Trifluorophosphine as a Bridging Ligand in Homoleptic Binuclear

Jun 4, 2010 - Peigang Hu , Qiong Luo , Qian-shu Li , Yaoming Xie , R. Bruce King , and Henry F. Schaefer. Inorganic Chemistry 2014 53 (10), 5300-5310...
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Trifluorophosphine as a Bridging Ligand in Homoleptic Binuclear Nickel Complexes† Hua-qing Yang,‡ Qian-shu Li,*,‡,§ Yaoming Xie,| R. Bruce King,*,‡,| and Henry F. Schaefer III| Center for Computational Quantum Chemistry, South China Normal UniVersity, Guangzhou 510631, P. R. China, Institute of Chemical Physics, Beijing Institute of Technology, Beijing 100081, P. R. China, and Department of Chemistry and Center for Computational Chemistry, UniVersity of Georgia, Athens, Georgia 30602 ReceiVed: April 5, 2010; ReVised Manuscript ReceiVed: May 14, 2010

The stable tetrahedral derivative Ni(PF3)4 is reported to generate binuclear Ni2(PF3)n ions in its mass spectrum emerging from ion-molecule reactions. Theoretical studies on such binuclear complexes indicate Ni2(PF3)7 to be energetically unfavorable with respect to decomposition into Ni(PF3)4 + Ni(PF3)3. However, viable rather unsymmetrical structures with two PF3 bridges for Ni2(PF3)n (n ) 6, 5, 4) are reported here as well as a triply semibridged Ni2(PF3)5 structure. The nickel-nickel distances in these Ni2(PF3)n derivatives suggest a formal double bond (2.51 Å), a formal triple bond (2.26 Å), and a formal double bond (2.34 Å) for n ) 6, 5, and 4, respectively. The mononuclear Ni(PF3)n derivatives are predicted to have tetrahedral, trigonal planar, and bent (139°) structures for n ) 4, 3, and 2, respectively. 1. Introduction Trifluorophosphine (PF3) is a strong π-acceptor ligand that can stabilize low formal oxidation states similar to carbon monoxide.1-10 In fact, the homoleptic zerovalent metal derivatives, such as Cr(PF3)6, Fe(PF3)5, and Ni(PF3)4, are even more thermally and oxidatively stable than the corresponding homoleptic metal carbonyls. Furthermore, their volatility is comparable to the analogous metal carbonyls despite their considerably higher molecular weights. These observations on the higher stability of metal trifluorophosphine complexes relative to corresponding metal carbonyls suggested originally that metal trifluorophosphine chemistry might develop into a more extensive area of inorganic chemistry than even metal carbonyl chemistry. However, as metal trifluorophosphine chemistry continued to evolve, metal trifluorophosphine complexes with bridging PF3 groups analogous to well-known metal carbonyls with bridging carbonyl groups such as Fe2(CO)9 [) Fe2(CO)6(µ-CO)3]11,12 and Co2(CO)8 [) Co2(CO)6(µ-CO)2]13-15 (Figure 1) remained unknown, even though metal trifluorophosphine complexes with terminal PF3 groups are generally more stable than their carbonyl counterparts. The only examples of a bridging PF3 group in the literature are the trinuclear palladium phosphine cations [Pd3(µPh2PCH2PPh2)3(µ-X)(µ3-PF3)]+ (X ) Cl, Br, I), in which a Pd3 triangle is bridged by a µ3-PF3 group bonding to all three palladium atoms.16,17 Metal carbonyl analogues of these compounds, in which all of the phosphorus ligands are replaced by carbonyl groups, are unknown. Our recent theoretical research on trifluorophosphine metal complexes has been directed toward understanding the reasons for lack of metal complexes with bridging PF3 groups analogous to metal carbonyl complexes with bridging carbonyl groups. Thus the stability of Fe2(CO)9 with three bridging CO groups suggested an investigation of Fe2(PF3)9 as well as formally unsaturated Fe2(PF3)n derivatives with fewer PF3 groups. †

Part of the “Klaus Ruedenberg Festschrift”. South China Normal University. § Beijing Institute of Technology. | University of Georgia. * To whom correspondence should be addressed. ‡

Figure 1. The stable homoleptic binuclear complexes of the first row transition metals including the unbridged Mn2(CO)10 structures as well as the Fe2(CO)9 and Co2(CO)8 structures, each with two bridging CO groups.

