7888
Journal o f t h e American Chemical Society
/
100:25
/
December 6, 1978
Electronic Structures and Spectra of Trinuclear Carbonyl Complexes David R. Tyler, Robert A. Levenson, and Harry B. Gray* Contribution No. 5789frotn the Arthur Amos Noyes Laboratory of Chemical Physics, California Institute of Technology, Pasadena, California 91 125. Receiced May 17, 1978
Abstract: Detailed experimental studies of the electronic spectra of trinuclear metal carbonyls have been made. An extended Hiickel molecular orbital calculation on D3h Ru3(C0)12 has been performed to aid in assigning the spectra of the triangular cluster complexes. A band at 390 nm (2.56 gum-]) in the spectrum of R u ~ ( C O ) IisI polarized in the plane of the Ru3 triangle, exhibits an MCD A term, and blue shifts and sharpens on cooling to 77 K. This band is assigned to the 'AI' 'E' (xz (bond'E' (z2 (antiing) x i (antibonding)), or u-n* transition. A weak band at 320 nm (3.12 pm-l) is assigned to the 'Al'
--
--
bonding) x z (antibonding)), or u*'-u* transition. Similar evidence suggests that bands at 330 (3.03 gm-I) and 385 nm (2.60 pn-l) in the spectrum of Os3(CO)12 be assigned to u - v * and u*'+u* transitions, respectively. The relative energies of the u-+u* and u*'--+u* transitions in the M3(C0)12, R u ~ ( C O ) ~ ( P Rand ~ ) FenRu3-,, ~, ( C 0 ) l ~( n = 0-3) molecules depend on the ligand field splitting of the xz and z 2 orbitals in the corresponding M(C0)d and Ru(CO)3PR3 fragments. A single crystal polarized spectrum of Fe3(C0)12 shows that the two lowest energy bands at 602 and 437 nm (1.66 and 2.29 gm-l) are polarized in the plane of the Fe3 triangle. These bands are assigned to the u*'-u* and u-u* transitions. respectively. Intense bands ( C M >25 000) at approximately 4.0 gm-l are assigned to MLCT transitions in the M,(C0)12 molecules. An MO scheme for the L)4h molecules M2Fe(CO)14( M = Mn, Re) is presented. The lowest energy allowed transition in each of these molecules is predicted to be of the metal-metal u to u* type (IAlg IA2" 2a1,)). The lowest energy band in the spect r u m of RezFe(CO),4 ( 2 63 p r i )is polarized along the ReFeRe axis, consistent with the ' A I , ' A l u assignment.
-
Previous work on the electronic spectra of dimanganese decacarbonyl and related species has shown that the lowest energy absorption bands are attributable to o+u* and d a - w * transitions, with the u and u* orbitals being the bonding and antibonding metal d o (mainly d22) combinations.' Excitation of r+u* has been shown to dictate the photochemistry of these complexes, by inducing homolytic cleavage of the metal-metal bond.2 Our interest in the photochemistry of larger carbonyl clusters has led us to investigate the electronic structures of M3(CO)12 ( M = Fe, Ru, Os) and M2Fe(C0)14 (M = Mn, Re) molecules. As photofragmentation of Ru3(C0)12 has been reported,' it was of interest to us to determine whether lowlying electronic transitions similar to u+u* in binuclear carbonyls would be observed in these larger clusters. Herein we report the results of our electronic spectral studies on several trinuclear carbonyl clusters. Extended Huckel molecular orbital calculations were performed on the R u ( C 0 ) 4 fragment and the R U J ( C O )molecule ~~ to aid in the interpretation of the spectra.
-
-
cules in the crystals. For the infrared measurements, crystals were grown on sapphire plates. Dah1 and Rundle have reportedi3 the polarized infrared spectrum of Fe3(C0)12. We found that their ''11 to b" corresponded to our colorless orientation and their ' 11 to CB" to our green orientation. The orientation of the molecules in the unit cell is such that the plane of the Fe3 triangle is nearly perpendicular to b. Thus, the green orientation is for polarized light with the electric vector in the plane of the Fe3 triangle and the colorless orientation for the electric vector perpendicular to the Fe3 plane.
