Bis(trimethylphosphine)silver(I) hexafluorophosphate

In the title two-coordinate silver compound, [Ag(C₃­H₉­P)₂]­PF₆, the cation has crystallographically imposed mirror symmetry, and approximates very closely to m (D_3d) symmetry with fully staggered methyl groups in the solid state. The Ag atom has a nearly linear coordination geometry, with a P-Ag-P angle of 178.70 (4)°. The Ag-P bond lengths are 2.3746 (12) and 2.3783 (12) Å, which are ­significantly longer than the Au-P bond length of 2.304 (1) Å in the analogous two-coordinate gold cation. The lack of intra­molecular steric effects within the present cations containing tri­methyl­phosphine (cone angle 118°), compared with those in known cations containing trimesityl­phosphine (cone angle 212°), provides a better comparison of M-P distances and thus more conclusive evidence that Au really is smaller than Ag.

S1. Comment

The concept of the `lanthanide contraction' is commonly invoked to explain some observed radius discontinuities within the periodic table (Pyykko, 1988). The phenomenon called the `relativistic contraction' (Schwerdtfeger et al., 1990), which is predictable by theoretical calculations involving relativistic and correlational effects, is usually thought to be the specific cause of a third-row element having an expected radius that is similiar to, if not less than, that of a second-row element. Experimental proof of this phenomenon has not been forthcoming, because a comparison of metal-to-ligand bond lengths is invalidated by changes in coordination number or geometry or crystal lattice in an apparently analogous set of second- and third-row complexes. Recently (Bayler et al., 1996), an attempt has been made to answer the question by making a direct comparison of analogous Ag and Au complexes. It was concluded from the crystal structure analyses of bis(trimesitylphosphine)silver(I) and bis(trimesitylphosphine)gold(I) tetrafluoroborate, that the Au^I atomic radius is almost 0.1 Å smaller than that of Ag^I. Although acknowledging that the Ag/Au—PR₃ distances were larger in these complexes than in other phosphine complexes with smaller R substituents, it was considered that the two trimesitylphosphine ligands were `in a strain-free staggered conformation with deviations in their geometrical parameters within the limits of experimental error', and thus that the comparison was valid.

Our previous structural analysis of the two-coordinate bis(trimesitylphosphine)silver(I) cation (Alyea et al., 1982) indicated, however, that significant intramolecular crowding occurs due to methyl···methyl interactions. The conformation adopted by the phosphine moieties in the molecule deviates from a fully staggered C₃P—Ag—PC₃ atom system by an average of 17°. Application of our `ligand profile' concept (Ferguson et al., 1998) also showed that repulsion led to displacement of the P atoms and o-methyl groups relative to the aromatic ring plane, as well as other angle bending. Similarly, our X-ray structural analysis of chloro(trimesitylphosphine)gold(I) (Alyea et al., 1993) clearly showed that significant attractive interaction occurs between the Au atom and the immediately adjacent mesityl methyl groups. Hence, any comparison of Ag/Au—P bond distances within M(Pmes₃) (Pmes₃ is trimesitylphosphine) moieties is flawed by the presence of steric interactions. Our recent comparison of the Ag/Au—P bond distances within the chloro[tris(2,4,6-trimethoxyphenyl)phosphine]silver(I) and chloro[tris(2,4,6-trimethoxyphenyl)phosphine]gold(I) complexes was still inadequate, due to the presence of weak M···O interactions (Alyea et al., 2000). The present work compares the title complex, (I), with an analogous two-coordinate Au complex also involving the small phosphine, trimethylphosphine (cone angle 118°), for which no steric hindrance is expected. \sch

The crystal structure of (I) contains discrete well resolved cations and anions located on independent mirror planes, with the PF₆ anion disordered (Fig. 1). While the cation in (I) has crystallographically imposed mirror symmetry, it approximates very closely to 3 m (D_{3 d}) symmetry with fully staggered methyl groups (Fig. 1). Examination of the structure with PLATON (Spek, 2002) showed that there were no solvent accessible voids in the crystal lattice.

