- Email: [email protected]
Electronic structure of Re2Cl82−
:’
The multiple suttedng Xcrmethod has been used tb calculate the ord&ng of both occupied a& ~~cc~pi~d one-electron energy states of ReaClg-. Single crystal polarized electkic spectra of [(nC&)4N] z[Re2C!ls] have been measured at 5 K. Principal band maxima are observed at i4 180 (z), 30 870 Izy), hnd 39 21% (z) cm-“. The calculation, ohserved polarizatiqns, and a comparison of band positions in RezCIi- and RezBrz- siiggest the following transition as&giments for the foimer complex: $4180 cmL1, bzg6 --c bru6w; 30870 cm-’ ,eg-c b@*; 39 215 cm-l , eufl --)Ie ir*. g
.*.
. . .’
:
1:Introduction
;
.’
ERe;CIS] at 5-K. Here we report the principal results and concl~io~s’s~~~ng from these ~yestiga~ons.
.
.: Discussions of strong metal-meCaI bonding’often : center around binuclear Re(II1) ~orn~Ie~es,~ partic- .‘ ,2:xcY ca&ationand results ular Re2CI~-, as the’properties ofthis system are con: sistent with a ~uad~pIe~b~nd formulation of the ground state. {l] . we have performed theoret&& caI- _. The,.multipIe scattering J& method has been formulated eisetihere [2,3] : The parameters u&d in the culations on RkiQi-, utilizing the multiple scatter-. ing Xo method [2,3] , ir. an effort to learn more.about 2-b calculation of Re2Cli- are summarized in table 1; ._. the ground state and the~nature of the low&t electronic tmnsitions. We have also examined the polarT&l;1 _’ ized electronic spectra of a crystal of ~(~-C~H~}~~]~’ -. Park&r5 for,the multiple suttering Xa calc&tion of
,’ ._, .Re;!&,, .‘ 1. : .; . .” . . .. : .. .‘ ,’ .. *Permanent address: Lekst&l fiir Thaoketkche Chetie :‘ Sphere radius (au) Alpha .. .der T&h&hen ~~ve~~~~~~~~~c~~~~.~ Munich 2,,, _. ,_I : West G&riany. :. .’ .2.42 ,.‘. j, ..,, o.ioooRe,‘ :. _. pi : ddd.re$i ~hysiwl S&ences Resure(, 3J.4 (‘&m_ :.w Perwent .__24% ... .’ : O ‘ -72325 ,‘ : : : : pany,-St. Pa~l;~~esot~, USA. .‘ : outer sphere : .7.82$ ,“‘ ,‘ :.,; ‘a.7186 : _. .I. ., _’ ,_, ,__ _, 7 fahbLmiNo* y44*’ .: ;. :_;: : : ,‘ _ :_’ ,, , _), ,_ r ,.), : :,,‘,: ..:. .’ .’ j .‘ : .‘ ,, ” ,._ .’ ,‘,.,. ,‘. ,..‘. . ..., ..’ ;, ,_,. ,.. _. ,’ _:, 283
_‘,
.‘ ..‘b ,‘.
._ ,, ::... ,‘(._..
,’ .‘ I;
_‘.
.‘._
:, .‘ ,.
.-.
...yq!urne 32,-r+mber 2
:.
. ...‘.
: 1 .. : :
:.
”
,, Table 2 .- ., ,Resuits,of the~n$Liple scatiering Xa’ckiculationof.RezCli-
Orbit&’ sjmm+j..a)
.I .a,g : ,. bPu big:
‘. .,-.
ku
I.0
6.4
-0.1?26
~4.5
32.8
,-d.2”46
50.8
34.5
.-OS2713
61.5
li.1
-0.4461 -0AR68 -0.$495 -0.5a90 --0_5982
2g
F e;
.‘“IU’.
‘.
azs:.
: eg .. .e,
azu
39
82.7
79.6
-0.6578
3.9 0.6
78.4 82.6
-0.6852
.’ .‘31,0’
33.8
_. % al-L-= U1U
.-0.7431
‘.