Molecular Orbitals for Ru(C0)4 and Ru~(CO)IZ
The molecular orbital energy levels for R u ( C 0 ) 4 were calculated using an iterative, extended-Huckel ( F = 2) procedure.14 The coordinate system used is shown in Figure 1. The geometry of the fragment was idealized to CzCsymmetry with the angle C ( l)-Ru-C(2) equal to 180' and all Ru-C-0 angles also equal to 180'. The value of 100' for the angle C(3)Ru-C(4) was taken from a crystal structure determination of Ru3(C0)12.15 The bond lengths used were Ru-C,, = 1.93 A, Ru-C,, = 1.89 A, and C - 0 = 1.14 A, where eq and ax refer Experimental Section to equatorial and axial, respectively. These values were taken from the mean bond lengths reported for Ru3(CO)12.I5 Fel(C0)12, Ru3(C0)12, and Os3(CO)12 were purchased from The C O ligand basis set consisted of the filled 4u, 5 o , IT, Strcm Chemical Co. Fe3(CO)12was purified by sublimation at 60 OC. Ru~(C0)12& a s recrystallized from toluene. Os3(CO)l2 was recrysand the unfilled 2s molecular 0rbita1s.I~These functions were tallized from ben7ene and then washed dry with ether. taken from S C F calculations.I6 Slater atomic orbitals were R U ( C O ) ~ .R ' U ~ ( C O ) ~ ( P P ~R~u) ~ (, ~C. O ' ) ~ ( P E ~ FezP ~ ~ ) ~used , ~ . for ~ the carbon and oxygen atomic basis set.16 The 4d, 5s, R U ( C O ) ~FeRu2(CO) ~.~ (PPN)MnFq(CO)12,7 Mn2Fe(C0)14,* and 5p R u atomic orbitals were taken from an earlier MnFeRe(CO),J.9Re2Fe(CO)lJ,'0and Fez(C0)9" were prepared paper." bb standard methods. The results of the Ru(C0)4 calculation are presented on the Ezlectronic absorption and M C D spectra were measured on Cary left-hand side of the scheme in Figure 2.18 Similar results have 17 and Car) 61 instruments. Spectra a t 77 K were obtained as debeen obtained19 in a calculation of the molecular orbitals of scribed previously;' for some measurements a Cryogenic Technology, Mn(C0)4. The Ru(C0)4 calculation shows that there are Inc., Model 21 cryocooler was employed. The contraction of 2methylpentane at 77 K was measured to be 22?6 relative to 300 K. The three low-lying d orbitals that are relatively unaffected by the use of EPA and 3-PIP ;is glassing solvents has been described previfour C O ligands. These orbitals are directed between the liously.' All the low-temperature spectra reported in this paper are gands. Several lobes of the xz orbital are directed toward the uncorrected for solvent contraction. Techniques for obtaining band equatorial ligands. The higher energy of this orbital reflects polari7ations using the nematic liquid crystal solvent BPC have been this antibonding character. The remaining d orbital, the x 2 described previously.' y 2 , is directed at the axial CO ligands and, as a result, possesses Thin single crystals of Fe3(CO)I2 were grown from hexane on extreme antibonding character. The x 2 - y 2 orbital is above quartz plates. The crystals are strongly dichroic, being green in one some of the C O T* orbitals, and it is not shown in Figure 2. The orientation and colorless in the other. Polarized spectra were measured lowest unoccupied MO is a hybrid ( I 5% x2 - y 2 , 25% z , 45% along the extinction directions of the crystal face. A polarized infrared a*)and is of a l cymmetry in Ru(C0)d. This orbital is involved spectrum was used to obtain t h e orientation of the Fe3(C0)12 mole0002-7863/78/1500-7888$01.00/0
0 1978 American Chemical Society
Tyler, Leuenson, Gray
/
7889
Electronic Structures of Trinuclear Carbonyl Complexes
t 2.