However, in the lowest energy Fe2(PF3)9 structure, one of the PF3 ligands splits into PF2 + F, leading to a (F3P)4Fe r PF2Fe(F)(PF3)4 structure with a bridging PF2 group coupled with formal oxidation of one of the iron atoms from Fe(0) to Fe(II).18 The prospects for obtaining trifluorophosphine metal complexes with bridging PF3 groups increase upon moving to the right of the Periodic Table in the d-block to the late transition metals, as illustrated by analogous metal carbonyl chemistry. A simple example is the unbridged structure19,20 of Mn2(CO)10 contrasted with the doubly bridged structure13-15 of Co2(CO)8 (Figure 1). The late transition metals require fewer electrons from the surrounding ligands to attain the favored 18-electron configuration. This leads to lower metal coordination numbers in stable derivatives, as indicated by the series of stable metal carbonyls Cr(CO)6, Fe(CO)5, and Ni(CO)4. In this connection, a dimetallic M2 system forms two M-C bonds with a bridging carbonyl group but only one M-C bond with a terminal carbonyl group. Application of similar ideas to a quest for homoleptic binuclear metal complexes with bridging PF3 groups suggests investigation of binuclear nickel derivatives of the type Ni2(PF3)n since nickel is the 3d transition metal furthest to the right in the Periodic Table to form a stable homoleptic trifluorophosphine complex. Thus Ni(PF3)4 is a stable but volatile liquid that was first synthesized in 1951 by Irvine and Wilkinson21 by the reaction of Ni(PCl3)4 with PF3. No binuclear Ni2(PF3)n derivatives have yet been synthesized but neither have any neutral homoleptic nickel carbonyls Ni2(CO)n. However, mass spectrometry experiments provide evidence for binuclear Ni2(PF3)n

10.1021/jp103051s  2010 American Chemical Society Published on Web 06/04/2010

PF3 as a Bridging Ligand in Binuclear Nickel Complexes

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Figure 2. The two optimized structures of Ni2(PF3)6 with only the BP86 distances.

derivatives (n ) 6, 5, 4, 3, 2) in the gas phase arising from ion-molecule reactions.22,23 Theoretical studies on binuclear nickel carbonyls24 predict low energy structures with one, two, and three bridging carbonyl groups for Ni2(CO)7, Ni2(CO)6, and Ni2(CO)5. This suggests that analogous Ni2(PF3)n (n ) 7, 6, 5) derivatives might have low-energy structures containing bridging PF3 groups. This paper describes a search for such structures using density functional theory (DFT) methods.

TABLE 1: Bond Distances (in Å), Total Energies (E in hartree), Relative Energies (∆E in kcal/mol), and Numbers of Imaginary Frequencies (Nimag) for the Optimized Ni2(PF3)6 Structures B3LYP

BP86

2. Theoretical Methods Electron correlation effects were considered using DFT methods, which have evolved as a practical and effective computational tool, especially for organometallic compounds.25-39 The reliability of such DFT methods is governed by the quality of the approximate exchange-correlation (XC) energy functional. We initially chose two DFT methods, the B3LYP and the BP86 methods, which are constructed in very different ways. The B3LYP method is a hybrid HF/DFT method using a combination of the three-parameter Becke functional (B3) with the Lee-Yang-Parr (LYP) generalized gradient correlation functional.40,41 The B3LYP method includes exact exchange and is calibrated by fitting three parameters to a set of experimental results. The BP86 method combines Becke’s 1988 exchange functional (B) with Perdew’s 1986 gradient corrected correlation functional method (P86).42,43 The BP86 method does not include exact exchange and is mainly deduced by forcing the functional to satisfy certain exact constraints based on first principles. When these two very different DFT methods agree, confident predictions can be made. All computations were performed using double-ζ plus polarization (DZP) basis sets. The DZP basis set used for fluorine adds one set of pure spherical harmonic d functions with orbital exponent Rd(F) ) 1.0 to the standard HuzinagaDunning contracted DZ sets44,45 and is designated (9s5p1d/ 4s2p1d). The DZP basis sets used for phosphorus add a set of polarization d functions with Rd(P) ) 0.6 to Dunning’s DZ set to yield (11s7p1d/6s4p1d).46 The loosely contracted DZP basis set for nickel is the Wachters primitive set47 augmented by two sets of p functions and a set of d functions, contracted following Hood, Pitzer, and Schaefer,48 and designated (14s11p6d/ 10s8p3d). The geometries of all structures were fully optimized using the DZP B3LYP and DZP BP86 methods. Vibrational frequencies were determined by analytically evaluating the second derivatives of the energy with respect to the nuclear coordinates. The corresponding infrared intensities were also evaluated analytically. Unless otherwise specified, none of the final optimized structures has any imaginary vibrational frequencies. All of the computations were carried out with the Gaussian 03