-0.7689 -0.7803
-32~
-0.7989
bzg. eu b2u big “‘6
:
--0.8083 -0.8297 -0.8562
~0..8680
a) ihe highest
72.5
15.3. 19.1 17.4
67.1 74.5
22.3
56.7
43.7 36.0
38.1
41.8 61.7 59.1
62.2
34.5
The 6 bondhig.orbital, b zg, is the highest-occupied level, folloH{ed closely by the e, ,Re-Re ‘ITbonding
orbital. However,the Re-Re Gbonding &b&l is not clkarly defined, sine the alg orbitd at LO.6852 Ryd.
this respect the present calculation accords well with XLY resultsreported [7] by Nor-manand Kolari for Mo2Cl;f-. It is interesting to note that in Re2Cli- the ligand ‘inorbit& of b& and b,b symmetries do have a fair amount of metal character. This is probably due to _ tie
the fact that the S and 6* orbitals are also bZ9 and , resjectively, and interaction occurs. The e, or-
b,,
bitals at -0.8083
62.9
orbiial b bzg at -0.4868
occupi$
or out& sphere region and hende
Re, whereas that at --0.8680 Ryd is 6Z!.2%F$e.Obviously, the metal-&and mixing in the als qrbitals is considerable. Nevertheless; formulation of the ground state in terms.of a quadruple ReRe ,bond, (b2$)2(e,,a)‘! (31gu)2, isquite reasonable. II-I
79.7
15.:
and -0.5495
Ryd have considerable
ligand-metal
mixing, indicating that they’are Re-Cl sigma bonding and antibonding, respectively. The sigma antibonding
character
of the eU orbital
at
-0.5495 Ryd strengthens its metaLmetal
pi bonding character by forcing the electronic density into the
region between the metal nuclei. The.eU
-0.7000
Ryd.
orbit&
at
anZ -0.6491 Ryd are-ligand localized and
non-b&ding, that is, they do not interact at all with the e, orbitals’at -0.8083 and.k.5495 Ryd. The ‘The atomic‘sphere radii were.chosen to be the dis-
Iowest unoccupied orbit& are the rhenium d funcThe highest alg orbital is very diffuse, as most of its density is either between the spheres or in the outer sphere region. This orbital is probably the syrrqztric (g) combination of the &s functions on the rhenium atoms.
tances where the superinposed atomic charge densi- ‘. .. ties place 72 electrons .jn the Re sphere and 17 in the Cl sphere. Hence,,the spheres overlapped along’the bonds [4] . The alpl!a f& Cl is iron-i Schwartz [S] , whereas the alpha for R: was &trapolated by us .fiom Schwartz’s table. The zlpha for tk outer sphere is a \yeigl!ted aveiage bf the Cl and Re alphas. The brbitals sf the Ne core of Cl as tiell as those of the ke core minus the’% and 5p electrons’of Re were frozen and not allowed to relax during the self-consistent-field calculation. The rotational ‘configuration ; ‘of Re,Cliis’eclipsed (Ddh) {6] . Principal angles a$ distarices are; ReReC1, 103.7”; ClR&l; 87”; ReFl; T.29 4 (4.33 au); ReRe, 2.2-i A (4.22 au) [6].
‘..-.The results of the c&ulaliok :28;l 1.’ ,‘..
: .)
,,.‘._!.-
...
.
;.
:
on the ground state
.-‘, :
..
-. ..,
.:
‘-
is ody33.8%
50.5
3.0
-0.7000 -0.7249
eu
_,
S8,2
5.8
-0.6692
.bl,
.,
8(i.5
-0.6491
.’
or-
cannot bk assigned unambiguously to either Re or Cl.
21.4 44.4 88.6 87.6
2.2
._
table 2. Only th’e higher ener&
listed.The reason that the percentages do
is-in the int~xsphere
19.3
i.2
- ‘-m,’
‘Cg
1.5April 1975
-not-add t? -.lOO% is betiu;e some electrcrnicdensity
18.0
,’
64.2 40.0 .g
-0,6023
‘- bzu.
74.8 : 70.4
”
,‘.’
are presentedin
_: 70Cl
-0.0:62
LJ~~ERS
bitals are
-.