Figure 1. The coordinate slstern for the Ru(C0)4 fragment
P E
w
Table I. Selected Eigenvalues of R u ~ ( C O ) I ~
-E, Wm-I
orbital character
symmetry
\
\
3.27 3.39 3.60 4.98 (LUMO)
e' a[' e' a2'
6.77 (HOMO) 7.14 7.18 7.20 7.41 7.50 7.73 8.32 10.71
al'
x 2 - L'2 z ,
e"
YZ
\
A*
\
T*
\
T*
x z (a*) I
e'
22
a]'' e" e'
XY XY
xz
a2"
YZ
a[' a['
22 (a')
T*
(a*')
(0)
Ru(CO)~
B
Iu, 2u,7r
Ru,(CO~,
Figure 2. MO scheme for Ru3(CO),?. Selected energy levels for the Ru(C0)4 fragment are shown at left (the Ru(C0)4 HOhlO is x z , and the L U M O is T * , z ) .
U
in the metal-metal bond network when the R u ( C 0 ) 4 fragments combine to form Ru3(C0)12. The RuRu bond distance in Ru3(C0)12 was taken to be 2.848 The F parameters used were F M , = F,, = 1.50, F M , = F,, = 2.2, Fg., = F M M= 2.0, Le., the same used for Mn2(CO) lo.' Corrections were applied' to the metal orbital H;; values. The calculation was not iterative owing to program limitations. Thus, VSIE values for the metal Hi, were adjusted to those from a charge-iterative calculation on Mo(CO)6.I4 These values are d = -7.4, s = -6.4, and p = -3.5 pm-I. A partial listing of the eigenvalues from the calculation is given in Table I. These results are represented as an MO scheme in Figure 2 . The metal orbitals in the cluster have been identified by their d orbital parentage because these basis set orbitals remain relatively unmixed in the cluster. The predicted ground state is ' A I ' which accords with the diamagnetism of the compound.20 The energy levels of the metal orbitals in the Ru3(C0)12 cluster are determined mainly by their energies in the Ru(C0)4 fragment (Figure 2). Every d atomic basis orbital in the fragment can be combined into an a-type and an e-type basis set of molecular orbitals in the cluster. Depending upon the d orbital involved, one of these molecular orbitals will be bonding and the other antibonding with respect to the metal-metal interaction. The al LUMO in R u ( C 0 ) 4 forms a bonding a ] ' orbital in Ru3(C0)12; this orbital is occupied in the ground state of the trinuclear carbonyl and accounts for a significant fraction of the metal-metal bonding. The following notation is introduced to simplify the electronic spectral discussion. By analogy to Mn2(CO)lo,' the e' ( x z ) bonding orbital is u, whereas the a2' ( x z )antibonding orbital is u* (u* is the LUMO
A(nrn)
Figure 3. Electronic absorption spectra of Ru?(C0)12 in 2-rnethqlpentane at 300 and 77 K .
in R u ~ ( C O ) I ~the ) ; a ) ' (z') bonding orbital is u' and the e' (z2) antibonding orbital is u*'. There are three allowed one-electron transitions from the d block of molecular orbitals to u*: u+u* ('A') 'E'), u*'+u* ( ' A I ' 'E') and al"(xy) AI' .+ 'A2").