Ni-Ni -E ∆E Nimag Ni-Ni -E ∆E Nimag

26-1 (C1)

26-2 (C2h)

2.546 6863.25186 0.0 none 2.483 6863.83784 0.0 none

2.899 6863.24926 1.6 none 2.518 6863.83068 4.5 3 (136i, 24i, 14i)

program,49 exercising the fine grid option (75 radial shells, 302 angular points) for evaluating integrals numerically, while the tight (10-8 hartree) designation is the default for the selfconsistent field (SCF) convergence. The optimized structures are depicted in Figures 2-5. A given Nia(PF3)b structure is designated as ab-c where a is the number of nickel atoms b is the number of PF3 groups, and c orders the structures according to their relative energies. Thus the lowest energy structure for Ni2(PF3)6 is designated 26-1. The distances and angles in the figures are those obtained by the BP86 method. All of the structures reported in this paper are singlet spin states. In all cases, the energies of the corresponding triplet spin states were found to be significantly higher. 3. Results 3.1. Structures of the Binuclear Derivatives. 3.1.1. Ni2(PF3)6. Two singlet structures were optimized for Ni2(PF3)6 (Figure 2 and Table 1). The lowest energy structure, namely 26-1, is a asymmetric structure, which is a genuine minimum. Structure 26-1 consists of a Ni(PF3)2 unit and a Ni(PF3)3 unit linked by a direct NidNi bond and a bridging PF3 group. A second PF3 group is a semibridging PF3 group with a short Ni-P distance of 2.142 Å (B3LYP) or 2.160 Å (BP86) and a long Ni-P distance of 2.815 Å (B3LYP) or 2.504 Å (BP86). The length of the NidNi bond in 26-1 is 2.546 Å (B3LYP) or 2.483 Å (BP86), corresponding to a formal double bond to give each nickel atoms the favored 18-electron configuration. Another singlet Ni2(PF3)6 structure 26-2 with C2h symmetry lies only 1.6 kcal/mol (B3LYP) or 4.5 kcal/mol (BP86) above 26-1. The length of the Ni-Ni bond in 26-2 is 2.899 Å (B3LYP) or 2.518 Å (BP86). Structure 26-2 has three imaginary vibrational frequencies, namely 136i, 24i, and 14i cm-1 (BP86). Following the normal mode corresponding to the largest of these imaginary vibrational frequencies leads to 26-1. 3.1.2. Ni2(PF3)5. Three structures were optimized for Ni2(PF3)5 (Figure 3 and Table 2). The lowest energy structure,

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Figure 3. The three optimized structures of Ni2(PF3)5.

TABLE 2: Bond Distances (in Å), Total Energies (E in hartree), Relative Energies (∆E in kcal/mol), and Numbers of Imaginary Frequencies (Nimag) for the Optimized Ni2(PF3)5 Structures B3LYP

BP86

Ni-Ni -E ∆E Nimag Ni-Ni -E ∆E Nimag

25-1 (Cs)

25-2 (C3V)

25-3 (C2)

2.274 6222.16780 0.0 17ia 2.238 6222.73669 0.0 none

2.258 6222.16704 0.5 3 (29i, 29i, 8i) 2.222 6222.73670 0.0 9i

2.749 6222.12571 26.4 29i 2.597 6622.68963 29.5 50i

a A smaller imaginary frequency of 12i cm-1 was obtained by using the finer integration grid (120,974).

