CH2405 – Multinuclear Metal Systems

Bond Order in Multinuclear Metal SystemsTypically bond order is considered to equate to the number of bonding electrons minus the number of anti-bonding electrons divided by 2. In metal systems this is not always true, orbital overlap contribution from σ orbitals is stronger than for π orbitals, which are in turn stronger than for δ orbitals, the result of this is a bond order that is often smaller than calculated. Bond strength would be expected to increase down a group however it is not always this straightforward, for example W2

4+ is sensitive to oxidation, in other words the bond strength can also depend on the equilibrium between M2

4+ and 2M2+.

A Few Historical Examples of Metal-Metal Bonds

1840; Chromium acetate [Cr2(OAc)4].2H2OSynthesis;

[CrII(H2O)6]2+ NaOAc

→ [Cr2(OAc)4].2H2O

The hexaaqua species is blue with chromium in the high spin d4 configuration, compared to chromium acetate which is red and diamagnetic, therefore containing no unpaired electrons.

1957; Decacarbonyl dimanganese Mn2(CO)10

1964; Octachloride dirhenium [Re2(Cl)8]2-

This species contains a Re-Re quadruple bond.Synthesis;

[ReO4]- + HCl + H3PO4 + Cs+ →

Cs2[Re2(Cl)8]

Alternative synthesis;

2[NBu4][ReO4] + 8PhCOCl →

[Re(Cl)8][NBu4]2

This method utilises NBu4 to facilitate solubility in organic solvents, the benzoyl chloride acts as a reducing agent, chlorinating agent and organises the Re units through the formation of an intermediate containing a benzoate bridge motif.

Figure 1 - Benzoate bridging motif

Criteria for a Metal-Metal BondIn order to form a metal-metal bond there must be orbitals capable of overlapping with the appropriate size, orientation and energy. Since 5d orbitals are more diffuse than 4d and 3d orbitals they are capable of forming better overlap and so metal-metal bonds between atoms with 5d orbitals will be stronger than those between 3d/4d orbitals (Table 1).

Table 1 - Analagous group VII complexes and the relative first excitation energies

Complex E (σ-σ*)Mn2(CO)10 29740 cm-1

Tc2(CO)10 32400 cm-1

Re2(CO)10 32800 cm-1

Multiple Metal-Metal Bonds

Table 2 - Electron configurations and the subsequent bond multiplicities

d1-d1 Single bondd2-d2 Double bondd3-d3 Triple bondd4-d4 Quadruple bond (Optimum)d5-d5 Triple bondd6-d6 Double bondd7-d7 Single bondd8-d8 No bond (due to symmetry interactions)

-[Re2(Cl)8] 2-

The structure of this species (Figure 2) consists of two pseudo-square planar ReCl4 units bound in an eclipsed formation. The chlorine atoms are within double the Van der Waals distance and so stearically this arrangement is disfavoured, the Re-Re bond must therefore be highly energetically favoured to account for this seemingly flawed structure. In rhenium metal the average Re-Re bond distance is approx. 2.78Å and so the distance of 2.24Å in this species is telling of multiply bonded metal atoms.

Figure 2- The structure of [Re2Cl8]2-

By treating each ReCl4 unit individually and applying a Jahn-Teller style distortion along the z-axis, Figure 3 is reached. This diagram shows that the overall bond order is calculated as 4, and can be described in terms of the nature of the bonding overlap in the form σ2π4δ2. This species actually has a measured bond order of 3.25.

Reactions of [Re2Cl8] 2- Retaining the quadruple bond;i) Halide exchange;

[Re2Cl8]2- + Bu4NX X=F ,Br→

[Re2X8]2-

ii) Other anions;

[Re2Cl8]2- + H2SO4 →

[Re2(SO4)4]2-

Here the SO4 units bridge the Re-Re system and force the Re-Re atoms even closer together with a bond distance of 2.14Å.

iii) Partial substitution by –PR3

[Re2Cl8]2- + 2PMePh2 →

[Re2Cl6(PMePh)2]

The structure of this species has one PR3 unit at each Re, trans to each other due to stearic interactions.