-0.3300 .’
:‘&
’
%Re
Energy (Rid1
:.
_,
CHEMICALPHYSICS
tions, as expected.
3. Electronic
spectra
Polarized :Ibsor&on spectra,of thin crystals of
:
!$$4
f9$$2~eZ~18 1 bate w”P nJi,t u = , c = 16.43 A; fl= 122.66 ) [8,9]
wek
heasureh on a Cary 17 spectrophotometei.
equipped.with an Andoni;in liquid double G!an-Taylor airqaced’cqlcite _’ I. :.... .. ‘. . 1
helium dewar and polarizerS ..
Volume 32, number-2
250
CHEMICAL PHYSICS LETTERS
350
650
nmFia. 1; Polarized absorption spectra on (100) of a single talof[(n.C4Hg)4N]z[Re2Cla] at5K:-,Ub;---,ti.
CIYS-
(fig. 1). Band positions and polarizations are set out in table 3. We shall concentrate attention on the three prominent bands, which have maxima at 14 180, 30870, and 39215 crrr-l .The bands at 14180 and 30870 cm-l exhibit extensive vibrational structure, whereas the absorption peaking at 39215 cm-l is rather broad and featureless. Discussion of the probable assignments of the several weak bands will be the subject of a subsequent paper. Enkgies of the allowed electronic transitions were calculated using Slater’s transition state approach, Table 3 Band positions and calculated transition energies for Re2CIi-
Obsd. (cm-‘)
Polb)
Assignment
14180 (27030)a) (28330)“) 30870
z z 2 XY
kg6 -‘b1u6
(31750)a) (34480jd woop) 39215 (39840)“) (42000)a)
xy XY
Calcd. (cm-‘) *
4488
‘kg+ 61”6P :
21722 .
.
XY 2 XY. XY
e”n+e
Tr* g
24702.
a) W&k band.,
-.
relative to the molecular axes, accordin$ to the follow_tigrelationships [ 91: A 1b = 0.6137 1 Ai+ o.3129.4,j;Alb _=0.3119x4,+: 0.6881Axy,
b) Polvintians
15 Aprii 1975
which allows for complkte relaxation ofthe orbitals [2,3,!0] . The lowest allowed transition, bzDf blu, is calculated to fall at 4488 cm-l ..TMs.is we.l? below the next higher transitions, whose predicted energies are all above 19 000 cm-r. Tke z poIarization of the 141EO-cm-1 band is consistent u-iLi its assiment as b2s + bl,, as is the fact that an analogous feature is found [8] in Re_,Bri- at 13 597 cm-l. The Iack of a significant shift in the transition enerw in going from Re,Cli- to ReiBri- fits nkeIy with theory, as both b,,S and b,,S* are predominantly Re-based orbitals. The vibrational structure of the 14180cm-i band of Re,Cli- is attributed [S] to progressions in alg ReRe (245 cm-l, ground state [ll] 174 cm-‘) and alg ReReCl(115 cm-l,groundstate [II] 115 cm-‘) modes;It may be noted here that a recent X0: calculation has shown that the lowest allowed electronic transition in Mo2Cli- is also bZ, * b,, [12] . Experiment has placed [8] the origin &f the allcwed b2, +%u transition in K4MozClg at 17 897 cm-‘: The 30870.cm- t band is xy polarized, indicating that the transition in question is to an E, excited state. The MCD spectrum of Re2C1i- shows that an A term is associated with this band [9] , thereby con firming the degenerate nature of the excited state. A similar band and MCD A term have been found [9 1 for Re, Bri- at 23 530 cm-l. The large red-shift in the tx&I iosition
on varying the tipand from cl&ride
to
bromide strontiiy suggests a Jigand-to-metal charge transfer transition. According to the &uIation, the lowest allowed charge transfer transition is eg + blU6*, whikh is of course predicted to be x3: polarized. The calculated transition energy is 21772 cmeL, which once again is too low. The eg orb&d of Re,Cli- is mainly chloride-based, which accords well with the experimental evidence. Furthermore, a progression attributable to an excited-state a1 ReCI mode (386 cm-l) is built on the 3057Ocm- g band. The intense band at 39215 cm-l in Re,C$- is z polarized. A similar band is observed [9] in Re,Briat 37735 cm-l’, indicating that the excitation involves primarily Re-bed orbit&: The Logical candidate is e,,rr + ear*, which is calculated at 24702 cm-’ . The e,n orbital does have 44.4% Cl character, which would account for the observation that the transition is siig,htly red-shifted in Re2B$. It should be noted that the to an inalgu-f a zuo,* transition should also &rise tense, z-polarized’b&d wi&h a smaU amount of halide,’
”
..
