O ) , IR and Raman Studies of Poly-nuclear Carbonyls Transition Metal Carbonyls : A DFT Application

DFT implemented in ADF 2012.01 was used to know about the relative spatial displacements of three/four metals and the surrounding12 terminal and bridging CO groups in 5 poly-nuclear carbo nyls:[M3(CO)12],(M=Ru,Os),[Ir4(CO)12], [Fe3(CO)12] and [Rh4(CO)12]. After optimization, the software was first run using the “NMR Program” with Single Point, Default, None, Collinear, Nosym using TZP or TZ2P Basis sets leaving Unrestricted command blank to obtain the Shielding Constants (s M , s 13C,s 17O), the Chemical Shifts (d M, d13C, d17O), 2 diamagnetic and 4 paramagnetic contributing terms in the s values of constituents.The k and j values of constituents were obtained from the same program by using a new Input File.There after, the software was run with Frequencies and Raman full to obtain frequencies of the normal modes of all the (3n-6) Fundamental vibration bands of the carbonyls. All the metals in the above named first three carbonyls were spatially equivalent while in the latter two carbonyls,all the metals were not equivalent.But no where, the two metals or any two COs were found to be magnetically equivalent.Excepting Ir4(CO)12 where all the12 CO groups were spatially equivalent,in other four carbonyls, CO groups were found to be two or more types spatially.The first metal and the responding spatially equivalent other metal/s possessed same k and j values. For CO groups attached to one metal and spatially equivalent CO groups attached to other spatially equivalent metal/s, k and j values of 13C nuclei possessed the same values. A perturbing metal and spatially equivalent responding metal/s along with spatially equivalent CO groups had same k and j values respectively. The study was important in four ways. Firstly,using these parameters, we could calculate quite a more number of parameters such as Effective Spin Hamiltonian (HSpin) of the metals and 13C nuclei,Coordination Shifts (∆d C,∆d 17O),atomic electron valence density(integrated)/ L value of 17O and charges on both the 17O and metals along with spatial displacements/stereochemical equivalences of constituents. Secondly, we could correlate these NMR parameters with their reported IR/Raman results which lent credence to their П–acid character.Thirdly,we classified their fundamental vibration bands types into four types.Fourthly, we could arrive at some optimization and thermal parameters of carbonyls.


INTRODUCTION
We had successfully applied DFT to study NMR parameters such as: Shielding Constants (s M ,s 13 C,s 17 O),Chemical Shifts (d M, d 13 C, d 17 O),two diamagnetic and four paramagnetic contributing terms in the s values of constituents, the k and j values, Effective Spin Hamiltonian (H Spin ) of the metals and 13 C nuclei and Coordination Shifts (∆ d 13 C,∆ d 17 O) for the 11 mononuclear 1-3 and 9 binuclear 4 carbonyls of the 1 st , 2 nd and 3 rd transition metals.Of course, Schreckenbach et al., had ,first of all, applied DFT to 13 C and 17 O NMR spectra to a few mono-nuclear transition metal carbonyls of the metals like M=Cr 5,6 , Mo 6 , W 5,6 , Fe 5,7 Ru 5,7 ,Os 5,7 to obtain their d, s and bond dissociation energies.
But unlike the vast variety of NMR parameters of 20 mono-and bi-nuclear [8][9][10][11][12][13][14] carbonyls studied by our group of workers 1,4 ,only a little had been reported on NMR studies by computational methods for poly-nuclear [18][19][20][21][22] carbonyls as follows: Variable temperature 13 C NMR spectra of the carbonyls M 3 (CO) 12 (M = Fe, Ru, and Os) and other related compounds 18 were reported.At least three CO scrambling processes had been shown to operate in these system.Mössbauer spectroscopic studies of iron carbonyls adsorbed on ү-Al 2 O 3 and SiO 2 were carried out by Iwai et al., 19 while 1 H NMR and IR spectra of ruthenium and osmium carbonyl clusters incorporating stannylene and stannyl ligands were studied by Shariff E .Kabir and his co-workers 20 .Fang 21 submitted his thesis (2012) on,"Studies of iridium carbonyl cluster complexes,".He prepared five polynuclear iridium-germanium mixed carbonyl-phenyl complexes and studied their X-ray strucuctures and 1 H NMR by applying DFT.The same properties were studied by Yuwei Kan 22 in his thesis(2013) entitled," New ruthenium and osmium carbonyl cluster complexes with main group bridging ligands having unusual structures and bonding".
The present study includes 5 poly-nuclear carbonyls of transition metals in their zero oxidation states as: [M 3 (CO)  12 ],(M=Ru,Os), [Ir 4 (CO) 12 ], [Fe 3 (CO)  12 ] and [Rh 4 (CO)  12 ].The first 3 carbonyls contained only terminal CO groups while the other two possessed both terminal and bridging CO groups.All these carbonyls obeyed "The 18 Electron Rule".The software did not work in [Co 3 (CO)  12 ].This manuscript was subdivided as follows: It would include structures of five polynuclear carbonyls and the explanation in the difference their NMR parameters, Spatial and Magnetic Equivalences of the three/ four metals and the 12 CO groups on the basis of their structures.