-
-+
-
Electronic Spectra The electronic spectra of Ru3(C0)12 at 300 and 77 K are shown in Figure 3. The most interesting feature is an intense band at 390 nm (2.56 pm-') that sharpens and blue shifts markedly upon cooling. In the nematic liquid crystal solvent
7890
Journal of the American Chemical Society
/ 100:25 / December 6 , I978
Table 11. Electronic Spectral Datao and Transition Assignments 300 K complex
a
A,,,
nm
Ymax, Km-I
390 320 sh
2.56 3.12
270 sh 238 385 sh 330 280 sh 240 602 431 sh 360 sh 310 sh
3.70 4.20 2.60 3.03 3.57 4.17 1.66 2.29 2.78 3.22
263 470 sh 390 492 370 507 387 43 1 380 403 260 sh 236
3.80 2.13 2.56 2.03 2.70 1.97 2.58 2.32 2.63 2.48 3.85 4.24
In 2-methylpentane unless noted otherwise,
CM
x 0.64
3.50 0.36 0.86 2.48 0.32
,,,x,
nm
v,,,,
77 K 1m-l
CV
x IO-^
MCD polarization A term assignment in-planeh
367 320 sh 292
2.72 3.12 3.42
1.02
38 I 317 275 sh
2.62 3.15 3.64
0.56 2.06 1.30
605 435 360 sh 320 sh 297
1.65 2.30 2.78 3.12 3.37
0.52 0.48
468 368 495 353 520 375 424 379
2.17 2.72 2.02 2.83 1.92 2.67 2.36 2.64
0.70 1.28 1.59 1.54 1.88 2.00 4.01 4.82
379 305
u+u* u*’+u*
385 327
g*‘-(J*
1.50 MLCT
MLCT in-plane in-plane
g*’-+(J*
u-u*
1.85
3 .OO 0.88 1.15 1.22 1.23 1.28 2.38 3.00
u+u*
MLCT 487 372 50 1 394
x(nm)
Figure 4. Electronic absorption spectra of Ru3(CO)s(PEtPhz)3 in EPA at 300 and 77 K.
BPC, the 2.56-pm-’ band is in-plane (X-Y) polarized (Table 11). This means that the transition must be either u+u* or u*’-*u*, either of which gives an E’ excited state and is thus X - Y polarized. The A term in the M C D spectrum of this band confirms the degeneracy of the excited state (Table 11). According to the calculation, the u*’ orbital is higher in energy than the u orbital (Figure 2). However, the calculated energy separation between the u*’ and u orbitals is too small (0.32 pm-l) to predict with any confidence which of the lAl’- ’E’ transitions lies to lower energy. In order to decide this question, we found it necessary to examine the spectra of several closely related derivatives. The spectra of Ru3(CO)g(PEtPh2)3 a t 300 and 77 K are shown in Figure 4. Spectral data for other Ru3(CO)g(PR3)3 complexes are given in Table 11. All these complexes have bands near 390 nm (-2.5 pm-l) that sharpen and blue shift
u+u* u-+u* u+u*
e‘-+a‘l MLCT
R . A. Levenson, Ph.D. Thesis, Columbia University, 1970. I n EPA.
..
u-+u* u*f-+u* UdU*
ReFeRe axis
0.57 0.84
77 K
u+a* u*f,u*
In 3-PIP.
on cooling. But, unlike Ru3(CO) 12,the phosphine-substituted complexes have an additional band near 500 nm (-2.0 pm-I). Since phosphine substitution occurs a t a site that is in the Ru3 plane, it is the energy of the xz orbital that is primarily affected (Figure 5). Replacement of C O by a phosphine makes the xz orbital less antibonding. This means that the xz-z2 energy separation in the Ru(C0)3(PR3) fragment will be less than in the R u ( C 0 ) d fragment. As a result of the decreased separation, when the fragments combine to form the trimer, the z 2 antibonding orbital (u*’) is higher in energy than the xz bonding orbital. Thus the energy of the u*’-*u* transition should decrease on going from Ru3(C0)12 to Ru3(C0)9(PR3)3. Accordingly, the “new” band a t -2.0 pm-I in the Ru3(CO)g(PR3)3 complexes is assigned to the u*’-*u* transition. Both the -2.0 and -2.6 pm-’ bands exhibit M C D A terms (Figure 6), which is consistent with our assignment. The above interpretation of the R u ~ ( C O ) ~ ( P spectrum R~)~ suggests an assignment for the 2.56-pm-’ band in Ru3(C0)12. In addition to having nearly the same energy, this band has the same temperature-dependent behavior (sharpening and blue shifting on cooling) as the -2.6-pm-I band in the Ru,(C0)9(PR3)3 complexes. These similarities suggest that the same transition is involved in each case and hence the 2.56-pm-l band is assigned to u-u* in the Ru3(C0)12 molecule. W e note that this u-u* assignment is consistent with the interpretation of the spectra of binuclear metal carbonyls.’ The observed temperature dependence of the band shape in M2(CO) 10 molecules is related to the substantial depopulation of excited M2 vibrational levels that occurs upon cooling the molecule to 77 K.! Similar temperature-dependent behavior is expected for u+u* bands in trinuclear clusters, as the electronic transition will be coupled to the low-frequency M3 stretching motions in these molecules. Phosphine substitution lowers the energy of the u+u* transition in Mn2(CO)lo. Thus, it is possible that the band assignments above are reversed, that is, phosphine substitution has shifted the u+u* transition from 2.56 pm-’ in Ru3(C0)12 to -2.00 pm-’ in R u ~ ( C O ) ~ ( P RTwo ~ ) ~lines . of evidence
Tyler, Levenson. Gray
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789 1
Electronic Structures of Trinuclear Carbonyl Complexes
!
