(CN)42- and A u ( N H & ~ +also. T o avoid the possibility of hydrolysis, measurements were made in nonaqueous solvents. Also, where possible, measurements were made in media that form rigid, transparent glasses when cooled to near liquid nitrogen temperature. Low-temperature glasses have advantages over single-crystal measurements ; solutions can be made sufficiently dilute so that the intense chargetransfer absorptions can be measured. Also, a solutionlike environment is preserved and specific crystal lattice interactions are avoided. Recent measurements on a variety of gold(II1) complexes have shown that lowtemperature measurements in rigid glasses are quite successful in increasing spectral resolution. l 4 The results reported here will be discussed in conjunction with those obtained previously for AuC14-, AuBr,-, and Au(CN)~-. 3*6,8v9
Experimental Section Preparation of Compounds. Starting material for the palladium and platinum complexes was hydrogen tetrachlorometalate(II), HZPdC14or H2PtCl4; the solution was prepared from metal sponge (Englehard Industries). 16, l7 The gold complex was prepared from (5) R. F. Fenske, D. S. Martin, Jr., a n d I 1200). Furthermore, a comparison of the rich spectrum of Pt(CN),?- with the notably empty spectrum of Au(CN)+- l 4 shows that the charge-transfer bands in the gold(II1) complex occur at higher energies than in the platinum(I1) complex. This establishes the transitions as metal to ligand (M-L) charge-transfer processes, It is characteristic of the M-L charge-transfer to shift to higher energy on increasing the metal oxidation number, which is in contrast to the shift to lower energy of the L+M transitions observed for the square-planar halides. The M+L transitions in the tetracyano complexes originate from the occupied d M O levels and terminate on levels derived from the 7r* ligand levels. Transitions from the e,(xz, y z ) and alg(z2)levels to the lowest 7r* level, a2,7r* (IAIg clE, and lAlg IA2,, respectively) are both allowed, but the transition from the bpg(xy)level to a2u7r*(IAIg + IBIu) is orbitally forbidden. This latter transition should be weaker than the former two. Furthermore, the clE, state may be split by distortions which lower the symmetry from D4h. Using these ideas, the first three bands of the Pt(CN)42- spectrum are assigned to the transitions to IAqu,clE,, and IBlu excited states, respectively.? Ion pairing in the solvent would probably cause a sufficient distortion of the Pt(CN)42- complex to split the clE,, state by the small amount observed. There is another more intense band in the Pt(CN)42spectrum at higher energy (46,000 cm-I). This chargetransfer transition is also logically assigned as M-+L, and it is presumed to originate from the bZg(xy)occupied d level and terminate on the next higher 7r* cyanide level, eu7r*. Therefore, the assignment of this band is 'Alg d'E,. In contrast to the transition from the b2,(xy) level to the a2,7r* cyanide level, this transition is fully allowed. The weaker ligand-field transitions are expected to be at quite high energy in the case of Pt(CN)42-, because of the high position of the cyanide ligand in the spectrochemical series and the very large d-orbital splitting characteristic of third-row transition metal ions. Ni(CN)42-. The spectrum of Ni(CN)42- contains a weak shoulder at about 22,500 cm-' ( E -2). The remaining bands are considerably more intense. Therefore, it is reasonable to assign this weak shoulder t o a spin-forbidden transition. At higher energy, there is a shoulder at about 30,500 cm-l which has been assigned to a d-d transition2 Spectra in single crystals have been measured in this region; it appears3that d-d excitations are accompanied by substantial changes in equilibrium internuclear configuration symmetry. The band maxima at 32,000, 34,000-35,000, and 36,00037,000 cm-I are more intense and are assigned as M+L -+
-+
-+
(32) C. I54.00
v
Table IV. Ligand-Field Parameters for Square-Planar Complexesa
CHBCN
Complex
[Pd(NH3)41(C104)2 33.55 (212) >53.00
[PdCL] in Pd2C1,'[PdBra] in Pd2BrsZPtCI41PtBrr2AuC14AuBr4Pd(NH3)r*+ Pt(NH3)a*+ Au(NHs)a 3+ Ni(CN)42Pd(CN)a *Pt(CN)a2Au(CN)4-
34.97 (42.5) 41.66 (143)b 45.25 (443)* 50.76 (11,070)
[Pt(NH3)al(C10& 34.60 (54) 43.10 (191)b 46.08 (560) 50.50 (12,300)
'Big c 'E,
...