In the cations, the Ag—P distances (Table 1) have a mean value of 2.376 (2) Å. For comparison, Ag—P distances of 2.461 (6) Å (Alyea et al., 1982) and 2.4409 (9) Å (Bayler et al., 1996) have been reported for the bis(trimesitylphosphine)silver(I) cation. The Ag—P bond distance based on the sum of tetrahedral covalent radii is calculated as 2.44 Å (Pauling, 1960) or 2.64 Å (Cambridge Structural Database, Release?; Allen & Kennard, 1993), so that a shorter distance might be anticipated for a two-coordinate complex.

Although several complexes of the type [L₂Ag]Y are primarily two-coordinate with other bulky phosphine ligands L, weak coordination of the anion Y occurs in the solid state. A search of the April 2002 release of the Cambridge Structural Database (CSD; Allen & Kennard 1993) for structures containing the P₂Ag fragment showed no instances other than the (mes₃P)₂Ag cation, wherein the P—Ag—P bond angle was larger than 170°. The Ag—P distances in the authentically two-coordinate complexes chloro[tris(2,4,6-trimethoxyphenyl)phosphine]silver(I) and bromo[tris(2,4,6-trimethoxypheny)phosphine]silver(I) are 2.379 (1) and 2.374 (2) Å, respectively (Baker et al., 1992).

In polymeric [Ag₄Cl₄(PMe₃)₃] (Bowmaker et al., 1999), the Ag—P distance is 2.362 (4) Å, though the coordination is not strictly two-coordinate. Likewise, for analogous gold complexes of the type [L₂Au]Y, the only truly two-coordinate example found without significant anion interactions was [(Me₃P)₂Au]Cl (Angermaier et al., 1994), with Au—P 2.2304 (1) Å and P—Au—P 175.4 (1)°; the Au···Cl distance of 3.167 (1) Å indicated a weak `ion-pair contact' interaction.

A `ligand profile' of the Me<sub>3</sub>P group in the present silver complex clearly indicates that steric hindrance does not affect the Ag—P bond distance. The mean cone angle was calculated to be 115°, compared with the `CPK model cone angle' of 118° (Tolman, 1977). For comparison, the cone angles were calculated for the Ag-PMe₃ moiety in some other known complexes of Ag with trimethylphosphine. In the polymeric complexes [Ag₄Cl₄(PMe₃)₃] (Bowmaker et al., 1999) and [Ag(PR₃)₂]₂[Ni(mnt)₂] (mnt is maleonitriledithiolate; Youm et al., 2000), similar cone angles of 116° and 115°, respectively, reflect the absence of steric congestion. The cone angle in polymeric [Me₃PAgCCSiMe₃], which has adjacent phosphine ligands [P—Ag—P 113.33 (9)°; Brasse et al., 1996], is 111°.

Since no intramolecular interactions exist between the methyl substituents in either of the current [(Me₃P)₂Ag] or [(Me₃P)₂Au] cations, a valid comparison of M—P distances is possible, allowing the conclusion that Au really is smaller than Ag.

The P—C bond lengths and Ag—P—C and P—C—P bond angles (Table 1) are comparable with those reported for other trimethylphosphine complexes of Ag. Comparison with the angles in the analogous Au complex is not possible due to the reported disorder of the phosphine ligands.

The shortest Ag···F interion distance of 3.392 (7) Å [Ag1···F7] is not compatible with any significant covalent bonding.

S2. Experimental

Compound (I) was prepared by combining silver hexafluorophosphate with two equivalents of trimethylphosphine in dichloromethane at ambient temperature under an N₂ atmosphere. The colourless crystals obtained were kept in the dark under N₂ to avoid possible decomposition. The ³¹P NMR solution spectrum in CH₂Cl₂/CDCl₃ (1:1) at a lowered temperature showed peaks at -34.07 p.p.m., attributable to the coordinated Me₃P, and at -143.28 p.p.m., due to the PF₆ anion. A superposition of two doublets, due to coupling of the P nucleus with the two magnetic isotopes of Ag, occurs for the cation signal; ¹J(¹⁰⁷Ag ³¹P) = 516 Hz, ¹J(¹⁰⁹Ag ³¹P) = 592 Hz. For comparison, the ¹J(¹⁰⁷Ag ³¹P) coupling constants for complexes [LnAg]Y of Ag(I) with triphenylphosphine are 755 (n = 1), 507 (n = 2), 319 (n = 3) and 224 (n = 4) Hz (Alyea et al., 1987). Hence, two-coordination is maintained in solution.