3@JCU
RU3( CO )9( PETPHZ )3 I
I
I
I
I
I
I
I
30003
I 27000
I
33000
24Om
21000
I
I ISOOD
I
I
1-
I 12000
ENERGY I N WRVENUMBERS
Figure 6. MCD spectrum of Ru3(CO)s(PEtPhz), in 2-methylpentane at 25 OC. The ordinate is in units of ellipticity G-l. IO4 wavenumbers (cm-I) = 1 jm-1.
C
frogmerot
trimer
Figure 5. Schematic of the interactions of xz and i 2 fragment orbitals to form trimer cluster orbitals (d.u*’.u,u*): (a) proposed interaction scheme for RuJ(CO)IZ; (b) the effect of increased metal-metal overlapon the u and u*’ orbital energies, as proposed for Os,(C0),2;and (c) the effect of decreased z*-xz energy separation on the u and u*’ orbital energies, as proposed for R u ~ ( C O ) ~ ( P R ~ ) ~ .
suggest that the latter has not happened. Firstly, the shifts in the Mnz(C0)lo a+u* transition energy with phosphine substitution are much smaller than 0.56 pm-’ (2.56-2.00 pm-l). Secondly, on cooling, the -2.6-pm-’ band sharpens and blue shifts, as is characteristic of a a+u* transition. Spectra of several Ru3(C0)9(ER3)3 (E = P, As, Sb) derivatives show that the lowest band increases in energy according to PPh3 < PEtPh2 < PEt2Ph < PEt3 (and also PPh3 < AsPh3 < SbPh3),5 following the a-donor ability of the ligands. With increasing a-donor ability, the d,2-dx, ligand field splitting increases in any given monomeric fragment, with a corresponding increase in energy for the u*‘+u* transition in the trimer. W e also note that the colors of the compounds M3(CO)I 1PR37 M3(CO)IO(PR3)2, and M3(C0)9(PR3)3 ( M = Ru, Os) are generally yellow, orange, and red, respectively.21 Increasing phosphine substitution likely results in a decrease in the average d,2-dxz (fragment) ligand field splitting, thereby lowering the energy of the a*’+a* transition in the trinuclear molecules. Electronic absorption spectral data for Os3(CO)l2 are summarized in Table 11. The peak at 330 nm (3.03 pm-l) and the shoulder at 385 nm (2.60 pm-l) give rise to M C D A terms (Figure 7), indicating degenerate excited states. The band a t 3.03 pm-’ sharpens and blue shifts, and is assigned to a-+u*. The weaker band a t 2.60 pm-’ is assigned to a*’+a*, which is the other low-lying transition to a degenerate excited state. The u+a* transition was assigned to the lowest energy band in Ru3(C0)12; however, in Os3(CO)12, the a*’+u* transition is below a+a*. We suggest that this pattern of relative transition energies is reasonable because the metal-metal orbital interactions should be greater in Os3(C0)12 than in Ru3(CO) 12, causing a greater bonding-antibonding energy splitting in the former molecule. As a result, in Os3(CO)12 the
Figure 7. MCD spectrum of O S J ( C O ) Iin~ 2-methylpentane at 25 O C . The ordinate is in units of ellipticity G-l, IO4 wavenumbers (cm-I) = 1 pm-‘.