namely 25-1, is a Cs structure with a predicted NitNi bond length of 2.274 Å (B3LYP) or 2.238 Å (BP86), corresponding to the formal triple bond required to give both nickel atoms the favored 18-electron configuration. Structure 25-1 consists of a Ni(PF3)2 unit and a Ni(PF3) unit linked by a direct NitNi bond and two bridging PF3 groups. Structure 25-1 has no imaginary vibrational frequencies by BP86 but a small imaginary vibrational frequency of 17i cm-1 by B3LYP. The latter is reduced to 12i cm-1 using the finer integration grid (120, 974). A more symmetrical C3V triply semibridged Ni2(PF3)5 structure 25-2 (Figure 3 and Table 2) has essentially the same energy as 25-1, indicating a highly fluxional system with a flat potential surface. The three equivalent semibridging PF3 groups in 25-2 have short Ni-P distances of 2.188 Å and long Ni-P distances of 2.475 Å (BP86). The NitNi bond length in 25-2 of 2.258 Å (B3LYP) or 2.222 Å (BP86) is very close to that in 25-1 and likewise can correspond to the formal triple bond needed to give each nickel atom the favored 18-electron configuraton. Structure 25-2 has some small imaginary vibrational frequencies, namely 29i, 29i, and 8i cm-1 (B3LYP) or 9i cm-1 (BP86).

The singly bridged Ni2(PF3)5 structure 25-3 (Figure 3 and Table 2) lies at the relatively high energy of 26.4 kcal/mol (B3LYP) or 29.5 kcal/mol (BP86) with a small imaginary vibrational frequency of 29i cm-1 (B3LYP) or 50i cm-1 (BP86). Its Ni-Ni distance of 2.749 Å (B3LYP) or 2.597 Å (BP86) corresponds to a formal single bond, thereby giving each nickel atoms a 16-electron configuration. 3.1.3. Ni2(PF3)4. Three structures were optimized for Ni2(PF3)4 (Figure 4 and Table 3). The lowest energy structure, namely 24-1, is asymmetric, consisting of two Ni(PF3) units linked by a direct NidNi bond and two semibridging PF3 groups. For the semibridging PF3 groups, the short Ni-P distance is ∼2.1 Å and the long Ni-P distance is ∼2.4 Å. The length of the NidNi bond is 2.350 Å (B3LYP) or 2.338 Å (BP86), corresponding to a double bond giving each nickel atom a 16-electron configuration. A C2V unbridged Ni2(PF3)4 structure 24-2 (Figure 4 and Table 3) lies 5.7 kcal/mol (B3LYP) or 9.2 kcal/mol (BP86) above 24-1 with several small imaginary vibrational frequencies less than 30i cm-1. The NidNi distance of 2.349 Å (B3LYP) or 2.286 Å (BP86) in 24-2 corresponds to a formal double bond, thereby giving each nickel atom a 16-electron configuration. A second doubly bridged Ni2(PF3)4 structure 24-3 with C2h symmetry was also found at 14.9 kcal/mol (B3LYP) or 19.7 kcal/mol (BP86) above 24-1 (Figure 4 and Table 3). The NidNi bond length in 24-3 of 2.351 Å (B3LYP) or 2.313 Å (BP86) is almost the same as that in the other Ni2(PF3)4 structures and can again correspond to a formal double bond to give each nickel atom a 16-electron configuration. Structure 24-3 has several small imaginary vibrational frequencies, namely 20i and 14i cm-1 (B3LYP) or 31i, 18i, and 4i cm-1 (BP86). 3.2. Structures of the Mononuclear Derivatives. 3.2.1. Ni(PF3)4. The only singlet structure found for the coordinately saturated 18-electron complex Ni(PF3)4 is the

PF3 as a Bridging Ligand in Binuclear Nickel Complexes

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Figure 4. The three optimized structures of Ni2(PF3)4.

