A molecular orbital study of the bonding between Th and Ni in a novel compound

Journal of Molecular Structure, (Theochem), 153 (1987) 241-248 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

A MOLECULAR ORBITAL STUDY OF THE BONDING BETWEEN Th AND Ni IN A NOVEL COMPOUND

M. A. MAKHYOUN, Department (Kuwait)

B. D. EL-ISSA and B. A. SALSA’

of Chemistry,

University of Kuwait, P. 0. Box 5969,13060

Khaldiyyah

(Received 30 March 1987)

ABSTRACT The Multiple Scattering Xa! method is used to discuss the bonding between Th and Ni in Th(r)5-C,H,)1(~-PH,)1Ni(CO),. INTRODUCTION

In the past few years, a large number of heteronuclear metallic complexes have been synthesized and well characterized [l-6] . The presence of hetero bimetallic bonding in this interesting class of compounds may be substantiated by X-ray structural determinations and supplemented by MO calculations 17, 81. In this work we report a relativistic MS-Xe calculation on the bimetallic compound Th(~5-C5H&(~-PH2)2Ni(C0)2 (Structure I) which is the analogue of the recently prepared complex Th(q5-C5(CH3)5)2(p-PPHz)zNi(CO), [9]. The most interesting structural feature of this compound is the presence of a short Th-Ni distance. This is estimated to be 0.5 A less than that in the non-bonded counterparts [lo] . A possible cause for such a short distance is steric hindrance but this has been ruled out on the basis of the geometry of the molecule [9]. Recently an Extended Hiickel calculation [lo] has been reported for Th($-C5H5)2(~-PH2)2Ni(C0)2. The work had been directed towards the proposed hetero metal-metal interaction in the compound. These calculations have indicated a negative reduced atomic overlap population over a range of Th-Ni distances of 3.2-3.7 A. This implies that a net antibonding interaction operates between the thorium and nickel atoms. In the present work we explore the possibility of the presence of thorium-nickel bonds by using a more sophisticated MO approach. METHOD OF CALCULATION

The relativistic MS-Xe method employed in this work has been discussed previously [ll, 121. It differs from the non-relativistic version in that it 0166-1280/87/$03.50

0 1987 Eisevier Science Publishers B.V.

242

Structure

I

includes the mass velocity and Darwin terms in the Hamiltonian. An updated MS-Xc [13] version was used in order to determine the ground state electronic structure of Th(~s-CSHs)2(p-PH2)2Ni(CO)z. The values for the atomic OLparameters used around all the atoms except thorium were taken from the Schwarz [14] values. The theoretical (Yvalue of Dohl et al. [15], however, had been used for thorium since such a value is not available from the Schwarz tabulation. In the intersphere and outersphere regions, weighted averages of the atomic cv values were used and the sphere radii were chosen according to the scheme described by Norman [16] and scaled by a factor of 0.88. The partial wave expansion was truncated at 1,, = 4 for the outersphere, 1,, = 3 for thorium, 1,, = 2 for nickel and phosphorous, I,, = 1 for carbon and oxygen and I,,,, = 0 for hydrogen. The 6s and 6p electrons of the Th atom were included during the SCF procedure as valence electrons. The various parameters used in this work are reported in Table 1. The bond distances and angles in the four membered ring ThPzNi were taken from the X-ray

243 TABLE 1 The sphere radii in au, quantum number (I-)

the exchange (Y values and the maximum angular momentum used in the MS-Xa calculation of Th(n W,H,),(p-PH,),Ni(CO),

Outersphere Th Ni P H(phosphido group) C(carbony1 group) O(carbony1 group) CWng) H(ring)

a

Sphere radius (au)

0.74449 0.69200

10.9284 3.2465 2.2106 2.4107 1.4492 1.6017 1.6222 1.6812 1.1761

0.7 0896 0.72620 0.77725 0.75928 0.74447 0.75928 0.77725

data of Th(q5-C5(CH3)5)2(~-PPh,),Ni(CO), [9] while other bond distances and angles were taken from similar molecules [ 4, 171. The complete geometry of the compound is depicted in Table 2. RESULTS