z L antibonding M O (a*’) moves above the xz bonding orbital (a), as shown in Figure 5b, and the a+u* transition occurs at higher energy (Ru3(CO)12, 2.56 pm-l < Os3(C0)12, 3.05 pm-l). The a*’--tu* transition therefore falls a t lower energy than u+u* in Os3(CO)I2. Independent evidence that the metal-metal interactions are stronger in Os3(CO) 12 than in Ru3(C0)12 comes from stretching force constants, which are 0.9 1 mdyn/A for the former molecule and 0.82 mdyn/A for the latter.22A similar correlation of u+u* transition energies and stretching force constants has been noted’ for M2(CO) I O ( M = Mn, Tc, Re) molecules. The spectral data for Os3(CO)12 show that the a*’-+u* band is much weaker than that associated with the u-*u* transition. Careful inspection of the Ru3(CO) 12 spectrum reveals a weak band, which appears as a shoulder at 3.1 2 pm-l, and the M C D spectrum of the molecule (Table 11) indicates the presence of an A term positioned a t 305 nm (3.20 pm-I). W e propose, therefore, that the weak band at 3.12 pm-l in Ru3(C0)12 is attributable to a*’+a*. W e turn next to the spectrum of FeRuz(C0)12, which possesses a triangular FeRu2 cluster and only terminal CO groups in solution.6 A band a t 390 nm (2.56 prn-’) in the spectrum of this molecule (Table 11) blue shifts and sharpens upon cooling. Owing to this characteristic temperature-dependent behavior, this band is assigned to the a+u* transition. The weaker band at 470 nm (2.13 ,urn-’) is assigned to the a*’+cr*
Journal of the American Chemical Society
7892
/ 100:25 / December 6, 1978
t u u 0 c
c"
e
W
UI
a n
FelCO),
(CO)5M...M(C0)5
Figure 9. MO scheme for a D4h M2Fe(CO)14 molecule.
A(nm)
Figure 8. Single crystal polarized electronic absorption spectra of Fe3(CO)12 at 25 "C. The spectra are for the electric vector of light perand in the plane (1'). pendicular to the Fe3 plane ( I )
Table 111. Comparison of Band Positions in the Series F e , R ~ j - ~ ( C 0 ) 1 2(n = 0-3) complex
transition. The presence of Fe in the FeRu2 triangle decreases the average ligand field splitting of the x z and z2 orbitals (Figure 5c), thereby placing the u*' orbital above u, as in the case of Ru3(CO)gL3. Consequently, the u*'-u* transition falls lower than u-u*. Electronic absorption spectral data for Fe3(C0)12are given in Table 11. I f the symmetry of CO-bridged Fe3(CO)12 is (both e' a2') idealized to Czr. then the u+u* and u*'-a* 'E', but are split into two transitions are no longer 'A' transitions, ' A I ' A I and ' A I IB1. Both transitions are polarized in the plane of the triangle, one component of each in the Z (C2u)direction ( ] A I ' A I )and the other in the X (C2(.) direction ( ' A ] 'Bl). Spectroscopic measurements on a single crystal of Fe3(C0)12 clearly show that the two lowest energy bands are polarized in the plane of the Fe3 triangle (Figure 8). Dah1 and Rundle have discussed the disordering of the Fe3(CO)12 molecules in the unit ~ e 1 1 . It l ~is this disorder that prevents us from distinguishing the Z and X polarizations. However, even our glass spectra a t 77 K revealed no splitting of either the 602- (1.66) or the 437-nm (2.29 pm-]) band. As in the other M3(C0)12molecules and consistent with the polarization data, we assign the lowest energy bands in Fe3(CO) 12 to the u-u* and CT*'+U* transitions. However, it is possible that one or both of these bands are low-energy charge transfer transitions involving the bridging C O ligands. T o examine this possibility, we studied the electronic spectrum of Fez(CO)g, a molecule with three bridging CO groups. The spectrum showed only rising absorption into the UV with no bands of appreciable intensity in the visible region. Likewise, our study of the electronic spectrum of Co2(C0)8 did not reveal any bands in the visible region attributable to metal bridging-CO transition^.^^ Further evidence that the two low-energy transitions in Fe3(C0)'2 are not charge transfer is provided by the spectrum of (PPN)MnFe2(CO)l*. The cluster structure of MnFer(C0)lz- is the same7 as that of Fe3(C0)]2. The spectra of the two complexes are similar except that the two lowest energy transitions in MnFez(CO)]2(1.74, 2.86 pm-l) are blue shifted with respect to their counterparts in Fe3(CO)l2 (1.66,2.29 pm-l). Substitution of Mnfor Fe(0) should red shift metal to ligand charge transfer ( M L C T ) transitions. Such a shift has been observed for the M L C T transitions when Cr- is substituted for Mn(0) in
- -- -
-
-
Ru3(C0)12 FeRu2(C0)12 Fe2Ru(C0)12 FellCO)I 7 77 K.