TABLE 3: Bond Distances (in Å), Total Energies (E in hartree), Relative Energies (∆E in kcal/mol), and Numbers of Imaginary Frequencies (Nimag) for the Optimized Ni2(PF3)4 Structures B3LYP

BP86

Ni-Ni -E ∆E Nimag Ni-Ni -E ∆E Nimag

24-1 (C1)

24-2 (C2V)

24-3 (C2h)

2.350 5581.06964 0.0 none 2.338 5581.61227 0.0 none

2.349 5581.06052 5.7 3(24i, 24i, 20i) 2.286 5581.59760 9.2 5(29i, 25i, 24i, 13i, 1i)

2.351 5581.04591 14.9 2(20i, 14i) 2.313 5581.58087 19.7 3 (31i, 18i, 4i)

expected tetrahedral structure 14-1 (Figure 5 and Table 4), which is a genuine minimum. The lengths of the Ni-P bonds are 2.131 Å (B3LYP) or 2.118 Å (BP86). The BP86 Ni-P distance is in perfect agreement with the experimental Ni-P distance of 2.116(10) Å in Ni(PF3)4, determined by gas-phase electron diffraction.50 3.2.2. Ni(PF3)3. The only singlet structure found for the coordinately unsaturated 16-electron complex Ni(PF3)3 is the C3h structure 13-1 (Figure 5 and Table 4), which is a genuine minimum. The lengths of the Ni-P bonds are 2.115 Å (B3LYP) or 2.105 Å (BP86). 3.2.3. Ni(PF3)2. The only singlet structure found for the highly coordinately unsaturated 14-electron complex Ni(PF3)2 is a C2V structure 12-1 (Figure 5 and Table 4), which is a genuine minimum without any imaginary frequencies. The structure 12-1 is not the expected linear structure but is bent with a P-Ni-P angle of 141.6° (B3LYP) or 136.0° (BP86) and Ni-P bond lengths of 2.081 Å (B3LYP) or 2.067 Å (BP86).

3.3. Dissociation Energies. Table 5 lists dissociation energies for three reactions of particular interest in the chemistry of these nickel trifluorophosphine complexes. Table 5 shows that the zero point vibrational energy (ZPVE) corrections are very small (< 2.5 kcal/mol). For the binuclear Ni2(PF3)n derivatives, the energies of PF3 dissociation from Ni2(PF3)n (n ) 6, 5) to give Ni2(PF3)n-1 are all predicted to be more than 15 kcal/mol. Thus the energy for PF3 dissociation from Ni2(PF3)6 (26-1) to give Ni2(PF3)5 (25-1) is predicted to be 15.6 kcal/mol (B3LYP) or 18.2 kcal/mol (BP86). The energy for PF3 dissociation from Ni2(PF3)5 (25-1) to give Ni2(PF3)4 (24-1) is predicted to be larger at 24.5 kcal/mol (B3LYP) or 32.7 kcal/mol (BP86). The homolytic dissociation energy of Ni2(PF3)6 (26-1) into two Ni(PF3)3 (13-1) moieties is predicted to be relatively low at 4.2 kcal/mol (B3LYP) or 15.7 kcal/mol (BP86). This suggests that the dissociation of Ni2(PF3)6 to 2Ni(PF3)3 is energetically accessible. However, the analogous homolytic dissociation energies of Ni2(PF3)n (n ) 5, 4) are significantly higher. Thus the homolytic dissociation energy of Ni2(PF3)5 (25-1) into Ni(PF3)3 (13-1) and Ni(PF3)2 (12-1) is predicted to be 17.3 kcal/mol (B3LYP) or 31.7 kcal/mol (BP86). Similarly, the homolytic dissociation energy of Ni2(PF3)4 (24-1) into two Ni(PF3)2 fragments (12-1) is predicted to be 21.5 kcal/mol (B3LYP) or 33.1 kcal/mol (BP86). The PF3 dissociation energies of the mononuclear Ni(PF3)n derivatives are also substantial. Thus the energy for dissociation of PF3 from Ni(PF3)4 (14-1) to give Ni(PF3)3 (13-1) is predicted to be 24.2 kcal/mol (B3LYP) or 31.6 kcal/mol (BP86), while the energy for dissociation of PF3 from Ni(PF3)3 (13-1) to give Ni(PF3)2 (12-1) is predicted to be 28.7 kcal/mol (B3LYP) or 34.1 kcal/mol (BP86).

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Figure 5. The optimized Ni(PF3)n (n ) 4, 3, 2) structures.

