Metal-to-ligand charge transfer bands observed in polarized Ni 2p photoabsorption spectra of [Ni(mnt)2]2−

Journal of Electron Spectroscopy and Related Phenomena 101–103 (1999) 827–832

Metal-to-ligand charge transfer bands observed in polarized Ni 2p photoabsorption spectra of [Ni(mnt) 2 ] 22 Takaki Hatsui, Yasutaka Takata, Nobuhiro Kosugi* The Graduate University for Advanced Studies, Institute for Molecular Science, Myodaiji, Okazaki 444 -8585, Japan

Abstract Linearly polarized Ni 2p photoabsorption spectra were measured for a planar Ni complex [(n-C 4 H 9 ) 4 N] 2 [Ni(mnt) 2 ] (mnt: 1,2-dicyanovinylene-1,2-dithiolato). Symmetries of the Ni 2p excited states are determined by examining the polarization dependence. Two satellite bands are found on the higher energy side of the lowest Ni 2p→3d* atomic line and are assigned to the metal-to-ligand charge transfer (MLCT) transitions with different polarization dependence, in good agreement with ab initio molecular orbital predictions. In [Ni(mnt) 2 ] 22 ion the in-plane backbonding is as important as the out-of-plane one.  1999 Elsevier Science B.V. All rights reserved. Keywords: Ni complex; Backbonding; Metal-to-ligand charge transfer; Ni 2p photoabsorption

1. Introduction Metal 2p photoabsorption spectra of 3d transition metal compounds are mainly associated with the excitation to unoccupied orbitals with d component. A satisfactory description of the metal 2p photoabsorption spectra of oxides and halides has been achieved on the basis of semiempirical approaches using some model Hamiltonian parameters [1]. For compounds with p backbonding such as organometallics, however, the metal 2p photoabsorption spectra are difficult to understand within model Hamiltonian schemes, because several unoccupied ligand orbitals are strongly combined with metal 3d (occupied) orbitals to form covalent bands between ligand and metal and can be responsible for relatively strong photoabsorption. *Corresponding author. Tel.: 181-564-55-7394; fax: 181-564-542254. E-mail address: [email protected] (N. Kosugi)

In the previous experimental and theoretical work on polarized Ni 2p photoabsorption spectra of K 2 Ni(CN) 4 .H 2 O [2] and Ni(Hdmg) 2 (Hdmg: 2,3butanedione dioximato) [3], we successfully revealed strong in-plane and out-of-plane MLCT transitions due to strong p backbonding. In the present work, we have measured linearly polarized Ni 2p photoabsorption spectra of [(n-C 4 H 9 ) 4 N] 2 [Ni(mnt) 2 ] (mnt: 1,2-dicyanovinylene-1,2-dithiolato), which is interesting from the viewpoint of electric conductivity [4]. The measurement for three polarization directions in the single crystals enables us to discuss electronic structure of the excited states in detail.

2. Methods [(n-C 4 H 9 ) 4 N] 2 [Ni(mnt) 2 ] was purchased from Tokyo Chemical Industry Co., Ltd. A large single crystal (about 831032 mm) was obtained by slow recrystallization from acetone solution. Lattice vec-

0368-2048 / 99 / $ – see front matter  1999 Elsevier Science B.V. All rights reserved. PII: S0368-2048( 98 )00432-0

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tors of the single crystal were determined by X-ray diffraction. All the planar [Ni(mnt) 2 ] 22 ions are parallel and well separated with the closest Ni–Ni ˚ [5]. distance of 9.84 A The photoabsorption spectra were measured by total electron yields at the soft X-ray double crystal beamline BL1A of the UVSOR facility, where a pair ¯ crystals was used. The photon energy of beryl (1010) resolution was 0.6 eV around the Ni 2p edge. All the spectra were calibrated with reference to the 2p 3 / 2 – 3d* main peak at 853.2 eV in NiO [6]. The spectra were normalized after background subtraction using a trinomial function in the 830–840 and 880–890 eV regions. Ab initio quantum chemical calculations were performed by using the GSCF3 code [7,8], on a [Ni(mnt) 2 ] 22 ion with D 2h symmetry [9], whereas the experimentally determined geometry belongs to C 2v symmetry [5]. The Ni 2p excited states were obtained with the improved virtual orbital (IVO) method [10] applied to the fully-relaxed self-consistent-field (SCF) potentials for the Ni 2p ionized states. The primitive basis functions given by Huzinaga et al. [11], Ni ( 3 F) (5333 / 53 / 5), S (533 / 53), C and N (63 / 5) were used where the innermost primitive basis functions of Ni 2s, 3s, and 4s are

replaced by 39.176, 3.8333, and 0.31845, respectively. This basis set is augmented with two polarization functions (z p 50.153, 0.049) and two diffuse functions (z s 50.01, z d 50.1481 [12]) for Ni atom and with one polarization function (z d 50.421) for S atoms. Finally, the basis functions contracted are [51111111111 d / 3111111*1* / 111111 d ] for Ni, [52121 / 521 / 1*] for S, and [621 / 41] for C and N. Oscillator strengths were evaluated in the dipole length form [13].