u+u*, pm-I (cM x 1 0 - ~ ; 0 f b )
2.72 (1.02; 0.13) 2.72 (1.28; 0.18)
2.63 (0.86; 0.15) 2.29 (0.48: 0.04)
u*'+u*, (tM
x
em-'
10-4;0p)
3.12 (cannot be measured) 2.13 (0.70; 0.06) 1.79 (0.68; 0.07) 1.66 (0.52: 0.08)
Oscillator strength v) as defined in ref 1
Mn2(C0)12.] Thus it is unlikely that the two lowest energy bands in Fe3(CO) 1 2 are due to M L C T transitions. It remains to assign the u-u* and u*'+u* transitions in Fe3(C0)12. For this, a comparison of the spectra of the complexes in ther series Fe,Ru3-, ( C 0 ) l l ( n = 0-3) is useful (Table 111). W e discussed above why the u*'+u* transition is lower in energy than u-u* in FeRu2(C0)12. Upon substitution of another Fe for R u to give FezRu(CO)l2, the u*'-+u* transition should fall to even lower energy. Indeed, the lowest energy band is now at 1.79 pm-' in comparison to 2.13 pm-' in FeRu2(CO)l2. The band is at lower energy still in Fe3(C0)12 (1.66 pm-l). Thus, the u*'-u* transition energy decreases as the average splitting of the xz and z2 orbitals decreases. The u-u* transition is assigned to the 2.29-pm-] band. This band is unresolved in solution at room temperature, so it is difficult to determine if the band blue shifts and sharpens on cooling. However, we note that the 1.66-pm-l band red shifts on cooling, which indicates that a u+u* transition is not involved.
Charge Transfer Bands Metal to ligand charge transfer transitions in metal carbonyls generally fall in the ultraviolet region of the spectrum.' The energies of such transitions are mainly determined' by the nature of the central metal atom; Le., the positions of d(Fe) x*CO and d(Ru) x * C O charge transfers in Fe3(CO) 12 and R u ~ ( C O )may I ~ be reasonably estimated from the M L C T band energies observed for F e ( C 0 ) s and Ru(CO)s, respectively. The lowest M L C T band in the spectrum of F e ( C 0 ) s falls at 242 nm (4.1 3 ~ m - ' ) whereas , ~ ~ that in R u ( C 0 ) s occurs at 236 nm (4.24 pm-l) (Table 11). Thus we assign the intense bands at 263 nm (3.80 pm-I) in Fe3(C0)12 and at 238 nm (4.20 pm-') in Ru3(C0)12 to M L C T transitions. By analogy, the band at 240 nm (4.17 pm-') in the spectrum of o s 3 ( c o ) 12 is attributed to a similar M L C T transition.
-
+
Tyler, Lecenson, Gray
/ Electronic Structures of Trinuclear CarbonJ'l Complexes
7893
;(pn-'i
Figure 10. Electronic absorption spectra of RezFe(C0)14 in 3-PIP at 300 and 77 I