(ii)
Then we would take up the calculation of Effective Spin Hamiltonian (H Spin ) of the metals and the 13 C nuclei, the division of (3n-6) fundamental vibration bands into Vibration Symmetry Classes and their IR/ Raman activities.(iii) Lastly, there would be a discussion on the corroboration of NMR results with the reported IR and Raman results, the higher AEVD {Atomic Electron Valence Density (integrated)/ L} and the higher negative charge on 17 O of CO of metal carbonyl than 17 O of CO(g) and slight increase in positive charge on the metals of the five carbonyls.

Need of the study
The following three points necessitated the use of a software for this study.

(I)
Computational Chemistry had, hardly, been used in ascertaining the pi-acid character of the poly-nuclear carbonyls by NMR technique though such studies were reported for mononuclear

Importance of the study (a)
The values of thermal parameters like zeropoint energy, moments of inertia, entropy, internal energy and heat capacity at constant volume which will be reported for the polynuclear transition metal carbonyls for the first time may prove helpful to the future scientists in studying their other physical properties.(b) Efforts will be made to exploit this technique to study Metal to LigandTransfer (MLCT) phenomenon for macro cyclic bis-and tris-complexes of 2, 2-bipyridine and 1,10phenanthroline ligands with transition metal ions.
Materials, Method and Experimental details [1][2][3] ADF software was installed on Windows XP platform as "ADF jobs".A new directory was created using "File menu" of ADF jobs.After optimization of the metal carbonyl , different commands were filled into the software to obtain a number NMR and IR/Raman parameter 1,4 such as d,s, k,j for M, 13 C and 17 O of carbonyls,values of two diamagnetic and four paramagnetic contributributing terms which constitute s values of these constituents along with atomic electron valence density (integrated)/ L value of 17 O and charges on the 17 O and metals.Diamagnetic terms were named as: {a} diamagnetic core and {b} diamagnetic valence tensor while the four paramagnetic terms were called:{c} paramagnetic (b^) tensor, {d} paramagnetic (u^) tensor, {e} paramagnetic(s^) tensor and {f} paramagnetic gauge tensors.Four more parameters namely Coordination Shifts (∆ d 13 C,∆ d 17 O), Effective Spin Hamiltonian (H^)of M and 13 C of carbonyls, atomic electron valence density(integrated)/L value of oxygen and charges on both the oxygen and metal atoms to elucidate the relative spatial arrangements of three /four metal ions and the 12 bridging and terminal CO groups was developed.
We obtained IR/ Raman parameters by giving different commands [1][2][3] to the software and corroborated them with NMR parameters to ascertain the П -acid nature of carbonyls.