S3. Refinement

Molecule (I) crystallized in the orthorhombic system; space group Pnma or Pn2₁a from the systematic absences. Pnma was chosen, and confirmed by the successful solution and refinement. The cation lies on a mirror plane, with atoms C11, P1, Ag1, P2 and C21 on the mirror. The PF₆^-1 anion also lies on a mirror plane and is disordered; atoms P3, F1, F2, F5 and F6 lie on the mirror plane. In the final refinement cycles, DFIX restraints (SHELXL97; Sheldrick, 1997) were employed to ensure octahedral geometry at P3. The P—F bond length was refined as a free-variable [final value 1.53 (1) Å]. H atoms were treated as riding atoms (C—H = 0.96 Å).

Computing details

Data collection: CAD-4-PC Software (Enraf-Nonius, 1992); cell refinement: SET4 and CELDIM in CAD-4-PC Software; data reduction: DATRD2 in NRCVAX96 (Gabe et al., 1989); program(s) used to solve structure: NRCVAX96 via Patterson heavy-atom method; program(s) used to refine structure: NRCVAX96 and SHELXL97 (Sheldrick, 1997); molecular graphics: NRCVAX96, ORTEPII (Johnson, 1976) and PLATON (Spek, 2002); software used to prepare material for publication: NRCVAX96, SHELXL97 and PRPCIF97 (Ferguson, 1997).

Figures

Fig. 1. A view of complex (I) with the atom-numbering scheme; the central P atom of the anion (P3) is not labelled. Displacement ellipsoids are drawn at the 30% probability level and H atoms are shown as small spheres of arbitrary radii [symmetry codes: (i) x, 3/2 - y, z; (ii) x, 1/2 - y, z].

Bis(trimethylphosphine)silver(I) hexafluorophosphate

Crystal data

[Ag(C3H9P)2]PF6	? #Insert any comments here.
Mr = 404.98	Dx = 1.823 Mg m^−3
Orthorhombic, Pnma	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ac 2n	Cell parameters from 25 reflections
a = 11.7912 (12) Å	θ = 10.0–18.9°
b = 8.4725 (7) Å	µ = 1.73 mm^−1
c = 14.768 (4) Å	T = 293 K
V = 1475.3 (4) Å^3	Block, colourless
Z = 4	0.55 × 0.31 × 0.25 mm
F(000) = 800

Data collection

Enraf-Nonius CAD-4 diffractometer	1387 reflections with I > 2σ(I)
Radiation source: X-ray tube	Rint = 0.000
Graphite monochromator	θmax = 27.4°, θmin = 2.2°
θ/2θ scans	h = 0→15
Absorption correction: empirical (using intensity measurements) via six ψ scans at 4° steps (North et al., 1968)	k = 0→10
Tmin = 0.536, Tmax = 0.648	l = 0→19
1795 measured reflections	3 standard reflections every 120 min
1795 independent reflections	intensity decay: variation +−1.0%

Refinement

Refinement on F^2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: inferred from neighbouring sites
R[F^2 > 2σ(F^2)] = 0.032	H-atom parameters constrained
wR(F^2) = 0.095	w = 1/[σ^2(Fo^2) + (0.0567P)^2 + 0.3935P] where P = (Fo^2 + 2Fc^2)/3
S = 1.09	(Δ/σ)max < 0.001
1795 reflections	Δρmax = 0.58 e Å^−3
108 parameters	Δρmin = −0.62 e Å^−3
23 restraints	Extinction correction: SHELXL97 (Sheldrick, 1997), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
Primary atom site location: structure-invariant direct methods	Extinction coefficient: 0.0107 (9)

