ClMPH3 and [MPH3]+ (M=Cu, Ag, Au); a density functional study

Inorganica Chimica Acta 262 (1997) 61–64

Note

ClMPH3 and [MPH3]q (MsCu, Ag, Au); a density functional study Guido Kickelbick, Ulrich Schubert U Institut fu¨r Anorganische Chemie, Technische Universita¨t Wien, Getreidemarkt 9, A-1060 Vienna, Austria Received 16 April 1996; revised 24 May 1996; accepted 16 December 1996

Abstract A gradient corrected density functional study on ClMPH3 and [MPH3]q (MsCu, Ag, Au) was performed. The calculated structures are in very good agreement with crystallographic data of closely related complexes. The M–P bond dissociation energies were calculated. They decrease in the order AuLn)CuLn)AgLn. The shapes of the LUMOs of all [MPH3]q are very similar, but the isolobality of the [AuPH3]q fragment with H cannot be expanded to the other [MPH3]q fragments because the energy criterion is not met. Keywords: Group 11 complexes; Phosphine complexes; Density functional study; Molecular orbital calculations

1. Introduction Phosphine substituted compounds of Cu, Ag and Au are widely used in preparative chemistry. In particular the complexes with the metal in the qI oxidation state often exhibit interesting structures [1]. The best experimentally as well as theoretically examined compounds of that type are the gold compounds [1–3]. Apart from the experimental work, some theoretical calculations especially of the properties of AuPR3 compounds were performed. Evans and Mingos demonstrated that the sequence of the orbital levels of the fragment determines whether an MPH3 fragment (MsAu, Pt) is isolobal with H [4]. Schwerdtfeger optimized different Au(I) complexes and found that accurate geometries are only achieved by the use of relativistic methods [5]. He found by Hartree Fock calculations on the [AuPH3]q system a contraction of the Au–P bond of more than 30 pm due to the relativistic effect. Ha¨berlen and Ro¨sch compared different substitution patterns at the phosphine ligand of MeAuPR3 and found that simplifying the calculations by using the PH3 ligand is a good structural model for the experimentally used PPh3 ligand [6]. Contrary to the Au(I) phosphine complexes there are only a few calculations on the copper and silver derivatives. Antes and Frenking calculated the structures and metal–carbon dissociation energies of MCH3 and MC6H5 (MsCu, Ag, Au) [7]. They found that the bond strength decreases in the order Au)Cu)Ag. In a recent study the same group compared the ClMCO (MsCu, Ag, Au) complexes with different ab U

Corresponding author. Tel.: q43-1-58801 4633; fax: q43-1-581 6668.

initio methods. The paper gives an overview of the method dependence of the calculations of geometries, dissociation energies and vibrational frequencies. It showed that the strength of the M–CO bond decreases in the same order as the M–C bond in the alkyl and aryl complexes [8]. In the first part of the present paper we evaluate the results of density functional calculations by comparison of the calculated structures with X-ray structure determinations [9] of ClMPR3 complexes. The gradient corrected density functional theory was applied, because recent methodical studies showed a good agreement of this theoretical method with experimental data for transition metal compound properties [10–13]. In the second part of the paper we extend these calculations to the structurally unknown cationic complexes [MPH3]q. After the geometry optimizations, calculations of M–P dissociation energies and properties of the complexes are investigated.

2. Computational methods The calculations were carried out at the gradient corrected density functional level by using the local spin density approximation [14], combined with Becke’s gradient correction [15], and with Lee, Yang and Parr’s correlation functional [16] (B3LYP) implemented in the GAUSSIAN92/ DFT [17] package. The small core effective core potential of Hay and Wadt [18] implemented in the GAUSSIAN92/ DFT package (LANL2DZ) was used for the transition metal atoms. It describes the inner electrons by a core and the (ny1)s(ny1)p(ny1)d(n)s orbitals as the valence space.