[A~(NHs)al(ClOa)s 40.32 (500)b
'Apg,
X
of v given).
cm-1 (E, 1. mole-' cm-').
3Eg, 3A~, 'AzK, 'E,
* Shoulder
(E
'Eg
is for value ~
A1
A2
A3
25,920
4240
b
20,450
3970
b
28,250 26,650 25,430 22,090 37,050 46,600 43,800 33,800 44,600 >50,000 >50,000
6250 5230 6090 4230 1500 1500 b b b b b
8030 3650 b b 500 3480 b b b b b
~~
a Energies in cm-' for interelectronic-repulsion parameter values quently, the band observed in the Pt(NH3)42+specB = 500. C = 3500 cm-1. * Insufficient data. trum at 34,600 cm-' must be assigned to a spinforbidden d-d transition. With a simple a-donor ligand such as NH3, there should be very little difference the bl,(x2 - y') level considerably more unstable than in the energies of the bzg(xy)and eg(xz, yz) levels. Thus the four occupied levels is well established. The values the band at 43,100 cm-I is assigned as a combination of of A1 are found to be larger than AZ and A3 combined lAIg -, lA2, and 'Alg --c lEg, whereas the maximum at (where data are available). Furthermore, the magni46,080 cm- is assigned as lAlg -, 'Big. tude of A, for a given metal varies with the nature of the In contrast t o the Pt(NH&*+ spectrum, only one ligand in the order CN- > NH3 > C1- > Br-. This broad band is observed for Pd(NH3)42+below 53,000 ordering is consistent with the placement of these ligands cm-'; this band is centered at 33,500 cm-I in acetoin the octahedral spectrochemical series. The size nitrile. In view of the extinction coefficient of this of Al depends on the relative energies of the blg(xz band it must be assigned as all three spin-allowed d-d - yz) a* and b,,(xy) n* levels and, therefore, both d transitions. 3 4 This would indicate that the alg(z2) orbital a , a n d n bonding are contributing factors, For level has moved t o higher energy relative t o the Ptexample, the r-acceptor cyanide ligand causes stabiliza(NH3)42+ case. tion of the b,,(xy) level by d n +. T*CN interaction. The aqueous solution spectrum of A u ( N H ~ ) ~ ~ The + simple a donor N H 3 ligand causes a marked deis very poorly resolved, and it is difficult t o visualize stabilization of the b',(.? - y 2 ) level while the bz,(xy) any shoulders that correspond to low-energy electronic level remains essentially nonbonding. The halides are transitions. In contrast, the acetonitrile spectrum weak a donors but good T donor ligands and will shows a distinct shoulder at 40,320 cm-'. This transidestabilize the bzg(xy) level with respect to blg(x2 tion is assigned to lAlg -, lAZg,lEgby analogy to the Y '). platinum(I1) complex. The transition is some 3000 Table IV shows that AI increases very markedly in the cm-' lower in energy for the gold(II1) complex than for order Ni(I1) < Pd(I1) < Pt(I1) for CN- as ligand. The the platinum(I1) complex. It is noteworthy that restrong increase is similar to that observedz6 for the cent on the AuCL- and PtCI12- complexes octahedral cyanides of Co(III), Rh(III), and Ir(II1) also place the first spin-allowed transition lower by a bonding in the and may be i n t e r ~ r e t e das~ increasing ~ about 3200 cm-' in the gold(II1) case. order 3da < 4da < 5da. It is of interest that the A At higher energy, a single intense band is observed results for cyanide complexes in the Ni and Co families at 50,500 cm-' for Pt(NH3)42+. Corresponding bands contrast with the slow increase in A in the Fe family in the Pd(NH3)42+and A u ( N H ~ ) ? ~spectra + are indicyanides. Apparently, the relatively contracted 5da cated, but at energies greater than 53,000 cm-'. This orbitals of Ir(II1) and Pt(I1) are better suited for a distinct red shift in the platinum(I1) complex with rebonding than are the larger 5da orbitals of Os(II1). spect to the palladium(I1) and gold(II1) complexes indiWe have previously argued that a relatively contracted cates that the logical assignment of the band at 50,500 5da orbital may give better da overlap than a more cm-l in the Pt(NH3)4z+spectrum is a d p transition. expanded one because of the cancelling effect of nodes a,"The formal assignment suggested is eg(xz, yz) in the latter situation. ( 'Alg + c'E,). There is a limit, however, to the degree to which contraction will aid 5da overlap. The data in Table IV show clearly that in every case A,[Pt(II)] > AI[Au(III)], Discussion as predicted from molecular orbital calculations. * Ligand-Field Parameters. Table IV summarizes The fact that this "inverse" relationship holds even the values of the ligand-field parameters, AI, A,, and for a simple a donor ligand such as N H 3 indicates that A3, derived from the d-d spectra of the complexes d orbital a bonding decreases in going from Pt(I1) t o under investigation. The d-MO splitting pattern with