Special details

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq	Occ. (<1)
Ag1	0.91326 (3)	0.7500	0.38115 (2)	0.05491 (17)
P1	0.84665 (10)	0.7500	0.53290 (7)	0.0451 (3)
P2	0.98427 (9)	0.7500	0.23041 (7)	0.0438 (3)
C11	0.6948 (4)	0.7500	0.5448 (4)	0.0650 (13)
H11A	0.6753	0.7500	0.6079	0.098*
H11B	0.6641	0.6575	0.5164	0.098*	0.50
H11C	0.6641	0.8425	0.5164	0.098*	0.50
C12	0.8910 (3)	0.5820 (5)	0.5993 (2)	0.0665 (9)
H12A	0.9722	0.5734	0.5974	0.100*
H12B	0.8578	0.4877	0.5749	0.100*
H12C	0.8668	0.5955	0.6608	0.100*
C21	1.1381 (4)	0.7500	0.2213 (4)	0.0619 (12)
H21A	1.1595	0.7500	0.1586	0.093*
H21B	1.1680	0.6575	0.2503	0.093*	0.50
H21C	1.1680	0.8425	0.2503	0.093*	0.50
C22	0.9411 (3)	0.9190 (5)	0.1643 (3)	0.0774 (11)
H22A	0.9627	1.0141	0.1953	0.116*
H22B	0.8604	0.9173	0.1563	0.116*
H22C	0.9775	0.9156	0.1062	0.116*
P3	0.73940 (12)	0.2500	0.38863 (9)	0.0618 (3)
F1	0.8347 (5)	0.2500	0.4604 (4)	0.173 (3)
F2	0.6456 (5)	0.2500	0.3162 (3)	0.161 (2)
F3	0.6751 (7)	0.3749 (10)	0.4426 (5)	0.147 (4)	0.50
F4	0.8003 (7)	0.3803 (11)	0.3375 (7)	0.199 (7)	0.50
F5	0.6516 (8)	0.2500	0.4632 (6)	0.268 (14)	0.50
F6	0.8288 (8)	0.2500	0.3135 (6)	0.201 (8)	0.50
F7	0.7402 (7)	0.4304 (6)	0.3892 (6)	0.182 (7)	0.50

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Ag1	0.0575 (2)	0.0660 (3)	0.0412 (2)	0.000	0.00881 (13)	0.000
P1	0.0579 (6)	0.0394 (5)	0.0379 (5)	0.000	0.0034 (4)	0.000
P2	0.0423 (5)	0.0493 (6)	0.0398 (5)	0.000	0.0028 (4)	0.000
C11	0.070 (3)	0.063 (3)	0.062 (3)	0.000	0.014 (2)	0.000
C12	0.093 (3)	0.0504 (19)	0.0560 (17)	0.0090 (18)	−0.0044 (17)	0.0068 (16)
C21	0.045 (2)	0.075 (3)	0.066 (3)	0.000	0.008 (2)	0.000
C22	0.074 (2)	0.081 (3)	0.077 (2)	0.014 (2)	0.0010 (19)	0.027 (2)
P3	0.0742 (8)	0.0502 (7)	0.0609 (7)	0.000	−0.0089 (6)	0.000
F1	0.185 (5)	0.138 (4)	0.195 (6)	0.000	−0.118 (5)	0.000
F2	0.179 (5)	0.181 (6)	0.122 (4)	0.000	−0.074 (4)	0.000
F3	0.184 (9)	0.123 (9)	0.133 (6)	0.052 (7)	0.012 (6)	−0.050 (6)
F4	0.185 (10)	0.137 (11)	0.275 (15)	−0.059 (8)	0.054 (10)	0.085 (10)
F5	0.34 (3)	0.23 (2)	0.23 (2)	0.000	0.21 (2)	0.000
F6	0.213 (17)	0.24 (2)	0.147 (12)	0.000	0.081 (12)	0.000
F7	0.248 (15)	0.052 (4)	0.247 (16)	0.015 (7)	−0.095 (12)	0.004 (7)

