Electronic structure / bonding in d-block complexes

Transcription

1 LN05-1 Electronic structure / bonding in d-block complexes Many, many properties of transition metal complexes (coordination number, structure, colour, magnetism, reactivity) are very sensitive to the number of d-electrons and how they are arranged in the d-orbitals. For the transition element valence orbitals the energy-ordering is: ns < (n-1)d < np H&S 19.2 BUT For higher oxidation states, M n+, the energies of (n-1)d orbitals tend to be lower in energy than the ns orbitals. Why? Recall: orbital energies affected by principal quantum number (n), effective nuclear charge experienced by electrons (Z eff ) and e -e repulsions as subshells are filled. Removal of one or more electrons (oxidation) reduces overall e repulsion and lowers energy; this effect is most pronounced for d-orbital energies, relative to s or p. e.g. Element free atom configuration configuration in complexes Scandium 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 1 1s 2 2s 2 2p 6 3s 2 3p 6 3d 3 Iron 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 6 1s 2 2s 2 2p 6 3s 2 3p 6 3d 8

2 LN05-2 Oxidation states and d-electron counts Transition metal ions (mostly) have no s-e, only d-e, in their valence shell so, by convention, we discuss the electron configurations of metal ions as d n where n is the number of d-electrons in the valence shell of the T.M. ion. H&S 20. Determining the d-electron count in a transition metal complex 1. Determine the oxidation state: oxidation state = - (Σ ligand charge) + (charge on inner sphere complex) Once the formal charge of the metal is determined, the d-electron count can be determined based on the group that the metal belongs to: for d n, n = (group number) (oxidation state of metal)

3 LN05-3 Examples: Rh(PPh 3 ) 3 Cl: Oxidation states and d-electron counts H&S 20. K 3 [Fe(CN) 6 ]

4 LN05-4 Oxidation states Group (Figure 19.3, H&S) Blue = common oxidation state. [ ] = rare oxidation state. Blank = non-existent

5 LN05-5 Oxidation states general comments 1. Most metals can adopt more than 1 oxidation state. Exceptions are groups 3 and Within a group, the heavier metals tend to (slightly) favour higher oxidation states. 3. In the early to mid part of the d-block (up to Group 8), oxidation states range up to and including the maximum possible oxidation state. 4. From Group 9 onwards, very high oxidation states become less favoured. For most metals in this part the preferred oxidation states are maximum +3 (and occasionally +4 for heavier metals)

6 LN05-6 The Electroneutrality principle H&S 19.6 Formal oxidation states are mainly useful for bookkeeping purposes and should not be interpreted as a representation of the real charge on a metal ion. old-fashioned way of representing Lewis acidbase interaction Purely covalent model Purely ionic model Partly ionic and covalent (polar covalent): electroneutrality principle

7 LN05-7 Bonding in transition metal complexes H&S 20.4 Various models have been used to discuss structure and bonding in relation to experimentally determined properties (structure, colour, magnetism). Valence bond theory (H&S 20.2) - not really used anymore; included for historical reasons Crystal field theory (H&S 20.3) - Conceptually simple and has some predictive power; but, based on dubious premise that there are NO covalent interactions between a metal and its ligands Molecular orbital theory (H&S 20.4) - The most complex, but most complete model. We will simply (but thoroughly) analyze them and focus on particular part of them (i.e. MOs that are based on the metal d orbitals) Ligand field theory (H&S 20.5) - Related to crystal field theory but with additional parameters ( fudge factors ) included to represent metal-ligand covalent bonding

8 LN05-8 Atomic orbitals H&S 1.6 3D representations of common atomic orbitals. Know these! 3d yz 3d xy 3d xz 3d z2 3d x2-y2

9 LN05-9 Molecular orbital diagram for octahedral complexesh&s 20.4

10 LN05-10 Octahedral MOs: a closer look M-L antibonding (σ*) orbitals H&S 20.4 e g * t 2g Non-bonding orbitals

11 LN05-11 MO theory: tetrahedral complexes H&S 20.4 y z x

12 LN05-12 MO theory: square planar complexes H&S 20.4 y z x

13 LN05-13 Qualitative comparison of MO diagrams

14 LN05-14 The 18 electron rule

15 LN05-15 The 18 electron rule H&S 20.4 & 24.3 Stable transition metal complexes tend to have a total of 18 valence electrons associated with the transition metal. Exceptions: 1. complexes of early transition metals (not enough d-electrons) 2. Late transition metals (too many d-electrons), particularly with weak sigma donors (more on this in a bit) 3. Square planar complexes these obey the 16 electron rule Note that we use the ionic model of M-L bonding, where formally anionic ligands are twoelectron donors (this is the model where we emphasize polar nature of the M-L bond). There is an alternative model for counting (mentioned in 24.3) in which M-L bonds are broken homolytically. We don t use this (but it s valid).