285
_‘( .-__ - v01ums 32,number,2
,‘:. ‘
-..,
,,‘
.: ;, ., .:
..,_:
:
...
..
... ‘
:” : i$~A~rjl19~$‘75 ._
>...
: ., ,. [2].J.C. %t&r, in: A$&&s iti.quantum,chemistry, YoL‘6, “’ ‘gc$tiotis, there iS no ieason to’beEeve iha’, algo 3’ 1. Z&P&L Lijwdin (Acaddmic hess,NeG.Yark, 197i). . ..I1 z&b* wi!.l be &.exceytion.‘Thtis we &fez to’ass& ., :.,.. p. 1, tLndIeferendestherein .’ .:. 1the 39~~,&rr-~.~tkd in.Re;C)$ .to the e,n -+ es”* ,131K.H. Loh.mon, i&Ad&& &q&n&m cl&nistry, Vol. ,‘. .; ‘1 2. t~nsj:~n~.~ ._ _,. 7, ed. P.-O. J&vdin(Au$eti’ic P&s, l&w Yo;k,,.i9?$‘. _‘ : .. :.:; ,’ : I .’ ,, _: .,p.145.,‘-‘. ,.( : ,*. -_ . . : .‘ : .“ Z&I &hrisaa, Chem. ; 14 ‘ J I$ IWsch;,W.G. &?$crerand .‘ . . .‘ . . .(. _: ,, .; . . . ” j Phys. Letters i3 fl97$149. .: . I ., :_ .._).~~~~~~~~~~.~~~~,, : I [S] ‘ K. Sctrwam,Pihys: Rev. iS (i$72) 2466. ‘. _. ;.’ ,,.. [6] F.k: Cotton and CB. Harris, Inorg. Chem. 4 !1965) ... . ; ; Tke .&hors acknoivlddge the ti A.&C. ccm; 330. jiut@qj’ce,nteq for a generous g&t cf &z-iput&‘time. ‘. _ [7]’ J.G. Sorrnan Jr..and I&J. KoI&, Che+ Commun. W! $jank JoI~‘Ca~a~y &!‘Johsj Wood fez a&~+:?-974:i3o3s . [St C.i)..Cowman and H.3, Gray, jr. Am. &e?rt! Soc.‘?S : ,_‘ingus tb use their cotipuier programs. Research at’ .. (lq?3:l81??. ” [Jf C.D. Cowman, Ph,D.“&e&, Caiii!ornia In&t&e bf 1:’ Nyo,&zq addit$onaIly supported by the National .‘. .,-I ‘:: S&nce Fdundation (GP-10331) and by@ Deutsche Technology (1974). [lo] ,A.P..Mortqla, J:Moskowitzand N. R&h, to be pub,I.’ ,~orschungsge,meinscflaftlResearch at t1le~CaJiforn.k ;‘ lished., 1’ Institut& of-Tec&ologLwas supported by the A&r.‘_. [;l] WK. Cr,tton, F.A. Cotton, hf. Debeau &I$ R.A. Waite?, .’ .: ,, -.‘, ~.eseafch_6ffice (Durham)‘. --.,.; 3. Coo:d.Chem.‘l ii9711 12i., . : 1121 J.G. Nor&~ Jr. and H_J.‘Kolarj, J. A& Chem. Sot., _ .-; , : .‘_. . I.‘ ‘submittea for publication. ,: _, .’ . ‘ ., .‘. ., .‘ : :. . : .. i.’ ., . :. .‘ _)_ .; : r,._ : . ‘ : _.‘. . : : .‘ ”‘ . ” .’ .‘
,., ,.-:
“
..’ ‘ :
,. ,.;
,’
._’
The multiple suttedng Xcrmethod has been used tb calculate the ord&ng of both occupied a& ~~cc~pi~d one-electron energy states of ReaClg-. Single crystal polarized electkic spectra of [(nC&)4N] z[Re2C!ls] have been measured at 5 K. Principal band maxima are observed at i4 180 (z), 30 870 Izy), hnd 39 21% (z) cm-“. The calculation, ohserved polarizatiqns, and a comparison of band positions in RezCIi- and RezBrz- siiggest the following transition as&giments for the foimer complex: $4180 cmL1, bzg6 --c bru6w; 30870 cm-’ ,eg-c b@*; 39 215 cm-l , eufl --)Ie ir*. g
.*.