Geometric parameters (Å, º)

Ag1—P1	2.3746 (12)	C21—H21B	0.96
Ag1—P2	2.3784 (12)	C21—H21C	0.96
P1—C11	1.799 (5)	C22—H22A	0.96
P1—C12^i	1.806 (4)	C22—H22B	0.96
P1—C12	1.806 (4)	C22—H22C	0.96
P2—C22	1.806 (4)	P3—F5	1.512 (7)
P2—C22^i	1.806 (4)	P3—F4^ii	1.518 (6)
P2—C21	1.819 (4)	P3—F4	1.518 (6)
C11—H11A	0.96	P3—F3^ii	1.526 (5)
C11—H11B	0.96	P3—F3	1.526 (5)
C11—H11C	0.96	P3—F7^ii	1.528 (5)
C12—H12A	0.96	P3—F7	1.528 (5)
C12—H12B	0.96	P3—F6	1.531 (7)
C12—H12C	0.96	P3—F2	1.539 (4)
C21—H21A	0.96	P3—F1	1.544 (4)
P1—Ag1—P2	178.70 (4)	H22A—C22—H22B	109.5
C11—P1—C12^i	103.63 (17)	P2—C22—H22C	109.5
C11—P1—C12	103.63 (17)	H22A—C22—H22C	109.5
C12^i—P1—C12	104.0 (3)	H22B—C22—H22C	109.5
C11—P1—Ag1	114.91 (18)	F4^ii—P3—F4	93.3 (10)
C12^i—P1—Ag1	114.61 (13)	F4^ii—P3—F3^ii	89.5 (6)
C12—P1—Ag1	114.61 (13)	F4—P3—F3^ii	177.2 (7)
C22—P2—C22^i	104.9 (3)	F4^ii—P3—F3	177.2 (7)
C22—P2—C21	103.94 (17)	F4—P3—F3	89.5 (6)
C22^i—P2—C21	103.94 (17)	F3^ii—P3—F3	87.8 (9)
C22—P2—Ag1	114.00 (15)	F5—P3—F7^ii	90.0 (4)
C22^i—P2—Ag1	114.00 (15)	F5—P3—F7	90.0 (4)
C21—P2—Ag1	114.84 (18)	F7^ii—P3—F7	179.0 (6)
P1—C11—H11A	109.5	F5—P3—F6	179.7 (5)
P1—C11—H11B	109.5	F7^ii—P3—F6	90.0 (4)
H11A—C11—H11B	109.5	F7—P3—F6	90.0 (4)
P1—C11—H11C	109.5	F5—P3—F2	90.8 (4)
H11A—C11—H11C	109.5	F4^ii—P3—F2	89.6 (3)
H11B—C11—H11C	109.5	F4—P3—F2	89.6 (3)
P1—C12—H12A	109.5	F3^ii—P3—F2	90.4 (3)
P1—C12—H12B	109.5	F3—P3—F2	90.4 (3)
H12A—C12—H12B	109.5	F7^ii—P3—F2	90.5 (3)
P1—C12—H12C	109.5	F7—P3—F2	90.5 (3)
H12A—C12—H12C	109.5	F6—P3—F2	89.5 (4)
H12B—C12—H12C	109.5	F5—P3—F1	89.9 (4)
P2—C21—H21A	109.5	F4^ii—P3—F1	89.9 (3)
P2—C21—H21B	109.5	F4—P3—F1	89.9 (3)
H21A—C21—H21B	109.5	F3^ii—P3—F1	90.1 (3)
P2—C21—H21C	109.5	F3—P3—F1	90.1 (3)
H21A—C21—H21C	109.5	F7^ii—P3—F1	89.5 (3)
H21B—C21—H21C	109.5	F7—P3—F1	89.5 (3)
P2—C22—H22A	109.5	F6—P3—F1	89.8 (4)
P2—C22—H22B	109.5	F2—P3—F1	179.3 (3)

Symmetry codes: (i) x, −y+3/2, z; (ii) x, −y+1/2, z.