TABLE 4: Bond Distances (in Å) and Total Energies (E in hartree) for the Optimized Ni(PF3)n (n ) 4, 3, 2) Structures B3LYP BP86

Ni-P -E Ni-P -E

14-1 (Td)

13-1 (C3h)

12-1 (C2V)

2.131 4072.72027 2.118 4073.02897

2.115 3431.62258 2.105 3431.90640

2.081 2790.51768 2.067 2790.77977

TABLE 5: Dissociation Energies (in kcal/mol) and Zero-Point Energy Corrections for Dissociation Reactions Ezpve correction

∆E Ni2(PF3)6 (26-1) f Ni2(PF3)5 (25-1) + PF3 Ni2(PF3)5 (25-1) f Ni2(PF3)4 (24-1) + PF3 Ni(PF3)4 (14-1) f Ni(PF3)3 (13-1) + PF3 Ni(PF3)3 (13-1) f Ni(PF3)2 (12-1) + PF3 Ni2(PF3)6 (26-1) f 2Ni(PF3)3 (13-1) Ni2(PF3)5 (25-1) f Ni(PF3)3 (13-1) + Ni(PF3)2 (12-1) Ni2(PF3)4 (24-1) f 2Ni(PF3)2 (12-1)

B3LYP

BP86

B3LYP

BP86

15.6

18.2

-1.0

-1.1

24.5

32.7

-1.3

-1.4

24.2

31.6

-1.6

-2.5

28.7

34.1

-1.1

-1.2

4.2

15.7

-0.7

0.0

17.3

31.7

-0.8

-0.9

21.5

33.1

-0.7

-0.6

4. Discussion A previous theoretical study24 on Ni2(CO)n (n ) 7, 6, 5) derivatives predicts a systematic series of structures (Figure 5) in which the formal nickel-nickel bond orders and numbers of bridging carbonyl groups increase as the total number of carbonyl groups decreases. This maintains an approximately tetrahedral arrangement of four Ni-C bonds to each nickel atom in the series of binuclear structures similar to that in the mononuclear Ni(CO)4. No viable structure was found for Ni2(PF3)7, since attempted optimization led to rupture into the mononuclear fragments Ni(PF3)4 + Ni(PF3)3. The low energy structures for the corresponding Ni2(PF3)n (n ) 6, 5) derivatives differ significantly from those of the carbonyl derivatives (Figure 6), but nevertheless provide examples of bridging PF3 ligands. The lowest energy structure of Ni2(PF3)6 (26-1 in Figure 2) is analogous to that of Ni2(CO)6 with two bridging PF3 groups. However, one of these PF3 groups in Ni2(PF3)6 is not a true bridging group but instead an unsymmetrical semibridging PF3 group. The NidNi double bond distance of 2.51 ( 0.04 Å in Ni2(PF3)6 is very similar to the previously predicted24 NidNi distance of 2.54 ( 0.02 Å in Ni2(CO)6. However, this NidNi bond in Ni2(PF3)6 is rather weak, as indicated by a low dissociation

Figure 6. The structures predicted for the Ni2(CO)n (n ) 7, 6, 5) derivatives in a previous theoretical study.24

energy of about 10 kcal/mol for the dissociation of Ni2(PF3)6 into 2 Ni(PF3)3 (Table 5). The previous theoretical study24 predicted a relatively symmetrical (D3h) triply bridged structure for Ni2(CO)5 with a short NitNi distance of 2.20 Å, indicative of the formal triple bond needed to give both nickel atoms the favored 18-electron configuration (Figure 6). A related, but less symmetrical, triply semibridged C3V structure was found for Ni2(PF3)5 (25-2 in Figure 3). At essentially the same energy is the unsymmetrical doubly bridged Ni2(PF3)5 structure 25-1 (Figure 3). The predicted NitNi bond distances of ∼2.23 Å in both 25-1 and 25-2 are significantly longer than the 2.19 Å distance for the triply bridged NitNi triple bond in Ni2(CO)5 but still can be interpreted as the formal triple bond needed to give each nickel atom the favored 18-electron configuration. The longer Ni≡Ni triple bond distances in the Ni2(PF3)5 structures relative to Ni2(CO)5 are likely to be related to the steric hindrance between the PF3 ligands on the two nickel atoms. We also investigated the more highly unsaturated Ni2(PF3)4, which requires a formal quadruple bond to give both nickel atoms the favored 18-electron configuration. Two unsymmetrical doubly bridged Ni2(PF3)4 structures 24-1 and 24-3 and one unbridged Ni2(PF3)4 structure 24-2 were found (Figure 4). The predicted NidNi distances of 2.32 ( 0.03 Å in each of these three Ni2(PF)4 structures are longer rather than shorter than the NitNi triple bond distances in the Ni2(PF3)5 structures 25-1 and 25-2 (Figure 3) and thus can be interpreted as a formal double bonds. This gives each nickel atom in the Ni2(PF3)4 structures a 16-electron configuration. No Ni2(PF3)4 structures were found having the very short Ni-Ni distances indicative of the formal quadruple bond required to give each nickel atoms the favored 18-electron configuration. This theoretical study of the binuclear Ni2(PF3)n derivatives coupled with the generation of binuclear Ni2(PF3)n ions in the mass spectrum22,23 of Ni(PF3)4 suggests the feasibility of synthesizing homoleptic Ni2(PF3)n derivatives with bridging PF3 groups. The thermodynamics (Table 5) suggests Ni2(PF3)6 or Ni2(PF3)5, particularly the latter, as the most promising synthetic