. . .’
:
1:Introduction
;
.’
ERe;CIS] at 5-K. Here we report the principal results and concl~io~s’s~~~ng from these ~yestiga~ons.
.
.: Discussions of strong metal-meCaI bonding’often : center around binuclear Re(II1) ~orn~Ie~es,~ partic- .‘ ,2:xcY ca&ationand results ular Re2CI~-, as the’properties ofthis system are con: sistent with a ~uad~pIe~b~nd formulation of the ground state. {l] . we have performed theoret&& caI- _. The,.multipIe scattering J& method has been formulated eisetihere [2,3] : The parameters u&d in the culations on RkiQi-, utilizing the multiple scatter-. ing Xo method [2,3] , ir. an effort to learn more.about 2-b calculation of Re2Cli- are summarized in table 1; ._. the ground state and the~nature of the low&t electronic tmnsitions. We have also examined the polarT&l;1 _’ ized electronic spectra of a crystal of ~(~-C~H~}~~]~’ -. Park&r5 for,the multiple suttering Xa calc&tion of
,’ ._, .Re;!&,, .‘ 1. : .; . .” . . .. : .. .‘ ,’ .. *Permanent address: Lekst&l fiir Thaoketkche Chetie :‘ Sphere radius (au) Alpha .. .der T&h&hen ~~ve~~~~~~~~~c~~~~.~ Munich 2,,, _. ,_I : West G&riany. :. .’ .2.42 ,.‘. j, ..,, o.ioooRe,‘ :. _. pi : ddd.re$i ~hysiwl S&ences Resure(, 3J.4 (‘&m_ :.w Perwent .__24% ... .’ : O ‘ -72325 ,‘ : : : : pany,-St. Pa~l;~~esot~, USA. .‘ : outer sphere : .7.82$ ,“‘ ,‘ :.,; ‘a.7186 : _. .I. ., _’ ,_, ,__ _, 7 fahbLmiNo* y44*’ .: ;. :_;: : : ,‘ _ :_’ ,, , _), ,_ r ,.), : :,,‘,: ..:. .’ .’ j .‘ : .‘ ,, ” ,._ .’ ,‘,.,. ,‘. ,..‘. . ..., ..’ ;, ,_,. ,.. _. ,’ _:, 283
_‘,
.‘ ..‘b ,‘.
._ ,, ::... ,‘(._..
,’ .‘ I;
_‘.
.‘._
:, .‘ ,.
.-.
...yq!urne 32,-r+mber 2
:.
. ...‘.
: 1 .. : :
:.
”
,, Table 2 .- ., ,Resuits,of the~n$Liple scatiering Xa’ckiculationof.RezCli-
Orbit&’ sjmm+j..a)
.I .a,g : ,. bPu big:
‘. .,-.
ku
I.0
6.4
-0.1?26
~4.5
32.8
,-d.2”46
50.8
34.5
.-OS2713
61.5
li.1
-0.4461 -0AR68 -0.$495 -0.5a90 --0_5982
2g
F e;
.‘“IU’.
‘.
azs:.
: eg .. .e,
azu
39
82.7
79.6
-0.6578
3.9 0.6
78.4 82.6
-0.6852
.’ .‘31,0’
33.8
_. % al-L-= U1U
.-0.7431
‘.