Deviations from a calculated bond order of 4;i) Reactions with dithioethers

[Re2Cl8]2- + MeS(CH2)2SMe →

[Re2Cl5(MeS(CH2)2SMe)2]

dz2dz2

dxz, dyzdxz, dyz

dxydxy

Figure 3 - MO diagram for [Re2(Cl)8]2-

Both dithioether units are found on one rhenium centre with the asymmetrical substitution a result of the trans-effect after substitution of the first dithioether. This species contains mixed-valence Re centres, ReII and ReIII are both present, since one rhenium centre has been oxidised the configuration can be rewritten as σ2π4δ2 δ*1 and so the calculated bond order is now 3.5.

ii) Reactions with tertiary phosphines

[Re2Cl8]2- + 4PMe2Ph →

[Re2Cl4(PMe2Ph)4]

Both Re centres are reduced from ReIII to ReII and so the electron configuration is now σ2π4δ2 δ*2

giving a total bond order of 3. With the δ component overall non-bonding there is some rotation about the Re-Re bond

iii) Reactions with bidentate phosphines (e.g. dppe)

[Re2Cl8]2- + Ph2P(CH2)2PPh2 →

[ReX2(Ph2P(CH2)2PPh2)]2 + [Re2X4(μ2- Ph2P(CH2)2PPh2)2]

The reaction gives rise to two products, one in which a bidentate phosphine is found at each rhenium forming a very stable 5-membered ring. The second product is one in which two bidentate ligands bridge both metal centres and forms a more staggered structure. The products can be differentiated by 31P NMR, the better the ligand is able to donate density, the more deshielded the phosphorus and so in this case the first product has a higher shift than both the second product and the free ligand system.

Breaking the Re-Re Quadruple Bond

[Re2Cl8]2- Δ→

[ReCl6]2-

[Re2Cl8]2- CO /Pressure

→[Re(CO)5Cl]

[Re2Cl8]2- R N+¿C

→−¿¿¿[Re(RNC)6]+ + [ReCl2(RNC)5]+

Similar Technetium Chemistry

[TcVIIO4]- + Zn/HCl →

[Tc2Cl8]3- electrochemistry→

[Tc2Cl8]2-

Initially technetium does not quite reduce entirely to TcIII with the mixed TcII/TcIII species achieved, this can then be oxidised electrochemically to give the analogous (cf. [Re2Cl8]2-) TcIII bimetallic species. The bond distance in the mixed valence species is shorter than the same-valence species suggesting the former to be more stable, despite a formal increase in bond order from 3.5 to 4.

Group VI ChemistryDimeric Chromium Species

Cr + H+(aq)

N2→

[Cr(H2O)6]2+ NaOAc→

[Cr2(OAc)4].2H2O

The product of this reaction again has a formal bond order of 4 and is a diamagnetic species, although paramagnetic impurities are not entirely uncommon. Particularly poor donors will lead to an increase in the energy of δ while decreasing the energy of δ*, this in turn allows for deviation from diamagnetism at low temperatures.

d(Cr-Cr) is susceptible to ligand changes, in particular through the use of bridging ligands it is possible to achieve bond lengths as short as 1.9Å.

Figure 4 - The structure of the dimeric chromium acetate species

Dimeric Molybdenum Species

Mo(CO)6 + HCl + RCO2H →

[Mo2(RCO2)4] HCl→

[Mo2Cl8]4-

The Mo-Mo quadruple bond is very very strong.

Trimeric Rhenium Systems

CsReCl4 →

Cs3[Re3(Cl)12]

The structure of this species includes bridging chlorides as well as axial and equatorial terminal chlorides. The species can then go on to react in different ways with different types of reagent. The bond distance between Re atoms in this species is 2.49Å, larger than that of the dimer as would be expected.

i)Reaction with phosphines, PR3

Cs3[Re3(Cl)12]→

[Re3Cl9(PR3)3]

Here the phosphines replace equatorial chlorides.

ii) Reaction with BBr3

Cs3[Re3(Cl)12]→

[Re3Cl3Br9]

The bromine replaces the terminal chlorides in this reaction.

iii) Reaction with sulphuric acid

Cs3[Re3(Cl)12]→

[Re3Cl9(SO4)]

The (SO4)2- replaces the axial chlorides and ‘caps’ the Re triangle.

These reactions provide insight into the relative reactivity of each type of chloride with reactivity increasing from bridging<axial<equatorial chlorides.