PF3 as a Bridging Ligand in Binuclear Nickel Complexes targets. The differences between the Ni2(PF3)n and Ni2(CO)n systems, such as the instability of Ni2(PF3)7 may relate to the large size of the PF3 ligand relative to the CO ligand. In the course of this work, the mononuclear Ni(PF3)n (n ) 4, 3, 2) structures were optimized. The expected tetrahedral structure 14-1 (Figure 5) was found for Ni(PF3)4 with predicted Ni-P distances of 2.12 Å essentially identical to the experimental value, determined by X-ray diffraction.50 A trigonal planar structure was found for the 16-electron complex Ni(PF3)3 completely analogous to the trigonal planar structure for Ni(CO)3 in the previous theoretical study.24 The 14-electron complex Ni(PF3)2 is predicted to have a P-Ni-P angle of 139 ( 3° and thus is slightly more bent than the corresponding Ni(CO)2 with a C-Ni-C angle of 148.5°. Acknowledgment. We are indebted to the 111 Project (B07012) and the National Natural Science Foundation (20973066) of China as well as the U.S. National Science Foundation (Grants CHE-0749868 and CHE-0716718) for support of this research. Supporting Information Available: Tables S1 to S12: Harmonic vibrational frequencies (in cm-1) and infrared intensities (in parentheses in km/mol) for the trifluorophosphine nickel complexes; Tables S13 to S23: Atomic coordinates of the optimized structures of the trifluorophosphine nickel complexes; complete Gaussian 03 reference (ref 49). This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Green, J. C.; King, D. I.; Eland, J. H. D. Chem. Commun. 1970, 1121. (2) Hillier, I. H.; Saunders, V. R.; Ware, M. J.; Bassett, P. J.; Lloyd, D. R.; Lynaugh, N. Chem. Commun. 1970, 1316. (3) Bassett, P. J.; Higginson, B. R.; Lloyd, D. R.; Lynaugh, N.; Roberts, P. J. J. Chem. Soc., Dalton Trans. 1974, 2316. (4) Mu¨ller, J.; Fenderl, K.; Mertschenk, B. Chem. Ber. 1971, 104, 700. (5) Head, R. A.; Nixon, J. F.; Sharp, G. J.; Clark, R. J. J. Chem. Soc., Dalton Trans. 1975, 2054. (6) Nixon, J. F.; Seddon, E. A.; Suffolk, R. J.; Taylor, M. J.; Green, J. C.; Clark, R. J. J. Chem. Soc., Dalton Trans. 1986, 765. (7) Savariault, J.-M.; Serafini, A.; Pellissier, M.; Cassoux, P. Theor. Chim. Acta 1976, 42, 155. (8) Braga, M. Inorg. Chem. 1985, 24, 2702. (9) Braga, M. J. Mol. Struct. 1992, 85, 167. (10) Frenking, G.; Wichmann, K.; Fro¨hlich, N.; Grobe, J.; Golla, W.; Le Van, D.; Krebs, B.; La¨ge, M. Organometallics 2002, 21, 2921. (11) Powell, H. M.; Ewens, R. V. G. J. Chem. Soc. 1939, 286. (12) Cotton, F. A.; Troup, J. M. J. Chem. Soc., Dalton Trans. 1974, 800.

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