3. Results and discussion Fig. 1 shows Ni 2p photoabsorption spectra of the powder sample. The most intense peak A with shoulder B, a weak peak C, and broad features D–H are observed in the Ni 2p 3 / 2 region. The spectral feature in the Ni 2p 1 / 2 region mimics the Ni 2p 3 / 2 one, and the intensity ratio of the Ni 2p 3 / 2 line (peak A) to the corresponding 2p 1 / 2 one is 2.0. These are characteristic of the 3d 8 low-spin ground state [14]. Fig. 2 shows polarized Ni 2p photoabsorption spectra of Eix, Eiy, and Eiz, where the polarization directions are defined in Fig. 1. The polarization dependence for the three directions enables us to

Fig. 1. Powder spectra of the Ni 2p photoabsorption of [(C 4 H 9 ) 4 N] 2 [Ni II (mnt) 2 ].

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Fig. 2. Polarized Ni 2p photoabsorption spectra of [(n-C 4 H 9 ) 4 N] 2 [Ni II (mnt) 2 ], where E denotes the electric vector of incident light. The molecular axes x, y, and z are chosen as in Fig. 1. The excited molecular orbitals, which are responsible for peaks A–D, are plotted.

identify weak peaks B, C and D and broad features E, F, G and H more clearly. In the following, we will focus on peaks A, B, C and D. Table 1 shows the polarization dependence of relative peak intensities obtained by fitting the spectra with five or six Voigt profiles in the 850–858 eV region. Considering the imperfect sample alignment in measuring the polarized spectra, peaks A–D are classified as follows:

1. forbidden for Eiz: peaks A and C,

Table 1 Experimental excitation energies and polarization dependencies of the Ni 2p 3 / 2 photoabsorption features of [(nC 4 H 9 ) 4 N] 2 [Ni II (mnt) 2 ] Peak

A B C D

Excitation

Relative

Relative intensities

energy (eV)

energy (eV)

Eix

Eiy

Eiz

853.8 855.5 856.7 857.7

0.0 1.7 2.9 3.9

1.000 0.018 0.100 0.023

0.940 0.059 0.092 0.032

0.091 0.110 0.009 0.097

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2. forbidden for Eix: peak B, 3. allowed for Eix,y,z: peak D (strongest in Eiz). Based on the one-electron picture, the transition is classified to excitations to unoccupied orbitals with d xy , d xz , d yz , d x 2 2y 2 , d z 2 and s symmetries, which * , b *3g , a *g , a *g , and a *g orbitals, correspond to b *1g , b 2g respectively, in D 2h symmetry (asterisk denotes unoccupied orbital). Therefore, the following assignments are possible: (i) b *1g (d xy ) or a *g (d x 2 2y 2 ), (ii) b *3g (d yz ), and (iii) a *g (d z 2 , s). Since [Ni(mnt) 2 ] 22 has a low-spin Ni 3d 8 configuration in the ground state and the Ni 3d xy orbital is regarded as completely vacant within the atomic picture, the most intense peak A is undoubtedly * orbital assigned to the Ni 2p excitation to the b 1g * component. A dimer system of with a strong Ni 3d xy [(mnt) 2 ] 42 has low-lying b *3g (L *yz ) and a *g (L *x 22y 2 ) * denotes an unoccupied ligand orbitals, where L yz ligand orbital to be mixed with Ni 3d yz in the [Ni(mnt) 2 ] 22 system. Therefore, peaks B and C can be assigned to the metal-to-ligand charge transfer * (L *yz ) and a *g (L x*22y 2 ) (MLCT) excitations to the b 3g orbitals, respectively. In the a g manifold of D 2h symmetry, the Ni 3d x 22y 2 , 3d z 2 , and 4s components can be hybridized with one another; we have to consider the hybridization in assignment of peak D. The Ni 2p transition to Ni 4s* is generally very weak. The theoretical calculation shows hybridization of Ni 4s* with Ni 3d z 2 , and the intensity of the Ni 2p→‘4s*’ transition is dominated by the Ni 2p→3d z 2 component. The assignment of Ni 2p→‘4s*’ to peak D is consistent with the polarization dependence (3). The Ni 2p→‘4s*’ transition was similarly observed for Ni(Hdmg) 2 [3]. Table 2 shows results of ab initio molecular orbital calculations for low-lying Ni 2p excited states. Although the * oscillator strengths in the transitions to the b 3g