16 LN05-16 Pi bonding in Octahedral complexes H&S 20.4 t 2g a 1g + e g + t 1u a 1g + e g + t 1u As before: L is a σ donor L is a π donor (and still a σ donor)

17 LN05-17 Pi bonding in Octahedral complexes H&S 20.4 Recall that, in the absence of π orbitals on the ligands, t 2g molecular orbitals are nonbonding and are just the dxy, dxz, and dyz orbitals from M. When the L have π donor orbitals, the t 2g become t2g* - i.e. M-L (π) antibonding: t 2g (no ligand π orbitals) t 2g * (ligand π donor orbitals)

18 LN05-18 Pi bonding in Octahedral complexes H&S 20.4 t 2g a 1g + e g + t 1u a 1g + e g + t 1u As before: L is a σ donor L is a π acceptor (and still a σ donor)

19 LN05-19 Pi bonding in Octahedral complexes H&S 20.4 Recall that, in the absence of π orbitals on the ligands, t 2g orbitals are non-bonding and are just the dxy, dxz, and dyz orbitals from M. When the L have π donor orbitals, the t 2g become t2g* - i.e. M-L (π) antibonding: t 2g (no ligand π orbitals) t 2g (ligand π acceptor orbitals) A ligand group π-acceptor orbital:

20 LN05-20 Bonding in organometallic pi complexes So far we have discussed ligands which can be pi donors, pi acceptors, or neither but all (so far) are sigma donors in which the donor orbital is a lone pair on the ligand: BUT we have also seen a few ligands that don t appear to have a sigma donor orbital: 2 -ethylene 6 -benzene 5 -cyclopentadienide How do we understand metal-ligand bonding in pi complexes? What is the correct way to think about these ligands in terms of the metal s coordination number? the metal s valence electron count?

21 LN05-21 Bonding in organometallic pi complexes Metal-ligand bonding in ethylene complexes: the Dewar/Chatt/Duncanson model LUMO HOMO 1. sigma-type interaction involving ethylene HOMO. Ethylene is using is π bond (HOMO) as a σ donor

22 LN05-22 Bonding in organometallic pi complexes Metal-ligand bonding in ethylene complexes: the Dewar/Chatt/Duncanson model H&S Fig 24.5 LUMO HOMO 2. π-type interaction involving backbonding from a metal d-orbital to the ethylene LUMO.

23 LN05-23 Bonding in organometallic pi complexes There is a sigma donating part and a pi backbonding part to the bonding in metal-ethylene complexes. For the purposes of electron counting, we focus mainly on the sigma donating part: the pair of electrons in the sigma bond come from ethylene. Therefore ethylene contributes two electrons as a neutral ligand. In general, a two-carbon unit bonded to a metal in a pi complex is described as occupying one coordination site and contributes two electrons to the complex Ligand number of e- charge number of coord. sites 2 -ethylene butadiene benzene benzene cyclopentadienide 6-1 3

24 LN05-24 Bonding in organometallic pi complexes M-L bonding in benzene and Cp complexes is more complicated these rings have more π molecular orbitals. For these ligands we can focus on the electronic and structural similarities to traditional σ donor ligands: both benzene and Cp- are best described as occupying three mutually cis L type positions:

25 LN05-25 Bonding in organometallic pi complexes Bis( 6 -benzene)chromium(0) Bis( 5 -cyclopentiadienyl)iron(ii) ferrocene

26 LN05-26 Representing pi complexes Pseudo-6-coordinate (NOT 2-coordinate) 12-coordinate 6-coordinate NO OK but harder to draw Pseudo-6-coordinate (NOT 2-coordinate) 10-coordinate NO 6-coordinate OK but harder to draw

27 LN05-27 Spectrochemical series H&S 20.4 The spectrochemical series is an empirical ordering of ligands in terms of their effect on Δ O : I- < Br- < Cl- < F- < OH- < H 2 O < NH 3 < en < bipy < phen < CN- < CO The parameter Δ O is an important determinant of the overall electronic structure, and hence properties (colour, magnetism, reactivity) of transition metal complexes.

28 LN05-28 High spin vs low spin: octahedral H&S 20.1 Orbitals are populated based on Hunds rule d 0 -d 3 t 2g e g * d 4 -d 7 e g * e g * t 2g t 2g LOW SPIN HIGH SPIN e g * d 8 -d 10 t 2g

29 LN05-29 High spin vs low spin: octahedral The spetrochemical series allows for general predictions of preferences for HS vs LS configurations, particularly when comparing two complexes with different ligands. Generally the spectrochemical series cannot be used to absolutely determine the HS/LS preference of one compound, except for ligands at either end of the series.

30 LN05-30 High spin vs low spin: other geometries H&S 20.1 In general Δ t tends to be about half the magnitude of Δ o (assuming equivalent ligands etc.) As a result, tetrahedral complexes are nearly always high spin when there is a choice (d 3 -d 6 ) Only populate this orbital if necessary (> 8 d electrons) Within these for orbitals, use high spin confiugurations..

31 LN05-31 High spin vs low spin: other geometries H&S 20.1 Only populate this orbital if necessary (> 8 d electrons) Within these for orbitals, use high spin confiugurations..

32 LN05-32 Jahn-Teller distortions H&S 20.3 Sometimes the structures of octahedral complexes deviate in subtle but important ways: regular octahedron: All Ni-O bonds 2.07 Å distorted octahedron: Red Ni-O bonds 1.95 Å Blue Ni-O bonds 2.38 Å Ni(II) = d 8 Cu(II) = d 9

33 LN05-33 Jahn-Teller distortions H&S 20.3 The Jahn-Teller theorem: Electronically degenerate states are susceptible to structural changes which remove the degeneracy Cu(II) = d 9 or Regular octahedron Distorted octahedron

34 LN05-34 Jahn-Teller distortions H&S 20.3 Distortions can be elongation of z-axis, or compression:

35 LN05-35 Jahn-Teller distortions H&S 20.3 Distortions can be predicted for the following d-electron counts in octahedral complexes: d 1 d 2 d 4 (HS and LS) d 5 (LS) d 6 (HS) d 7 (HS and LS) d 9 Consequences of Jahn-Teller distortions: 1. Structures 2. spectroscopy (see later) 3. Reactivity (see later)