AND DISCUSSION

The converged statistical potential of the ground state of Th(r)5-C5HS)2(pPH2)2Ni(C0)2 leads to the following populations and net charges (9) for the Th and Ni atoms: Th: (6s, 7~)~*~~‘,(6p, 7~) 6.0656& 185p 816,Q~ 4#.665, Ni: 3d s.744sO.454

~~~ =

=

6.712

6.141

The oxidation numbers of thorium and nickel in this compound are +4 and 0 respectively. In the case of thorium, one expects the formal valence configuration to be doto. The net non-zero populations in these orbit& is apparently a result of charge transfer from the ligand moieties. As suggested earlier [ 151, the f and d populations in this case may be taken as a measure of the covalent bonding in the compound. If this is true, then one can conclude that the Th d orbitals are most significant in representing the bonding between the thorium and the surrounding ligands. This conclusion is further supported by considering the contribution of the Th d components in the individual bonding molecular orbitals (MOs) (Table 3). The other important interactions are contained in the orbitals that have preponderant Th f character. We bear in mind that the Th 6s and 6p orbitals are corelike and that their populations will be expected to be very close to the maximum occupation numbers of 2 and 6 respectively. In a distorted tetrahedral environment, the Ni atom is expected to exhibit the formal valence configuration of d los ’p ‘. The observed non-zero populations in the s and p orbitals of nickel may be explained in line with our interpretation of such a population in the case of the thorium atom. In the case

244 TABLE 2 Geometry of Th(775-C,H,),(p-PH,),Ni(CO),. dienyl {C,H,) rings. Distances are in .k

Cp represents the center of the cyclopenta-

Dis tunces Ni-P = 2.245 Th-Ni = 3.206 Th-P = 2.880 Th-Cp = 2.540 c-o= 1.130 Ni-C = 1.790 P-H = 1.450 c-c = 1.430 C-H = 0.960 outersphere-Ni = 2.7 09 outersphere-Th = 0.497 Angles Th-P-Ni P-Th-P C-Ni-C Ni-C-0

= 7 6.3” = 85.8” = 116O = 180”

P-Ni-P = 121.6” H-P-H = 109.5” Cp-Th-Cp = 139”

of the Ni d orbitals, however, the observed population may indicate a partial electron transfer in a backbonding donation to the ligands and possibly to thorium There are 53 doubly occupiedvalence MOs in Th(r) 5-CsHs)z(p-PH2)2Ni(C0)2 and in view of the complexity of the molecule, we shall concentrate on those that have an appreciable Th and/or Ni character. A part of the MOs spectrum of the Th-Ni complex and the corresponding orbital charge distribution over the various atoms is shown in Table 3. These are the MOs where appreciable interaction between the metal centers and ligand moieties is expected. Eight of the virtual MOs displayed in Table 3 are mainly the vacant 5f-like MOs of thorium. Because the molecule has no center of symmetry, both the f and d orbitals of Th are mixed together in the same symmetry representation and consequently one of the,virtual molecular orbit& ( 21al) is Th d-like and intrudes well into the f-manifold. The other d-like orbitals are expected to occur at much higher energy. The Ni 3d-like MOs are expected to be completely filled because of the zero formal charge of Ni in this compound. An inspection of Table 3 reveals that the MOs of preponderant Ni 3d character are in descending order with respect to the energy: 14b2 > 9bI > 17~~ > 7~~ > 16Q1. Judging from the composition of the Ni d, component in the 7~ level, it can be assumed that this orbital is primarily non-bonding. The bonding counterparts of the others, however, are the 8&, 8bl, 14aI and the lOal molecular orbitals. The orbitals contained in the lOal and 8bz sets are well-stabilized and are separated from the orbitals that are associated with bonding interactions involving the Ni-3d orbitals due to the strong interaction between the s and d orbit& centered on Ni and the hybridized sp orbit& centered on the carbon atoms of the [CO] groups. The 14aI, 8bI and 16~~ orbitals are higher in energy than the latter set and are associated with interactions between the Ni-d components and the PH, groups. An important point here is that the Th-Ni interactions are manifested mainly in the orbitals that contain preponderant Ni-3d character and consitute bonding interactions with the Th d orbit&. The interactions