Platinum Complexes and ApplicationsPt(II) (d8) complexes regularly form square planar complexes, these complexes often tend towards Pt-Pt interactions resulting in stacked Pt(II) units.The three most important families of complexes include;

i. [PtX4]2- (X=Cl, Br, I, CN, SCN-S)ii. [PtX2L2] (X=Cl, Br; L= NH3, NR3, RCN, py, PR3, SR2)

iii. [PtL4]2+ (L=PR3, NH3, NR3, MeCN)

Synthesis of mixed amine complexes;

[PtCl4]2- + SO2 ; HCl → H2PtCl4 + NH3 + heat → [Pt(NH3)4]Cl2

[PtCl4]2- + 2NH3 → cis-[PtCl2(NH3)2] + 2Cl-

The latter reaction proceeds via a trigonal bipyramidal intermediate, this then resolves to give the cis product (after two additions) as a result of the trans-effect.

Precipitation of colourless [Pt(NH3)4]Cl2 and pink [PtCl4]2- gives Magnus’ green salt, a polymerisation isomer of [PtCl2(NH3)2]. This salt contains alternating, eclipsed cations and anions with significant Pt-Pt interactions (3.24 Å), this interaction is the cause of the colour change. Despite the metal-metal interactions, Magnus’ salt is not a metallic conductor.

The related cyano complex is synthesised by reaction of [MCl6]2- and potassium cyanide, giving the potassium hydrate – a yellow crystal that forms colourless aqueous solutions. The colour change arises from the non-eclipsed stacking of the square planar anions in the solid (Pt-Pt distances 2.78 Å). Replacement of the two potassium cations with a single group II cation will alter the colour from yellow-green for barium to violet for strontium.

The potassium salt can be partially oxidised with Cl2 or Br2 to give bronze complexes (K2(Pt(CN)4]X0.3.2.5 H2O), these contain isolated X ions and stacks of staggered [Pt(CN)4]2- anions with short Pt-Pt contacts (2.88 Å) that act as very effective one-dimensional metallic conductors.

Pt-Pt interactions form a class of vapochromic salts – these are materials that display pronounced and reversible changes of colour and/or emission in the presence of volatile organic compounds. They can be used as chemical sensors and incorporated into electronic devices. The complexes are typically designed with planar ‘terpy’ type ligands (Figure 5) with strong sigma-donors and pi-acceptors in order to favour the Pt-Pt interaction.

Figure 5 - A 'terpy' platinum complex

The Cl- salt of the complex above responds to ether, methanol, acetonitrile, dichloromethane, ethanol, benzene, water and propanol. The PF6

- salt responds only to acetonitrile and can be analysed by IR and gravimetric analysis.

Palladium (II) and platinum (II) stacks and piezochromic responses

A piezochromic response is a change in colour and/or emission as a result of pressure perturbation. Stacked, non-eclipsed complexes will show a reduction in bond length at higher pressures, as the energy of the vacant px orbital is reduced until overlap is achieved with the dz2.

Gold Complexes and Aurophilic Interactions

Closed shell Au(I) complexes (d10) are known to aggregate, this is thought to be the result of relativistic effects and is known as aurophilicity. These interactions are important in understanding the solid state structures of many Au(I) compounds. Metallic gold has d(Au-Au) of 2.89 Å while double the Van der Waals distance is 3.6 Å, if the Au-Au distance is less than 3.6 Å it is likely that an aurophilic interaction is occurring. Au(I) typically adopts a 2-coordinate linear structure, preferring soft donors e.g. thiols and phosphines.

Synthesis;

AuBr(THT) + dppe → [Au2(μ-dppe)2Br2]

Recrystallization from different solvents will lead to different emission characteristics in the colourless crystal; DMSO/acetone/DCM/DMF will lead to orange emission, Et2O/MeCN will give green emission. Removal and addition of solvate can alter the characteristic emissions of these crystals. The flexibility of the complex determines the vapoluminescent properties.

Luminescence in these complexes can also be activated under irradiation with gentle grinding of the solid sample (this process is called luminescence tribochromism).

Figure 6 - An example of an Au(I) tribochromic species

The complex shown in Figure 6 forms an aurophilic helix with the hydrophilic thiouracilates stacked in the centre with the hydrophobic dppm ligands on the helix exterior. The neighbouring dinuclear Au-Au cations link head-to-tail (d(Au-Au)=3.3321 Å). Typically this complex is very weakly emissive at 375 nm following irradiation, gentle crushing with a spatula leads to bright blue emission at 480 nm.