Structures and explanation of NMR parameters of M 3 (CO) 12 {M=Ru,Os}
Their formation, reasons for change in NMR parameters and large differences in s 13  Os 3 (CO) 12 {Fig:2} and its lighter analogue Ru 3 (CO) 12 {Fig:1} possessed D 3h symmetr y consisting of an equilateral triangle of Os (0) or Ru (0) respectively.Each metal was directly bonded to two more metals, two axial and two equatorial CO ligands in a six coordinate trigonal prism geometry having sp 3 d 2 hybridization 27 .The d orbitals involved in this hybridization were: n d x z, n d y z {n=4,5; M=Ru, Os} with major lobes pointing towards the vertices though not as directly as in the case of an octahedron.In the excited state, these two d orbitals on each metal being half filled would take part in forming bonds with each one of the other two metals with their respective half filled orbitals.Each one of the remaining four vacant hybrid orbitals received a lone pair of electrons from the carbon of each one of four COs to form sigma bonds.The two axial COs attached to each metal being ^,did not back accept electron cloud from the filled nd x y, n d x 2 -y 2 , n d z 2 {n=4,5; M=Ru,Os}.Rather, they would lose electron density.The П* molecular orbitals of carbon of two equatorial COs were geometrically favorable and energetically suitable with the above named filled orbitals of metal to back accept electron     bonded to the remaining three Rh (0); each having coordination number seven (Rh 7 (0)) to form three Rh 6 -Rh 7 bonds.In addition, the {Rh 6 (0)} was further bonded to three terminal COs.On the other hand, each one of the three Rh symmetry symbol was given to each one of their (3n-6) fundamental vibration bands.The bands were classified as IR-active, Raman-active and both IR-and Raman-active.Some Raman-active bands which possessed negligibly small intensities or had depolarization ratios ≈0 were not observed in the spectra (Table :13).Unlike their experimental determination [29][30][31][32] , here the Raman intensities were calculated from the polaziabilities [33][34][35][36][37]

Confirmation of П-back acceptor character of carbonyls from NMR
This NMR study confirmed the П-backacceptor nature of carbonyls in two ways:

Corroboration between total coordination shift (∆ d C T ) and [υ CO ] values
Since, the carbonyls possessed stereo chemically different CO groups with different dC values,it would be better to correlate their [υ CO ] values with their Total Coordination Shift (∆ d C T ) values which were the averaged values of δC for all the carbonyl groups in any metal carbonyl.Depending upon the similarities in their symmetries, the discussion was divided into three headings as follows:

M 3 (CO) 12 {M=Ru, Os}
As the Total Coordination Shift (∆ d C T ) increased, the nCO (cm -1 ) also increased to decrease the П -back accepting capacity of electron cloud by the metals.The NMR studies corroborated well with IR studies because like υCO, the (∆ d 13 C T ) values 38,39 were also found to be higher in Os 3 (CO) 12 than Ru 3 (CO) 12 as both possessed the same symmetry point group (D 3h ) (Table :14).

Ir 4 (CO) 12
Since all the 12 carbonyl groups of Ir(0) were found to stereo chemically equivalent; each having higher s 13 C and s 17 O values(5.41and -41.57p pm) than the reference values(-34.44 and -129.53ppm) and thus confirmed the higher electron density on carbon of each carbonyl group than that on CO(g) to prove the back accepting nature of Ir 4 (CO) 12 with T d symmetry (Tables:4,5).

Fe 3 (CO) 12 and M 4 (CO) 12
With different point group symmetries (C 2v , C 3v ) and different number of bridging and terminal carbonyl groups, no comparison was possible in their Total Coordination Shift (∆ d C T ).
But, their П-back acceptor character was confirmed by (B) as follows: From the Charges and AEVD Values on 17

O and Metals
This NMR study proved two facts simultaneously:

Acceptance of electron density by CO groups
Since 17 O became more negative(though small) than 17 O of CO(g)} to show higher AEVD {Atomic Electron Valence Density (integrated)/L} than17O of CO(g) (Table :9) to confirm the acceptance of electron cloud from the metal/s.

back donation of electron cloud by the metals
Again, each metal acquired a very small positive charge(Table:8) to prove that the metal/s would donate electron density to CO groups.
Hence like vibration spectral studies, the NMR studies also confirmed the synergic nature of the metal carbonyls.

