Electronic structure and bonding in L-Ni-PH3 (LCO, N2) complexes

Journal of Molecular

Structure

(Theochemj,

304 (1994) 41-44

41

0166-1280/94/$07.00 0 1994 - Elsevier Science Publishers B.V. All rights reserved

Electronic structure and bonding in L-Ni-PH3 (L = CO, N2) complexes Manuel

Braga

Department (Received

of Physical

Chemistry,

14 July 1993; accepted

Chalmers

University

of Technology,

S-412 96 Gothenburg,

Sweden

24 July 1993)

Abstract Ab initio MO calculations were carried out for the model systems OCNi-PH, and N2-Ni-PH3. The magnitude of the ligand + metal 0 donation and metal + ligand n back-donation was calculated and the contribution of both processes to the bonding in these systems analysed in detail.

Introduction In recent papers [1,2] the nature of the bonding in transition metal complexes containing phosphorus compounds has been discussed in detail. In these studies attention was mainly focused on the relative extent of the 0 and r charge transfer processes and the nature of the phosphorus 7r-acceptor level. It was found that in the case of the Ni(PX,),, where X=H, F or Cl, the extent of the P --) Ni 0 donation and the Ni -+ P r backdonation are clearly dependent on the electronegativity of the atoms attached to the phosphorus atom [2]. Moreover, it was found that the phosphorus 3p7r level is involved in the metal + P 7r back-donation with a comparatively small participation of the P3d orbital. This conclusion is in agreement with other recent theoretical studies [3,4]. A comparative study of the (T-T donor acceptor capabilities of the CO, N2 and PF, ligands using ab initio MO calculations has also been done [2]. In this paper we report an improved ab initio MO study for the model systems OC-Ni-PH3 and N2-Ni-PH3. The interest in these systems lies in the fact that they allow a direct comparison SSDI 0166-1280(93)03452-D

of different ligands, whilst at the same time they are small enough to permit an accurate theoretical treatment. In the study, the ligand + metal 0 donation and metal -+ ligand 7r back-donation charge transfers for the various ligands attached to the metal atom are calculated and their relative importance for the bonding was analysed in detail. It was found that the Ni --f ligand 7r back-donation makes the largest contribution to the bonding in the carbonyl complex. In the dinitrogen complex, the ligand + Ni 0 donation is the most important bonding mechanism. Finally, the influence of the P3d level in the 7r back-donation was found to be almost negligible. Computational

details

Ab initio Hartree-Fock calculations were carried out using the MOLECULE program. For carbon, nitrogen and oxygen atoms the Dunning double-c (9s5p/4s2p) basis set was used [5]. For hydrogen we used the (4s/2s) double-c basis set with a stalling factor of 1.2 [5]. The phosphorus atom was described using the Veillard double-c (12s9p/6s4p) basis set [6] to which a 3d polarization function (a = 0.48) was added. For nickel the

M. BrugqJ.

42 0.c

OC- Ni-PH3

co

Mol. Struct.

N2

3d p.4~ -

d

16a,

16a,

3dz2,4s

z

P W

7e

z

W

501 / /

15a, -

\ Ill_ __--

6e

XY

Jdxz, YZ

‘1 ,

1501

2e _--------__

5e ---

3d x2-&

-‘\.

,x’-

0.5 5u

304 (1994) 41-44

N2-Ni-PH3

PH3

5

i Theochem)

5e

\ \ \ , 14al

3uo

-4e

-.

4e _/_,r=-- 1% 140,

4u _---

_-

2%

130,

/

-’

4a1 12al --

__

-L--_

---

130,

,/’

12al

- 1.0

Fig. 1. Orbital energy levels for the OC-Ni-PH3

and N,-Ni--PH,

Wachters (14s9pSd) basis set [7], increased by the addition of one 4p function with an exponent of o = 0.22, was used; the final basis set for the nickel atom was (14slOpSd/Ss6p3d). As an accurate description of the P3d shell was particularly important in the present study, we also tried using a double-c polarization basis for the phosphorus atom with exponents 0.825 and 0.275 [8]. However, the use of such a “double-<” polarization basis did not change to any significant extent the results obtained with the use of a single 3d function (the participation of the diffuse 3d function is totally negligible). All the calculations were carried out assuming CsV symmetry and the PH, group was kept fixed. The geometries of the PH3, N2 and CO ligands

complexes and PH?, N2 and CO ligands

were taken as those of the free molecules. For the Ni-P distance we used the same value as in our previous study of the Ni(PX,), series [2]. For the Ni-C

and Ni-N

distances

the values reported

in

Ref. 9 were employed. Results and discussion In Fig. 1 we show the highest occupied valence orbitals which are relevant for the bonding in the OCNi-PH3 and NJ-Ni-PH3 complexes. The HOMO 16a, is a hybrid Ni4s-Ni3d,2 orbital with 42.6% and 56.3% of 4s and 3d,z character, respectively, for the carbonyl complex and 54.7% and 48.6%, respectively, for the dinitrogen complex. The next two MOs (7e and 6e) are Ni3dn and

M. BragalJ.

Mol. Struct.

(Theochem)

Table 1 Gross atomic populations

304 (1994) 41-44

for OC-Ni-PH,

43

and N,-Ni-PHs

complexes

Atom

Orbital

OCNiPHs

N*NiPH,

Ni

4s 4PXJ 4P, 3dzz 3dX2_,z,yy 3dzyi

0.82 0.00 0.08 1.46 4.00 3.58

1.10 0.00 0.09 1.18 4.00 3.14

C(Nl)a

2s 2P; 2PXJ

1.62 1.09 1.20

O(N2)

2s 2Pz 2Pw

P

H

co

N?