-0.7689 -0.7803
-32~
-0.7989
bzg. eu b2u big “‘6
:
--0.8083 -0.8297 -0.8562
~0..8680
a) ihe highest
72.5
15.3. 19.1 17.4
67.1 74.5
22.3
56.7
43.7 36.0
38.1
41.8 61.7 59.1
62.2
34.5
The 6 bondhig.orbital, b zg, is the highest-occupied level, folloH{ed closely by the e, ,Re-Re ‘ITbonding
orbital. However,the Re-Re Gbonding &b&l is not clkarly defined, sine the alg orbitd at LO.6852 Ryd.
this respect the present calculation accords well with XLY resultsreported [7] by Nor-manand Kolari for Mo2Cl;f-. It is interesting to note that in Re2Cli- the ligand ‘inorbit& of b& and b,b symmetries do have a fair amount of metal character. This is probably due to _ tie
the fact that the S and 6* orbitals are also bZ9 and , resjectively, and interaction occurs. The e, or-
b,,
bitals at -0.8083
62.9
orbiial b bzg at -0.4868
occupi$
or out& sphere region and hende
Re, whereas that at --0.8680 Ryd is 6Z!.2%F$e.Obviously, the metal-&and mixing in the als qrbitals is considerable. Nevertheless; formulation of the ground state in terms.of a quadruple ReRe ,bond, (b2$)2(e,,a)‘! (31gu)2, isquite reasonable. II-I
79.7
15.:
and -0.5495
Ryd have considerable
ligand-metal
mixing, indicating that they’are Re-Cl sigma bonding and antibonding, respectively. The sigma antibonding
character
of the eU orbital
at
-0.5495 Ryd strengthens its metaLmetal
pi bonding character by forcing the electronic density into the
region between the metal nuclei. The.eU
-0.7000
Ryd.
orbit&
at
anZ -0.6491 Ryd are-ligand localized and
non-b&ding, that is, they do not interact at all with the e, orbitals’at -0.8083 and.k.5495 Ryd. The ‘The atomic‘sphere radii were.chosen to be the dis-
Iowest unoccupied orbit& are the rhenium d funcThe highest alg orbital is very diffuse, as most of its density is either between the spheres or in the outer sphere region. This orbital is probably the syrrqztric (g) combination of the &s functions on the rhenium atoms.
tances where the superinposed atomic charge densi- ‘. .. ties place 72 electrons .jn the Re sphere and 17 in the Cl sphere. Hence,,the spheres overlapped along’the bonds [4] . The alpl!a f& Cl is iron-i Schwartz [S] , whereas the alpha for R: was &trapolated by us .fiom Schwartz’s table. The zlpha for tk outer sphere is a \yeigl!ted aveiage bf the Cl and Re alphas. The brbitals sf the Ne core of Cl as tiell as those of the ke core minus the’% and 5p electrons’of Re were frozen and not allowed to relax during the self-consistent-field calculation. The rotational ‘configuration ; ‘of Re,Cliis’eclipsed (Ddh) {6] . Principal angles a$ distarices are; ReReC1, 103.7”; ClR&l; 87”; ReFl; T.29 4 (4.33 au); ReRe, 2.2-i A (4.22 au) [6].
‘..-.The results of the c&ulaliok :28;l 1.’ ,‘..
: .)
,,.‘._!.-
...
.
;.
:
on the ground state
.-‘, :
..
-. ..,
.:
‘-
is ody33.8%
50.5
3.0
-0.7000 -0.7249
eu
_,
S8,2
5.8
-0.6692
.bl,
.,
8(i.5
-0.6491
.’
or-
cannot bk assigned unambiguously to either Re or Cl.
21.4 44.4 88.6 87.6
2.2
._
table 2. Only th’e higher ener&
listed.The reason that the percentages do
is-in the int~xsphere
19.3
i.2
- ‘-m,’
‘Cg
1.5April 1975
-not-add t? -.lOO% is betiu;e some electrcrnicdensity
18.0
,’
64.2 40.0 .g
-0,6023
‘- bzu.