CONCLUSION
We are able to reaffirm the relative spatial displacements of both the terminal and bridging carbonyl groups and the П -acid character of the carbonyls from the NMR parameters of 13 C and 17 O nuclei such as s 13 C,s 17 O, d 13 C, d 17 O along with the six diamagnetic and paramagnetic constituting terms of sM, s 13 C and s 17 O.We were also able to confirm the spatial equivalence/nonequivalence of the three/ four metal ions of the carbonyls.This NMR studies corroborated with the results already obtained from their IR/Raman studies.Lastly, we could identify some bands which, no doubt, were Raman active but because of their negligible Raman intensities or linear Depolarization ratios could not be observed in their Raman spectra.

Fig. 1 : 12 Fig. 2 : 13 C 17 OFig. 3 : 12 Fig. 4 :
Fig.1: D 3 stereochemistry of Ru 3 (CO) 12 Fig.2: D 3 stereochemistry of Os 3 (CO) 12 on metal.But due to a strong П back donation from the filled d orbitals of metal to energetically favorable and geometrically suitable vacant П* molecular orbitals of CO (OC →M), electron density should be reduced on the metal to cause an increase in the electron density on CO.As s of a nucleus was directly related to electron density, any change in the value of its s should serve as an indicator to the change in electron density.Hence,if CO was to act as a back acceptor, s13 C of metal carbonyls should become more than s 13 C of CO (g).But according to vibration (IR/Raman) spectroscopy, the П back donation of CO would causes a decrease in nCO in metal carbonyls with respect to pure CO(g){nCO= 2143 cm -1 } 26 .(h) Some of this increased electron density on carbon was also transmitted to oxygen of CO group to make their s 17 O also more than that that in free CO. (i)Relative spatial displacements of constituting species were reaffirmed from shielding constants of the M, C and O {s M, s 13 C (M CO), s 17 O (MCO)} simply by the fact that the spatially equivalent species should have same values of shielding constants along with their constituting two diamagnetic and four paramagnetic terms respectively.Structures, NMR and IR/ Raman Parameters of poly-nuclear carbonylsTheir discussion was subdivided into nine headings (4.2.1-4.2.9) as follows:Trends in NMR parameters of 5 polynuclear carbonyls(1)  Even if the 3 or 4 metals were found to be spatially equivalent, their CO groups might possess different NMR parameters like s13 C s 13 O, d 13 C d 13 O. (2) Depending upon the geometry, even the COs attached to the same metal might differ spatially to give more than one value of s z C, d 13 C and s 17 O, d 17 O.O f c o u r s e, t h e s p a t i a l l y e q u i va l e n t species were always expected to have the same values of the 2 diamagnetic and 4 paramagnetic terms which contribute to the Fig.5: C 3v stereochemistry of Rh 4 (CO)12 ,23,25)= -4.99; T-2 (21) T-1 Numbers in parentheses [ Figs: 1-5 ] ;T,T-1,T-2,T-3-terminal, B-bridging 7 (0) was directly bonded three Rh (0) {one Rh6 (0) and two Rh 7 (0)}, two terminal COs and two bridging COs as explained below: (ii)Rh6 (0) with sp 3 d 2 hybridization possessed three half filled hybrid orbitals and three vacant hybrid orbitals.In addition, Rh 6 (0) possessed three completely filled atomic orbitals.Each one of the three half filled sp 3 d 2 hybrid orbitals formed three Rh 6 -Rh 7 bonds with each one of the three other Rh 7 (0) by overlapping with each one of their half filled sp 3 d 3 hybrid orbital respectively.(iii) Each one of the remaining three vacant sp 3 d 2 hybrid orbitals received a lone pair of electrons from carbon atom of each one of the three COs.These three COs, then, back accepted electron cloud from the three filled d orbitals of Rh 6 (0) to increase the electron density which was confirmed by their increased s 13 C and s 17 O values.Thus the three COs acted as the terminal groups (T-3) with Rh 6 (0).(iv) Now each one of the three Rh 7 (0) with sp 3 d 3 hybridization (having three half filled hybrid orbitals and four vacant hybrid orbitals) with two completely filled atomic orbitals was left with two half filled and two vacant sp 3 d 3 hybrid orbitals after forming a Rh 6 -Rh 7 bond.Each one of the two half filled hybrid orbitals on each Rh 7 (0) formed two Rh 7 -Rh 7 bonds with each of the two similar Rh 7 (0).(v) Then, each one of the other two vacant hybrid orbitals received the lone pair from carbon of CO; back accepted the electron cloud from two filled atomic orbitals to increase the electron density as indicated by the increased s 13 C and s 17 O values.The two terminals COs on each one of these three Rh 7 (0) were of two types spatially (one axial and other^).So, the 9 terminal COs were of three types.Six COs bonded to three Rh 7 (0) were divided into two types of terminal COs (T-1,T-2);one T-1 and oneT-2 were bonded to each one of the three Rh 7 (0)}.As already explained,three terminals Cos (T-3) were bonded to Rh 6 (0).(vi) There were still left six half filled atomic orbitals {two on each one of the three Rh 7 (0)} and three lone pairs of electrons on three COs which acted as bridging COs with these three Rh 7 (0) involving two Rh 7 (0) at a time.Thus, each one of three Rh 7 (0) was bonded to two bridging COs.Various spatial displacements of COs having different shielding constants were given in (Tables:10,11).