1.69 1.05 2.18

1.84 1.01 0.93

1.83 1.17 2.00

1.I1 1.35 3.08

1.85 1.21 I .96

1.74 1.41 3.06

1.83 1.17 2.00

3s 3Pz 3PX.V 3dZz3d,I -I.>,q 3dxz.q

1.51 1.47 1.85 0.11 0.05 0.13

1.49 1.83 0.08 0.05 0.12

1s

0.96

0.96

0.97

Ni3dS orbitals with a remarkably low ligand admixture in the case of the 7e MO. The g and x orbitals of PHs, CO and N2 are distributed in the remaining al and e representations. The HOMO levels of the PHs, N, and CO ligands are shifted to lower energies due to interaction with the metal 3d,2 and 4s orbitals. The analysis of the two lowest lying unoccupied orbitals of e symmetry (8e and 9e) is relevant to the understanding of the rr back-bonding mechanism and the role of the P3d orbital in the bonding. In both complexes these two orbitals are characterized by strong mixing between the CO(N2)27r*( 17rg) level and the P3p7r level to such an extent that it is difficult to identify the nature of each orbital. The P3dr component is found to be very small in both levels. This result is consistent with a previous theoretical study on the nature of the LUMO in PHs and other substituted phosphines [lo]. Both orbitals also have some metal character (about 10%); however, the Ni3drr component is found mainly in

9e. In the Se orbital the metal contribution corresponds to the Ni4pn orbital which gives to this orbital some kind of “diffuse” character. How-

H

1.46

PH3

1.64 1.50 1.74 0.07 0.05 0.09

“Nl is the nitrogen atom closest to the nickel atom.

ever, this may not necessarily be a computational artefact. The mixing of some metal 4p diffuse character into the 3d levels may result in a more extended orbital, which in turn would be more suited to interaction and charge transfer (X backdonation) with the diffuse 7r* ligand levels. Table 1 gives the Mulliken gross atomic populations for the valence orbitals in both complexes. The total Ni3dr back-donation is 0.42 and 0.26 electrons for the carbonyl and dinitrogen complexes, respectively. In the OC-Ni-PHs complex, 0.29 electrons are transferred to the CO ligand and 0.13 to the PH3 ligand. In the dinitrogen complex, 0.14 and 0.12 electrons go to the N2 and PHs ligands, respectively. The T-acceptor capability of the PHs molecule is then considerably less than that of the CO, and of about the same magnitude as that of the NZ. It is worth noting that, of the

M. BragajJ. Mol. Struct. (TheochemJ 304 11994) 41-44

44

total charge back-donated a very small

fraction

to the PH3 ligand, goes into

only

the P3d orbital

(< 5% of one electron). Removing the 3d function from the phosphorus atom does not change to any significant degree the extension of the Ni3dn + P back-donation. The 0 donation

to the metal 3dZ2, 4s and 4p shells

follows a trend opposite to that of the 7r backdonation, i.e. is significantly larger for the dinitrogen than for the carbonyl complex (0.38 and 0.29 electrons, respectively). In the dinitrogen complex the N2 and PH3 ligands have very similar a-transfer capabilities (0.20 and 0.18 electrons, respectively). For the carbonyl complex, the CO + Ni c donation is considerably smaller than the Ni3d + CO 7r back-donation (0.17 compared with 0.29 electrons). For the PH3 molecule in the same complex, both values are of about the same magnitude (0.12 and 0.13 electrons, respectively). As a consequence of the different c and T chargetransfer capabilities of the different ligands, we obtain a very small positive charge on the nickel atom in the carbonyl complex and a slightly larger negative charge in the dinitrogen complex. Finally, the electronic configuration of the nickel atom is close to 3d94s’ with a 4p occupancy close to zero.

and

method

the OC-Ni-PH,

and

existence

using

accurate

ab

ab initio

calculations

of a P3d7r acceptor

done

with

a

the importance and the non-

orbital.

The 7r back-

donation is the most important bonding mechanism in the case of the carbonyl complex. However, in the case of the dinitrogen complex the ligand -+ metal c donation makes the most important contribution to the bonding.

References I 2 3 4 5 6 7 8

10

In this work we have studied

complexes

smaller basis set [2]. We confirm of the metal + P 7r back-donation

9

Conclusion

N2-Ni-PH3

initio calculations and found that our results are in agreement with previous conclusions obtained using the local exchange multiple-scattering Xa

M. Braga, Inorg. Chem., 24 (1985) 2702; Quim. Nova, 11 (1988) 71. M. Braga, J. Mol. Struct. (Theochem), 253 (1992) 167. D.S. Marynick, J. Am. Chem. Sot., 106 (1984) 4064. A.G. Orpen and N.G. Connelly, J. Chem. Sot., Chem. Commun., (1985) 1310. T.H. Dunning, J. Chem. Phys., 53 (1970) 2823. A. Veillard, Theor. Chim. Acta, 12 (1968) 405. A.J.H. Wachters, J. Chem. Phys., 52 (1970) 1033. P.W. Ahlrichs, F. Keil, H. Lischka, W. Kutzelnizg and V. Stalmmler, J. Chem. Phys., 63 (1973) 455. M. Braga, S. Larsson and J.R. Leite, J. Am. Chem. Sot., 101 (1979) 3867. S.X. Xiao, W.C. Togler, D.E. Ellis and Z. BerkovitchYellin, J. Am. Chem. Sot., 105 (1983) 7033.