8b, 10% 3b, 4% lb, 3a, 26,

8b, 14a,

16% 150,

7e, 13b,

9b, 17a,

8% lob, 14b,

18% 15b,

9% llb, 19a,

1.50

12.00

1.60

3.9

31.70 11.00 97.40 85.00 66.20

1.00

5.60 1.20

1.00 1.10

1.00

1.00

4.90 4.80

1.1 2.9 1.1 8.4

6.30 5.50 4.80 3.90 1.90

27.5 7.4 9.1 80.0 5.9 20.5 10.5 6.20 1.0 12.10 22.20 20.30 35.90 9.40 2.00 14.20 7.00 0.50 0.50 0.50 1.40 0.40

88.8 75.0 84.9 87.5 16.1 4.60 5.80 10.60 2.10 1.10 0.30 2.20 2.40

15.5 76.7 81.7

16.20

8.20

6.60

9.6

3.10 2.10 11.10 4.00

3.00

11.70 0.30

P

2.70

1.7 1.30 1.50

9.6

72.50 4.20 33.60 22.70 22.10 11.90

7.40 66.90 48.90 43.00 90.80

22.60 2.20

d

1.50

3.10 2.20

2.40 1.60

2.00

s

1.00

10.90 2.50 38.60 41.30

19.80 25.10

59.10

3.90 1.80 28.80

1.40

16.00 1.20

p

2.10 2.90

1.00

3.90 2.80 1.10 2.30

2.30

2.80

7.20 2.40 3.10 1.00

d

4.60

33.90 32.10

s

1.60

2.70 1.80

3.10 5.40

3.60

8.80 1.40

29.90 29.20

p

sp

0

2.80 2.60

1.90

3.20 3.50

3.30 2.60

1.70

2.90

2.30 1.90 3.00

2.80 5.50

3.40 5.60 1.30 3.60

11.30 28.10

1.80 87.30 2.90 75.30

73.60 71.40 1.40 64.70 60.40

p

58.40 75.00

s

Clkls

1.50 2.20 5.10 3.20 1.60

4.30 1.00 1.20

1.40

13.40 15.00

s

-0.2845 -0.3054 -0.3140 -0.3185 -0.3238 -0.3269 -0.3273 -0.3300 -0.4435 -0.4589 -0.4787 -0.4847 -0.5027 -0.5477 -0.5681 -0.5994 -0.6141 -0.6663 -0.67 00 -0.6830 -0.7234 -0.8028 -0.8288 -1.1323 -1.1734 -1.7103 -1.7600 -1.8333 -1.8449 -1.8786

f

C

22a, 13b, 17b, 21a, 16b, 20a, 12b,

d

s

P

s

P

H

Ni

Energy

State

Th

orbital point

3

Part of the spectrum of the orbital energies and the charge composition in Th(qW,H,),(cc-PH,),Ni(CO),. The highest occupied is (lib,) and the lowest unoccupied orbital is (9a,). The designation of the molecular orbitals are according to the C,,symmetry group and the energies are in Rydberg units

TABLE

lw

246

that deserve special attention here are those contained within the 16a1, 12~~ and 9b1 orbitals. Because the Th 5f components have an insignificant contribution in these particular MOs, the proposed Th-Ni bonding is best described in terms of d-d interactions. In the 16al MO the bonding is mainly through the dz2 orbitals centered on Th and Ni and since both atoms lie along the z axis, the bonding involved here may be construed as being of a u-u nature as may be verified by Fig. 1. In the case of the 9b1 orbital, however, the orientation of the d, orbit& on both metals are in favor of dn-dn interactions. The presence of at least partial n-bonding between the two metal centers may be the cause of dn-pn weakening between the Ni and the CO group in the xz plane. The d, component of Ni which is expected to be involved in n-bonding with the CO group is now engaged in dn-dn interactions with the other side. Only the d, components can participate in n-bonding with the CO group as may be deduced from the charge composition of the 14bz MO (Table 1). (No significant Ni-C n-back donation was detected in the Xol results of Ni(C0)4 [ 181.)

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0.3-

0.1-

-0.k

-0.3-

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-0.7‘\

,

-0.9

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-0.7

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-0.5

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-0. I

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0. I

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, 0.3

0.5

0.7

0.9

X Axe +I0 Y AXIS x10

Fig. 1. A contour map of the (161~‘) molecular orbital showing a 0 interaction between the Th and Ni atoms in Th(q5-C,H,),(p-PH,),Ni(CO),. Broken lines represent negative contours. Contour levels are drawn from 0.00 to to.15 with an increment of +O.Ol units.

247

Fig. 2. A contour map of the highest occupied orbital (2b,g) in Re,Cli- showing a quadruple bond. Broken lines represent negative contours. Contour levels are drawn from 0.00 to f 0.15 with an increment of kO.01 units.