Table 1 : Optimization Parameters of Poly-nuclear Carbonyls Carbonyl Point Total Total Energy: Nucleus I Dipole group bonding X c** (LDA)** moment (D) Energy* k J mol -1
*Bonding energy is computed as an energy difference between molecule and fragments**Xc contains LDA and GGA Components; both contain Exchange and Correlation parts; GGA is zero in all the three carbonyls

Table 8 : Charges and Types of Metal Ions Carbonyl ADF numbers of Metal Ions Charges on Metal Ions Types of Metal Ions
Two bridging COs (Table: 10) possessing lowest set of values of s 13 C and d 17 O (highest set of d 13 C, d 17 O) respectively were of the first type.Second type consisted of four terminal COs (Table-10) with another set of values of s 13 C, s 17 O and d 13 C, d 17 O.Each one of the remaining three types of CO s (Table-10) contained two similar terminal COs with three different sets of s 13 C, s 17 O and d 13 C, d 17 O values respectively.(10) [Rh 4 CO) 12 ] contained four types of COs.Three bridging COs, though bonded to two different sets of Rh (0) showed lowest but same set of s 13 C and s 17 O (highest set of d 13 C, d 17 O) values and thus belonged to the same type (Table-10).Each one of the remaining three types of COs containing three similar terminal COs (Table-10) with three different sets of s 13 C, s 17 O, d 13 C, d 17 O values belonged to three different types.
12 ] contained five types of CO s with 5 sets of values of s 13 C, s 17 O and d 13 C, d 17 O respectively .

Table 9 : Atomic Electron Valence Density (integrated)/ L Value and Charges on O Carbonyl ADF Numbers of O
C, s 17 O, d 13 C, d 17 O values in these carbonyls, was explained as follows: *AEVD (integrated)/ L value at O=6.359 and **Charge on O= -0.359 in uncoordinated CO (g)

Table 14 : Coordination Shifts of [M 3 (CO) 12 ] (M= Ru, Os)
Each one of its remaining three vacant sp 3 d 2 hybrid orbitals would receive a lone pair of electrons from each one of the three carbon atoms of the COs and back accept electron cloud from the three filled d orbitals of Ir (0) to form bonds with three equivalent terminals COs.The increase in electron density was indicated by the increased s 13 C and s 17 O values.The 12 terminal COs were found to be spatially equivalent (Table:10) having the same s 13 C and s 17 O values respectively.