* orbital are orbital for Eiy direction and to the b 1g overestimated, the results are consistent with the above assignments based on the polarization dependence as shown in Fig. 2 and Table 1. This indicates that the one-electron picture is basically appropriate in investigating peaks A, B, C, and D. Let us discuss many-electron effects such as shake-up or down accompanying ligand-to-metal charge transfer (LMCT) and electron correlation effects, though these effects were not taken into account in the present theoretical calculations. The LMCT shake-up or -down satellite state is described 5 10 21 21 by mixing of 2p 3d L xy (L : ligand hole) with the main 2p–3d* configuration, 2p 5 3d 8 (3d *xy )1 [1], and might be assigned to peak C. This explanation is, however, unacceptable because no strong satellite is observed in the photoemission [15], where the shakeup transition is generally larger in probability than in the photoexcitation [16]. In the ground state of [Ni(mnt) 2 ] 22 , a mixing of [(3d yz )(3d xz )(3d x 2 2y 2 )(3d z 2 )] 6 (3d *xy )2 with the main [(3d yz )(3d xz )(3d x 2 2y 2 )(3d z 2 )] 8 (3d *xy )0 configuration is possible due to intra-atomic electron correlation. This causes extra peaks with * )2 configura2p 5 [(3d yz )(3d xz )(3d x 22y 2 )(3d z 2 )] 7 (3d xy tions in the Ni 2p photoabsorption [17]. If peak D arises in electron correlation, the Ni 2p transition to (3d z 2 )0 will be possible due to the ground-state correlation through mixing with (3d yz )2 (3d xz )2 (3d x 2 2y 2 )2 (3d z 2 )0 (3d *xy )2 . * (3d *xy ), b *3g (L *yz ), Fig. 2 shows plots of the b 1g a *g (L x*2 2y 2 ), and a *g (4s*13d z 2 ) orbitals of [Ni(mnt) 2 ] 22 . The b *1g (3d *xy ), b *3g (L *yz ), and a g* (L x*2 2y 2 ) orbitals have ligand components. Fig. 3 shows some ligand orbitals in one mnt 22 unit with C 2v symmetry. The occupied ligand orbital 14b 2 is * covalently combined with the unoccupied Ni 3d xy orbital, where the in-phase (bonding) and out-of-

Table 2 Calculated energies and oscillator strengths of the Ni 2p 3 / 2 photoabsorption features of [Ni II (mnt) 2 ] 22 (D 2h ) Peak

A B C D

Relative

Oscillator strength

energy (eV)

Eix

Eiy

Eiz

Orbital character

0.0 1.3 3.5 4.0

0.06780 – 0.00150 0.00081

0.06556 0.00103 0.00230 0.00063

– 0.00073 0.00016 0.00308

b *1g Ni 3d *xy 2[14b 2 214b 2 ] b *3g [4a *2 14a *2 ]2Ni 3d yz a *g [16a *1 116a *1 ]2Ni 3d x 2 2y 2 a *g Ni 4s* / 3d z 2 2[15a 1 115a 1 ]

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Ni 2p photoabsorption spectra of the planar complex of [Ni(mnt) 2 ] 22 to determine the symmetries of the excited states. Based on the polarization dependence and ab initio molecular orbital calculations, we have found that the strong satellite features have different symmetries from the intra-atomic transitions and arise from the MLCT transitions to ligand unoccupied orbitals. The polarized soft X-ray absorption spectroscopy can be a powerful tool to investigate chemical bonds between 3d transition metals and ligand molecules.

Acknowledgements The authors acknowledge valuable advice on the sample preparation by Dr. Hideki Fujiwara and kind support for the measurement of X-ray diffraction by Dr. Masaaki Tomura.

References

Fig. 3. Some molecular orbitals of a ligand system (mnt 22) with C 2v symmetry. The orbital energies of 15a 1 , 14b 2 , 4a 2* and 16a 1* were 20.67, 0.89, 9.9 and 12.1 eV, respectively.

phase (anti-bonding) combinations in [Ni(mnt) 2 ] 22 * 1[14b 2 214b 2 ] and Ni3d xy * 2[14b 2 2 such as Ni3d xy 14b 2 ] result in the s donation (LMCT) within the * (3d *xy ) occupied manifold and in the unoccupied b 1g orbital, respectively. The a g* (4s*13d z 2 ) orbital is described as Ni 4s* / 3d z 2 2[15a 1 115a 1 ], where the ligand 15a 1 component has a very small contribution; on the other hand, the 15a 1 ligand component is * (L *yz ) and dominant in the occupied manifold. The b 3g a g* (L x*2 2y 2 ) orbitals are described as [4a *2 14a *2 ]2Ni 3d yz and [16a *1 216a *1 ]2Ni 3d x 2 2y 2 , respectively, with dominant ligand contributions in the unoccupied manifold; peaks B and C arise from the out-of-plane p and in-plane p MLCT transitions, where the 16a 1 * orbital has larger contribution of the in-plane p CN orbital. Therefore, [Ni(mnt) 2 ] 22 has two kinds of backbonding in the occupied manifold. In summary, we have measured linearly polarized

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