74.8 : 70.4
”
,‘.’
are presentedin
_: 70Cl
-0.0:62
LJ~~ERS
bitals are
-.
-0.3300 .’
:‘&
’
%Re
Energy (Rid1
:.
_,
CHEMICALPHYSICS
tions, as expected.
3. Electronic
spectra
Polarized :Ibsor&on spectra,of thin crystals of
:
!$$4
f9$$2~eZ~18 1 bate w”P nJi,t u = , c = 16.43 A; fl= 122.66 ) [8,9]
wek
heasureh on a Cary 17 spectrophotometei.
equipped.with an Andoni;in liquid double G!an-Taylor airqaced’cqlcite _’ I. :.... .. ‘. . 1
helium dewar and polarizerS ..
Volume 32, number-2
250
CHEMICAL PHYSICS LETTERS
350
650
nmFia. 1; Polarized absorption spectra on (100) of a single talof[(n.C4Hg)4N]z[Re2Cla] at5K:-,Ub;---,ti.
CIYS-
(fig. 1). Band positions and polarizations are set out in table 3. We shall concentrate attention on the three prominent bands, which have maxima at 14 180, 30870, and 39215 crrr-l .The bands at 14180 and 30870 cm-l exhibit extensive vibrational structure, whereas the absorption peaking at 39215 cm-l is rather broad and featureless. Discussion of the probable assignments of the several weak bands will be the subject of a subsequent paper. Enkgies of the allowed electronic transitions were calculated using Slater’s transition state approach, Table 3 Band positions and calculated transition energies for Re2CIi-
Obsd. (cm-‘)
Polb)
Assignment
14180 (27030)a) (28330)“) 30870
z z 2 XY
kg6 -‘b1u6
(31750)a) (34480jd woop) 39215 (39840)“) (42000)a)
xy XY
Calcd. (cm-‘) *
4488
‘kg+ 61”6P :
21722 .
.
XY 2 XY. XY
e”n+e
Tr* g
24702.
a) W&k band.,
-.
relative to the molecular axes, accordin$ to the follow_tigrelationships [ 91: A 1b = 0.6137 1 Ai+ o.3129.4,j;Alb _=0.3119x4,+: 0.6881Axy,
b) Polvintians
15 Aprii 1975
which allows for complkte relaxation ofthe orbitals [2,3,!0] . The lowest allowed transition, bzDf blu, is calculated to fall at 4488 cm-l ..TMs.is we.l? below the next higher transitions, whose predicted energies are all above 19 000 cm-r. Tke z poIarization of the 141EO-cm-1 band is consistent u-iLi its assiment as b2s + bl,, as is the fact that an analogous feature is found [8] in Re_,Bri- at 13 597 cm-l. The Iack of a significant shift in the transition enerw in going from Re,Cli- to ReiBri- fits nkeIy with theory, as both b,,S and b,,S* are predominantly Re-based orbitals. The vibrational structure of the 14180cm-i band of Re,Cli- is attributed [S] to progressions in alg ReRe (245 cm-l, ground state [ll] 174 cm-‘) and alg ReReCl(115 cm-l,groundstate [II] 115 cm-‘) modes;It may be noted here that a recent X0: calculation has shown that the lowest allowed electronic transition in Mo2Cli- is also bZ, * b,, [12] . Experiment has placed [8] the origin &f the allcwed b2, +%u transition in K4MozClg at 17 897 cm-‘: The 30870.cm- t band is xy polarized, indicating that the transition in question is to an E, excited state. The MCD spectrum of Re2C1i- shows that an A term is associated with this band [9] , thereby con firming the degenerate nature of the excited state. A similar band and MCD A term have been found [9 1 for Re, Bri- at 23 530 cm-l. The large red-shift in the tx&I iosition