The type of interaction between the Th and Ni atoms which we have discussed so far is not expected to be as strong as those between the ligands and the two metals especially if we take into consideration the Th-Ni antibonding interaction (particularly between the d,z _ Y* components on both sides) that are contained within the l&q level. Because this level is occupied, a bond weakening is expected to occur but since we have three bonding as compared to one antibonding levels, we conclude that a final bonding effect will persist. This is probably the cause of the short Th-Ni distance in this system. Bearing in mind that all these bonding levels have a preponderant Ni 3d character, the interaction here may be considered as resulting from a net flow of charge from the nickel to the thorium atoms. The important nature of the bonding between the thorium and the surrounding ligand moieties is exhibited by the Th 6d orbitals. Less contribution is manifested by the 5f, 6p and 7s orbitals. The Th Gp-like MOs are energetically very stable with respect to the rest of the orbitals. There is a small bonding interaction between the Th 6p orbitals and the cyclopentadienyl rings. The bonding contributions of the 6d and 5f orbitals of thorium start to show at much higher energy.

248 CONCLUSION

This study attempts to explain the interaction inherent between the Th and Ni atoms in Th(q5-CSH5)2(p-PH2)2Ni(CO)Z. Our results show beyond any doubt that (I interactions between the dzz component of the Th and Ni atoms play an important role in stabilizing the molecule and are responsible for the unexpectedly short Th-Ni distances. Metal-metal interactions have become extremely important nowadays and many catalytic processes have been explained by the nature of the bonding between the metal centers, These interactions can be either of u or n nature involving the d orbit& of the transition metals. An extreme case of such interaction are quadruple 6 bonds involving the d orbitals of two transition metals in a square bipyramidal environment. The presence of such a bond had been postulated by Cotton and co-workers [7] in the case of Tc,C13,- and has been confirmed by recent multiple scattering calculations on Re2Cl~- [ 191. We report in Fig. 2 a contour map that depicts such an interesting bond. The metal-metal u bonding interaction alluded to in this study and the metal-metal 6 interactions reported in the case of Re,ClZ,- are perhaps an indication of the involvement of the d orbitals of the transition metals in bonding interactions which result in stabilizing the molecular system in a particular conformation. ACKNOWLEDGEMENT

The authors express their gratitude to the Research Management Unit at Kuwait University for supporting this work (Project SC033). REFERENCES 1 A. Albinati, A. Musco, R. Naegeh and L. M. Venanzi, Angew Chem., Int. Ed. Engl., 20 (1981) 958. 2 W. Clegg, C. D. Ganer, J. R. Nicholson and P. R. Raithby, Acta Crystallogr., Sect. C, 39 (1983) 1007. 3M. C. Azar, M. J. Chetcuti, C. Eigenbrot and K. A. Green, J. Am. Chem. Sot., 107 (1985) 7209. 4 R. T. Baker, T. H. Tulip and S. S. Wreford, Inorg. Chem., 24 (1985) 1379. 5L. G. Gelmimi, L. C. Matassa and D. W. Stephan, Inorg. Chem., 24 (1985) 2585. 6 A. Fumagalli, S. Matinengo, G. Ciani and G. Marturano, Inorg. Chem., 25 (1986) 592. 7 F. A. Cotton and B. J. Kalbacher, Inorg. Chem., 16 (1977) 2386. 8 L. Szterenberg and B. Jezowska-Trzebiatowska, Inorg. Chim. Acta, 86 (1984) L29. 9 J. M. Ritchey, A. J. Zozuhn, D. A. Wrobleski, R. R. Ryan, H. J. Wasserman, D. C. Moody and R. T. Paine, J. Am. Chem. Sot., 107 (1985) 501. 10 J. V. Grtiz, J. Am Chem. Sot., 108 (1986) 550. 11 J. H. Wood and A. M. Boring, Phys. Rev. B, 18 (1978) 2701. 12 B. D. El-Issa, M. A. Makhyoun and B. A. Salsa, Int. J. Quant. Chem., 31 (1987) 295. 13 M. Cook and D. A. Case, QCPE, 14 (1982) 465. 14 K. Schwarz, Phys. Rev. B, 5 (1972) 2466. 15 D. Dohl and N. Rosch, Inorg. Chem., 25 (1986) 2711. 16 J. G. Norman, Jr., J. Chem. Phys., 61 (1974) 4360. 17 E. A. Mintz, K. G. Moloy and T. J. Marks, J. Am. Chem. Sot., 104 (1982) 4692. 18 K. H. Johnson and V. Wahlgren, Int. J. Quant. Chem., 6s (1972) 243. 19 A. Katrib, B. D. El-Issa and R. Ghodsian, Int. J. Quant. Chem., 29 (1986) 993.