- -
(34) Magnetic circular dichroism measurements performed by A. J. McCaffery and P. N. Schatz at the University of Virginia indicate that the 33,550-cm-I band in Pd(NH3)r2+shows a small but distinct A term. This is consistent with a degenerate excited state for the transition(s) involved.
-
(35) The alternative interpretation, involving increasing d s'CN bonding (3d < 4d < 5d), may be ruled out in this case because infrared spectra of [(n-CaH9),NI1[M(CN~)] in acetonitrile show 38 B(CEN) higher in both Pd(CN)r*- (2127 cm-1) and Pt(CN)?*-(2121 cm-l) than in Ni(CN)42- (2113 cm-1).
Journal of the American Chemical Society 1 90:21 1 October 9, 1968
5729 Table V. Charge-Transfer Energies for Halide Complexes Complexb
Ln + do* system" Lu -.t. du* system 'A1, + alTl,,blT1, 'A1, -+ clT1,
IrBrC3PdC16'PtCl6'PtBr6*-
41 ,ooO 28,600 48,500 37,000 (41,500) 29,400 37,700c 30,580, 31, 54OC
41,700 49,2W 44,4m
[PdCl,] in Pd2Cle2[PdBrrl in Pd2BrsZPtC142PtBrr2AuClrAuBr4-
'A1, + lA2,,a1E, 34,700d 30, 720d 43,670, 46,30Oe 33,330, 36,040 31,050 25,250
'Al, -.t. blE, -50,000d 46, 3Wd >54,000 47,380 44,250 39,060
R hC16 3RhBrs3m 6 3 -
... 38,600
... ...
a Energies in cm-l. Data for MX8"- taken from ref 32, except as indicated. From ref 36. L v M . e See ref 28.
Au(II1). Thus it appears that maximum 5da overlap occurs for Ir(II1) and Pt(I1) and is somewhat lower for both Os(II1) and Au(II1). Charge-Transfer Energies. Table V compares L-M charge-transfer energies in d6 o ~ t a h e d r a l ~ " and ~ 6 d6 square-planar halide complexes. It is notable that the charge-transfer band system in the square-planar Pd(I1) and Pt(I1) complexes is observed at higher energy than the band system in the octahedral Pd(IV) and Pt(1V) complexes. The trend is expected because of the lower stability of the M(I1) metal orbitals. The importance of metal orbital stability in determining charge-transfer energies is further shown by the 9250cm-I shift to higher energy of the first L-M band in going from PdCL2- to PtC1dz-. This shift is substantially larger than the corresponding A, change of 2300 cni-'; it must, therefore, be largely due to the difference in atomic 4d and 5d orbital energies. In all cases the bromo complex shows lower energy charge transfer than does the corresponding chloro complex as expected from ligand orbital stability, but the difference in charge-transfer energy between these two ligands is dependent on the metal; for example, the difference between MC1,"- and MBr4"- for Pd(I1) is 4000 cm-l; Au(III), 5800 cm-'; Pt(II), 10,340 cm-'. The increase in difference parallels the decrease in metal orbital stability, but it is not at all clear why bromide and chloride should be so differently affected. From Table V it appears that a two-band L-M charge-transfer system with a separation of 10,00016,000 cm-l is characteristic of both octahedral MX6"and square-planar MX4"- halide complexes. The lower energy band is assigned to LT du* while the higher energy transition is assigned to La da*. The resolution of the lower energy band into two components, IAIg- 'A zu and lA16- alE, in the case of PtX,?-, establishes the fact that the ligand P levels are indeed at higher energy than ligand u levels. The assignments of the two transitions indicate relative stability for the e,T level; this may be taken as evidence of the importance of the participation of the 6p orbital in bonding in Pt(II) complexes. Table VI compares M+L charge-transfer energies of d6 octahedralz6 and d6 square-planar cyanide complexes. The transitions to the aZuT*C N level of the
--
( 3 6 ) W. R. Mason, unpublished results, 1967.