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This basis set has a quasirelativistic character for Ag and Au, but not for Cu. The well known relativistic effect of Cu [19] was taken into consideration by a comparative calculation using the quasirelativistic effective core potential of Stevens et al. [20]. The phosphorus and chlorine atoms were treated by using the 6-31g(d) [21] basis set. Dunnings (4s)/[2s] description for H was used throughout. All structures were optimized in C3v symmetry. 3. Results and discussion 3.1. The chloro complexes ClMPH3 To get an idea about the accuracy of the basis sets, we optimized the structures of the ClMPH3 (MsCu, Ag, Au) complexes and compared the obtained results with experimental data from crystal structure analyses. Because no PH3 substituted complexes were structurally characterized, the data from the triarylphosphine substituted compounds chloro[tris(2,4,6- trimethoxyphenyl)phosphine]copper(I), chloro[tris(2,4,6-trimethoxyphenyl)phosphine]silver(I) and chloro(triphenylphosphine)gold(I) were used [9]. The error in the theoretical geometry optimizations should be small when a PPh3 ligand is substituted by a PH3 ligand, especially in linear compounds, where sterical effects are small. The available crystallographic data show, for example, that the bond lengths in R3PAuCl compounds differ by only about 0.5 pm in substituting R3s(tBu3C6H5)H2 [22a] for R3siPr3 [22b] or R3sPh3 [9b]. This supports the results of the calculations of Ha¨berlen and Ro¨sch who found only little geometrical effects in changing from PPh3 to PH3 in MeAuPR3 [6]. Nevertheless, a substitution could cause bigger effects in bond dissociation energy calculations. The experimental M–P bond lengths in the reference ClMPR3 complexes increase from copper (217.7(1) pm) and gold (223.3(1) pm) to silver (237.9(1) pm) (Table 1). The observed large bond contraction of the gold compound is due to the well known relativistic effect. The M–L bond length sequence CufAu-Ag was also observed in the neutral (C2H4)M systems calculated by Nicolas and Spiegelmann [23]. The experimentally observed trend of the M–P distances can be found in our calculations, too. Table 1 Optimized distances (pm) and angles (8) for the ClMPH3 compounds (MsCu, Ag, Au) Molecule

Basis

M–P

P–H

M–Cl

M–P–H

ClCuPH3

NR R

222.9 218.6 217.7 243.6 237.9 228.3 223.3

141.1 141.2

214.0 210.3 211.8 235.1 234.2 232.5 227.8

119.0 118.9

Exp. [9a] ClAgPH3 Exp. [9b] ClAuPH3 Exp. [9c]

R R

141.0 140.9

NRsnon-relativistic, Rsrelativistic basis set.

The calculated M–P bond lengths in the copper and gold compounds (Cu–P 222.9 pm; Au–P 228.3 pm) are close to the experimental ones. Using the quasirelativistic pseudopotential on copper, an additional shortening of the Cu–P bond to 218.6 pm occurs. These values are in good agreement with the crystal structure analyses. The silver system shows an Ag–P bond length of 243.6 pm, which is about 6.7 pm longer than the experimental value. The M–Cl bond length is also in very good agreement with the experimental values, where the ClAuPH3 complex has the biggest deviation (4.7 pm) from the crystallographic data. The P–H distance differs in all calculations only by 1–2 pm around 141 pm. There is also only a small effect on the M–P–H angle, which in all cases is around 118–1198. 3.2. The cations [MPH3]q A general bond lengthening of about 2–3 pm in going from the chlorides to the cationic complexes is observed. As in the calculations of the chloro compounds, the copper complex shows an additional shortening of the bond length when the quasirelativistic pseudopotential description is used. The M– P distances are 221.2 pm for MsCu, 241.3 pm for Ag, and 226.6 pm for Au (Table 2). 3.3. Bond dissociation energies The gradient corrected density functional method is well known to give very good results in the calculation of bond dissociation energies (BDE) in organometallic complexes [6,12,24]. The calculations often reach the same quality as ab initio calculations with inclusion of a high electron correlation [24]. We calculated the bond dissociation energies for the reactions ClMPH3™ClMqPH3 and [MPH3]q ™MqqPH3 (Table 3). The energies were not corrected for zero-point vibrational energy contributions because these are Table 2 Optimized distances (pm) and angles (8) for the [MPH3]q cations (MsCu, Ag, Au) Molecule

Basis

M–P

P–H

M–P–H

[CuPH3]q

NR R B3LYP/B2 B3LYP/B2

225.2 221.2 241.3 226.6

140.7 140.7 140.6 140.2

115.8 115.5 116.1 113.2

[AgPH3]q [AuPH3]q

NRsnon-relativistic, Rsrelativistic basis set. Table 3 Dissociation energies De (in kJ moly1) for the metal phosphorus bond in ClMPH3 and [MPH3]q

