Vibrational spectra of dodecacarbonyltetrarhodium

NORG.

HUCL.

CHEM.

LETTERS

Vel.

S, pp.

M~'M4

1969.

Pergentea Prua.

Pdntad

I,

Greet

Britain.

VIBRATIONAL SPECTRA OF DODECACARBONYLTETRARHODIUM E.W. Abel and R.A.N. McLean. Department of Inorganic Chemistry, The University, Bristol, England.

Continuing our study I of tetranuclear metal carbonyl clusters, we have recorded the solid-state infrared and Raman spectra of Rh4(CO)I2.

In Ir4(CO)12

the Raman spectrum 1'2 has three very strong bands at 208. 164. and 105 cm-I. which are assigned to the AI. T 2 and E vibrations respectively of the Ir 4 metal cluster.

This is in accord with the T d structure of Ir4(CO)12 in which

four It(CO) 3 units are joined solely by metal-metal bonds 3. and the assignment is further confirmed by the appearance of only one band at 162 cm

-I

.

xn this

region of the infrared spectrum I.

Oc ~ /CO

l\

A\

l ~ ~-.

/

oc/ I ~ R h I

\

OC

OC\l~r/CO

oC/

CO

I

.co _It--co

/\\ 0c

~

CO

The crystal structure of Rh4(CO)I2 differs from Ir4(CO)12 in that there are present three basal bridging carbonyls as shown in the figure.

3

This

reduces the symmetry for Rh4(CO)I2 to C3v , and we would expect four (2A I ÷ 2E) vibration bands in both the infrared and the Raman spectra. frequencies for Rh4(CO)I2 below 250 cm

-I

The observed

are shown in the Table.

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SPECTRA OF DODECACARBONyLTETRARHODIUM

Vol. 5, No. 5.

TABLE Observed Infrared

Raman

and Raman Bands o f Rh4(CO)I2_ _ below 250 cm

225 (90), 200 (iO)

Infrared

197 (IOO)

176 (IOO)

-I

128 (45)

174 (20)

(F~gures in brackets after each band position (in cm -I) are the approximate relative intensities.)

A qualitative examination of the results in the table shows that except for the weak band at 200 cm -I the presence of the basal bridging carbonyl groups does not cause any great change between the Raman spectra of Ir4(CO)12 and Rh4(CO)12 in the metal vibration region.

In the infrared spectrum, however,

the reduction in s~mnetry has been sufficient to split the T 2 mode of a perfectly T d molecule into an E mode (197 cm -I) and an A I mode (174 cm -I) (assignments based on relative intensities in Raman and I.R.).

Overall,

however, the presence of three bridging carbonyl groups does not produce

two significantly different types of metal-metal bonds, as if

this were so, we would expect the other E and A 1 modes at 225 cm

-1

and 128 cm

to appear in the infrared spectrum. Despite the lower symmetry of Rh4(CO)I2 being only marginally reflected in the metal-metal bonding, a single metal-metal force constant does not predlct observed frequencies well.

Using the simple cluster model 2, a single

force constant of O.9 m.dynes/~ predicts frequencies for the AI, T 2 and E modes of 242, 173 and 122 cm

-1

respectively, compared with the observed

frequencies of 225, 188 (200 and 176 have been averaged for comparison) and 128 cm-l.

This poor agreement may reflect that interaction constants are

larger in Rh4(CO)12 than in Ir4(CO)12.

Alternatively,

although a qualitative

analysis of the spectra of Rh4(CO)12 suggests that it may be treated as an M 4 tetrahedral cluster for metal-metal bond vibrations, the finer quantitative

-1

Vol. 5, No. 5.

SPECTRA OF DODECACARBONYLTETRARHODIUM

15

details of the spectra must be derived from an M3M' trlgonal pyramidal cluster. In the M3M' cltmter model the A I and E vibrations of the M 3 triangle with bridging carbonyl groups should have frequencies in the ratio /2:1, and this is well satisfied by the bands at 176 and 128 cm -I, giving a metal-metal stretching %

force constant of

~0.95 m.dynes/~ for the metal-metal bonds bridged by carbonyls.

A further condition requires the two remaining modes to be very close, which is only partially satisfied by the 225 and 200 cm "I bands.

The position of these

two bands is, however, predictable to within i0 cm-I with a M-M' metal free constant of 1.35 m.dynes/~. Vibrational spectra of binary metal carbonyls without bridging groups are usually free of absorptions between 230 cm -I and 350 cm -I.

Rh4(CO)I2 has

strong absorptions in this region both in the Raman spectrum at 316 (2) p 339 (10) and 360 (5) cm -I and the infrared spectrum at 337 (8) and 358 (10) cm -I. --1

Whilst the band at ~360 cm

•

is very likely a terminal M-C stretching mode

(a band also appears I in Ir4(C0)12 at 365 cm-l)p the absorptions at 316 and 339 cm

-i

we assign to mainly bridging M-C stretching vibrations.

(Co2(C0) 8

has a similar band at 329 cm-l.) The C3v sym=netry of Rh4(CO)12 is further reflected in the region 600-360 cm

-i

, where although not all the possible bands were observedp the Ranmn

absorptions at 490, 440, 400 and 360 cm

-i

were co-incident with the cortes-

ponding active bands observed in the infrared, such not being the case in Ir4(C0)12 of T d symmetry. The seven (3A 1 and 4E) CO-stretching vibrations are observable in both the Raman and infrared spectra of Rh4(CO)I2. We have been unable to record the Raman spectrum of the corresponding Co4(C0)12 due to rapid decomposition caused by absorption of the Laser energy due to the very deep colour.

ACKNOWLEDCE~NTS We are grateful to Drs. P. L. Goggin, P. J. Hendra and M. M. Qurashi for help with some of the spectra determination, and to the Salter's Institute for a scholarship (R.A.N. McL.).

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REFERENCES I.

EoWo ABEL, P . J . HENDRA, R.AoNo HcLEAN, M.H. QURASHI, I n o r g . Chim. Actat 1969 o i n p r e s s .

2°

C°O° QUICKSALL and T. $PIROs Chem. Comm. (1967) 839.

3.

C.ll. NEI and L.F° DAHL, J . Amer. Chem. Soc. ~

1821 (1966).