on varying the tipand from cl&ride
to
bromide strontiiy suggests a Jigand-to-metal charge transfer transition. According to the &uIation, the lowest allowed charge transfer transition is eg + blU6*, whikh is of course predicted to be x3: polarized. The calculated transition energy is 21772 cmeL, which once again is too low. The eg orb&d of Re,Cli- is mainly chloride-based, which accords well with the experimental evidence. Furthermore, a progression attributable to an excited-state a1 ReCI mode (386 cm-l) is built on the 3057Ocm- g band. The intense band at 39215 cm-l in Re,C$- is z polarized. A similar band is observed [9] in Re,Briat 37735 cm-l’, indicating that the excitation involves primarily Re-bed orbit&: The Logical candidate is e,,rr + ear*, which is calculated at 24702 cm-’ . The e,n orbital does have 44.4% Cl character, which would account for the observation that the transition is siig,htly red-shifted in Re2B$. It should be noted that the to an inalgu-f a zuo,* transition should also &rise tense, z-polarized’b&d wi&h a smaU amount of halide,’
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: ., ,. [2].J.C. %t&r, in: A$&&s iti.quantum,chemistry, YoL‘6, “’ ‘gc$tiotis, there iS no ieason to’beEeve iha’, algo 3’ 1. Z&P&L Lijwdin (Acaddmic hess,NeG.Yark, 197i). . ..I1 z&b* wi!.l be &.exceytion.‘Thtis we &fez to’ass& ., :.,.. p. 1, tLndIeferendestherein .’ .:. 1the 39~~,&rr-~.~tkd in.Re;C)$ .to the e,n -+ es”* ,131K.H. Loh.mon, i&Ad&& &q&n&m cl&nistry, Vol. ,‘. .; ‘1 2. t~nsj:~n~.~ ._ _,. 7, ed. P.-O. J&vdin(Au$eti’ic P&s, l&w Yo;k,,.i9?$‘. _‘ : .. :.:; ,’ : I .’ ,, _: .,p.145.,‘-‘. ,.( : ,*. -_ . . : .‘ : .“ Z&I &hrisaa, Chem. ; 14 ‘ J I$ IWsch;,W.G. &?$crerand .‘ . . .‘ . . .(. _: ,, .; . . . ” j Phys. Letters i3 fl97$149. .: . I ., :_ .._).~~~~~~~~~~.~~~~,, : I [S] ‘ K. Sctrwam,Pihys: Rev. iS (i$72) 2466. ‘. _. ;.’ ,,.. [6] F.k: Cotton and CB. Harris, Inorg. Chem. 4 !1965) ... . ; ; Tke .&hors acknoivlddge the ti A.&C. ccm; 330. jiut@qj’ce,nteq for a generous g&t cf &z-iput&‘time. ‘. _ [7]’ J.G. Sorrnan Jr..and I&J. KoI&, Che+ Commun. W! $jank JoI~‘Ca~a~y &!‘Johsj Wood fez a&~+:?-974:i3o3s . [St C.i)..Cowman and H.3, Gray, jr. Am. &e?rt! Soc.‘?S : ,_‘ingus tb use their cotipuier programs. Research at’ .. (lq?3:l81??. ” [Jf C.D. Cowman, Ph,D.“&e&, Caiii!ornia In&t&e bf 1:’ Nyo,&zq addit$onaIly supported by the National .‘. .,-I ‘:: S&nce Fdundation (GP-10331) and by@ Deutsche Technology (1974). [lo] ,A.P..Mortqla, J:Moskowitzand N. R&h, to be pub,I.’ ,~orschungsge,meinscflaftlResearch at t1le~CaJiforn.k ;‘ lished., 1’ Institut& of-Tec&ologLwas supported by the A&r.‘_. [;l] WK. Cr,tton, F.A. Cotton, hf. Debeau &I$ R.A. Waite?, .’ .: ,, -.‘, ~.eseafch_6ffice (Durham)‘. --.,.; 3. Coo:d.Chem.‘l ii9711 12i., . : 1121 J.G. Nor&~ Jr. and H_J.‘Kolarj, J. A& Chem. Sot., _ .-; , : .‘_. . I.‘ ‘submittea for publication. ,: _, .’ . ‘ ., .‘. ., .‘ : :. . : .. i.’ ., . :. .‘ _)_ .; : r,._ : . ‘ : _.‘. . : : .‘ ”‘ . ” .’ .‘
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