Table VI. Charge-Transfer Energies for Cyanide Complexes in Aqueous Solutions M(CN)$"complexb
'Alg clT1,
+
Fe(CNh445,870 RU(CN)e448,500 OS(CN)6447,950 CO(CN)B~49,500d >50,000 Rh(CN)s3Ir(CN)s3>50,000 a
Energies in cm-l.
M(CN)ancomplex
'AM -+ c~E,
'AiK d'E,
Ni(CN)r2Pd(CN)4'Pt(CN)a2-
37,300 45,450 39,000
50,500 49,000 46,190
>54,000
>54,000
Au(CN)a-
+
Data for M(CN)6n- taken from ref 26.
-
M(CN)l"- complexes, 'Al, clE,, are observed at lower energies than the M-L transitions of the M(CN)6n- complexes of either the Fe(I1) or Co(II1) group. This shift t o lower energy is attributed to the increased importance of the a,, np metal orbital in square-planar systems. The transitions to the e,T* C N orbital in the planar systems, on the other hand, are observed at energies more comparable to those of the M(CN)6"- complexes; the e,P* C N level is not as sensitive t o the metal as the level and the observed energies are about what would be expected by extrapolation from the Co(II1) family. Furthermore, the clEu energy ordering of Au(II1) > Pt(I1) < Pd(I1) can be explained in terms of the expected increased participation of the 6p over 5p orbitals. Direct evidence for the lower 6p(a2,) than 5p(azu)level is obtained from the d + p spectra of Pt(NH3)d2+ and Pd(NH3)4z+. In the latter case the transition bl,(xy) + a245p) comes at higher a2,(6p) transition in the energy than the bZg(Xy) former complex. The movement of the alg(z2)level with respect to the other occupied d levels in the series Ni(II), Pd(II), and Pt(I1) can be described from the charge-transfer data of the M(CN)**- complexes. In Ni(CN),*-, the al,(zz) level is approximately 2000 cm-' more energetic relative to the eg(xz, yz) level than it is in Pd(CN),"-; and it is 3200 cm-l higher in the same comparison with Pt(CN)42-. It is not likely that the small differences in interelectronic repulsion among these complexes will alter this trend. A similar movement of the a1,(z2) level is observed from the A3 values of Pd(NH3)42+and Pt(NH3),*+; the alg(zz) level is about 500 cm-I below eg(xz, vz) in Pd(I1) and moves to about 3500 cm-l below eg(xz, y z ) in Pt(I1). Thus the alg(zo)a*level is a stronger function of the period in complexes of ammine ligands than in the cyanide case. Although no quantitative data are available for A 3 values of the Pd(1I) halide complexes, it is probable that a similar trend would be found for halide ligands. The fact that al,(z2) becomes less antibonding relative to the other d orbital levels on proceeding from Ni(I1) t o Pt(I1) may be interpreted as due to increased participation of the valence s orbital in the a16a bonding. Presumably, ns bonding is very strong in the case of the 6s Pt(I1) orbital; the result is less involvement of 5dZg and increased stability in alg(z2).
-
Acknowledgments. We thank the National Science Foundation for support of this research. Acknowledgment is due Professor P. N . Schatz and Dr. A. J. McCaffery for communicating results of magnetic circular dichroism measurements to us. Mason, Gray
I Electronic Structures of Square-Planar Complexes