119.0 117.9

MsCu MsAg MsAu

ClMPH3

[MPH3]q

143 96 174

245 176 315

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Fig. 1. Electron density plots of the LUMO of the three different [MPH3]q compounds.

usually small in comparison to the errors of the calculations. The calculations were carried out with the quasirelativistic pseudopotential of Stevens on the copper atom. The ClAuPH3 complex has the strongest M–P bond with a bond dissociation energy of 173 kJ moly1. This is in good agreement with the M–P dissociation energy in MeAuPH3 of 182 kJ moly1 calculated by Ha¨berlen and Ro¨sch [6]. The energy for the bond cleavage in the copper complex is 143 kJ moly1 and in the silver compound it is 96 kJ moly1. This behavior shows the relatively weak M–P bond in the silver compound. The comparison of the related carbonyl complexes of Frenking and co-workers displays the same stability behavior [8]. A comparison of calculated data for different Auq(L) (LsH2O, CO, C2H4, C6H6 NH3, C3H6) dissociation energies was published by Hertwig et al. [12]. Our results show that the Au–PH3 bond in the cationic complexes seems to be the strongest bond in the series of the calculated systems. This supports the good stability and the frequent use of the AuPH3 fragment. The copper and the silver compounds display a definitely lower BDE. That of [CuPH3]q is about 244 kJ moly1 and that of [AgPH3]q is 176 kJ moly1 which is only about half that of the gold compound. This confirms a low stability of these fragments in comparison to the gold system.

The LUMO shapes of the [MPH3]q cations are very similar to each other (Figs. 1 and 2). The criterion for isolobality is not only the shapes of the orbitals but also the energies [4]. Compared with the [CuPH3]q complex, the LUMO energy of the silver compound is about 4.35 eV less and that of the gold compound 6.74 eV less. Because of the big differences between the LUMO energies, the isolobality argument cannot be extended to all three cations. A parameter for the donor/acceptor behavior of the phosphine ligand in the [MPH3]q systems is the natural charge on the atoms (Table 4). A very similar charge distribution is observed in the copper and silver complexes. In both compounds the metal atom shows a natural charge in the range of 0.75 while the phosphorus has nearly no charge, and the charge of the H atoms is in the range of 0.1. In the gold system there is a slightly different situation, as the gold atom has a positive charge of 0.5 and the phosphorus atom of 0.16. The observed charges support the high Pauling electronegativity

3.4. NBO analysis of the optimized [MPH3]q fragments (MsCu, Ag, Au) A single point MP2 natural bond (NBO) analysis [25,26] of the optimized (B3LYP) [MPH3]q geometries was performed to gain more insight into the electronic structure of the cations. The NBO study has the advantage of being not as basis set dependent as the Mulliken population analysis and that the orbital description corresponds to the Lewis structure picture. The LUMO in all three cations has an overall s-character on the metal mixed in an antibonding manner with a spz-type hybrid orbital on the phosphorus. Because of the high degree of s-character at the metal, the orbitals exhibit the a1 spherical symmetry typical for a hydrogen atom orbital.

Fig. 2. Three-dimensional electron density plot of a typical LUMO. Table 4 Natural charges on the atoms in the [MPH3]q cations Charge on

[CuPH3]q

[AgPH3]q

[AuPH3]q

M P H

0.75 y0.05 0.10

0.75 y0.05 0.10

0.51 0.17 0.11

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of the gold atom. This could be explained by the relativistic contraction of the orbitals at the gold atom which shifts the electron density closer to the core.

4. Conclusions The B3LYP gradient corrected density functional theory, combined with effective core potentials on the transition metals, is a good method for optimization of neutral and cationic Group 11 phosphino compounds. The optimization results show that consideration of the relativistic effect for copper is necesssary for a good agreement of the calculated with the experimental M–P bond length. The calculation of the bond dissociation energies displays the high stability of the Au–P bond which should be one major reason for the many possibilities to use this fragment in preparative chemistry. The dissociation energy decreases in the order Au–P)Cu–P)Ag–P in the neutral chlorides as well as in the cationic complexes.

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