- an approach to bonding that is useful for making estimates of E of orbitals in coordination complexes

Transcription

1 10.4 Angular Overlap - an approach to bonding that is useful for making estimates of E of orbitals in coordination complexes - estimate the strength of interaction b/w ligand orbitals & metal d orbitals based on the overlap - Why this term?? : amount of overlap depends strongly on the angular arrangement of the M orbitals & the angles of ligand Sigma-Donor Interactions - Strongest interaction: b/w M-d z2 & ligand-p reference interaction for other σ-interaction bonding orbital: larger ligand component decreased MO E by e σ antibonding orbital: larger M component increased MO E by e σ Fig Sigma Donor Interactions - Table 10.10: changes in orbital E due to other interactions b/w M d orbitals & ligand orbitals need to justify the # qualitatively 1

2 Sigma Donor Interactions - Example on p.390) [M(NH 3 ) 6 ] n+, octahedral only σ-interaction (NH 3 no π orbital) donor orbital on NH 3 p z p x, p y : used in bonding w/ H - calculation of the orbital E in a complex,, 1) d orbital: of # for the appropriate ligands in the vertical column 2) ligand orbital: of # for all d orbitals in the horizontal row Fig Sigma Donor Interactions - Metal d orbitals - 1) d z2 : strongest w/ 1 & 6 raise E by e σ : weak w/ 2, 3, 4, 5 raise E by 1/4 e σ increase by total 3 e σ 2) d x2-y2 : position 1, 6 no interaction position 2, 3, 4, 5 raise E by 3/4 e σ increased by total 3 e σ 3) d xy, d xz, d yz : no interaction w/ ligands remains unchanged - Ligand orbitals - 1) ligand 1, 6 w/ d z2 : lowered by e σ w/ other d: no interaction 2) ligands 2, 3, 4, 5 w/ d z2 : lowered by 1/4 e σ w/ d x2-y2 : lowered by 3/4 e σ Each ligand orbital is lowered by e σ!! 2

3 Sigma Donor Interactions - Fig.10.22: resulting E pattern decribe how the M complex is stabilized (X 2) d orbitals of M: increase E (X 3) d orbitals of M: remains unchanged (X 6) ligand orbitals: lower E * net stabilization: 12 e σ for the bonding pair Fig Pi-Acceptor Interactions -CO, CN -, PR 3 (phosphine): π acceptors w/ empty orbitals - Strongest π interaction: b/w d xz of M & π* of ligand - Fig.10.23: -e π < e σ π overlap is weaker than σ-overlap 3

4 Pi-Acceptor Interactions - Table 10.11: Pi interaction 1) M d orbital d z2, d x2-y2 : no π interaction at all (1-6) d xy, d xy, d yz : stabilized by 4e π 2) ligand orbitals raise E - d e - occupy the bonding MO w/ - 4e π - Fig : σ-donor & π-acceptor ligands o = 3e σ + 4e π Pi-Acceptor Interactions - Example on p. 394) [M(CN) 6 ] n- M s d xy, d xz, d yz lowered by 4e π X 6 ligands increased by 2e π o (t 2g -e g split) = 3e σ + 4e π 4

5 Pi-Donor Interactions - Interaction b/w occupied ligand p, d or π-orbitals & metal d-orbitals - Similar to π-acceptor case except the reversed E b/w M & ligands M d orbitals: E ligand π-orbitals: E - Fig.10.25: - Fig.10.26: Pi-Donor Interactions - Example on p.395) [MX 6 ] n- halide ion: σ-interaction donate e - via p y : π-interaction donate e - via p x, p z both σ- & π-donor d z2 & d x2-y2 orbitals: no π-interaction no effect on the E of these d orbitals d xy, d xz and d yz orbitals: π-interaction w/ four ligands d xy w/ 2, 3, 4, 5 by total 4e π 5

6 Types of Ligands and the Spectrochemical Series - Ligands can be classified by their donor & acceptor capabilities -NH 3 : σ-donors only, no π-interactions simple bonding (Fig.10.4) - (ligand field split): depends on the relative E of M & ligand the degree of overlap - (en) > (NH 3 ) strongest effect = order of proton basicity - Ligand field strength of halide ions F - > Cl - > Br - > I - = order of proton basicity - Ligands w/ occupied p orbitals: π-donors d (+ σ-bonding b e - ) (section ) most halide complexes: high-spin configuration - ex) the order of other examples: H 2 O > F - > RCO 2- > OH - > Cl - > Br - > I Types of Ligands and the Spectrochemical Series -Ligands w/ π* or d orbitals: π back-bonding is possible becomes π acceptors ex) CO, CN - > phenanthroline > NO 2- > NCS - - When these lists are combined,, spectrochemical series (from strong π-acceptor to strong π-donor) CO, CN - > phen > NO 2- > en > NH 3 > NCS - > H 2 O > F - > RCO 2- > OH - > Cl - > Br - > I - Low spin High spin Strong field Weak field Large donor only Small π acceptors π donors Strong interactions w/ TM s orbitals 6

7 Magnitudes of e σ, e π, and Charge on metal - Changing the ligands or the M affects the magnitudes of e σ, e π, result in a change in the # unpaired e - -ex) H 2 O: weak field ligand w/ Co 2+ in O h [Co(H 2 O) 6 ] 2+, high-spin, 3 unpaired e - w/ Co 3+ in O h [Co(H 2 O) 6 ] 3+, low-spin, no unpaired e - Fig Magnitudes of e σ, e π, and Different Ligands - ex) [Fe(H 2 O) 6 ] 3+ vs [Fe(CN) 6 ] 3+ high-spin low-spin balance among, Π c, Π e determine high- or low-spin - Tetrahedral complex: t is small low-spin tetrahedral complex is unlikely!! instead,, low-spin octahedral complex!! - Table 10.12, Table 10.13: angular overlap parameters observed trends 1) e σ > e π σ interaction: more direct orbital overlap b/w nuclei π interaction: small overlap, no direct toward each other 2) σ, π as size bond length overlap as EN of halide ion the pull 7

8 Magnitudes of e σ, e π, and - Table 10.12: ligands listed in the spectrochemical series order ex) for Cr 3+ CN - is listed first highest in the spectrochemical series π-acceptor (e π is - ) en : affected only by e π values (σ-donor ability) NH 3 halide ions: at the bottom of the series π-donor & σ-donor Special Cases - The angular overlap model: can describe the electronic E of complexes w/ diff. shapes or w/ combinations of diff. ligands : estimate the magnitudes of e g & e π w/ diff. ligands -ex)[co(nh 3 ) 4 Cl 2 ] + : low-spin magnetic property does not depend on o but, o effect on the visible spectrum (Chapter 11) - Helpful to compare E of diff. geometries ex) for 4-coordinate complex tetrahedral or sqaure-planar?? (section 10.6) 10.5 The John-Teller Effect - There can not be unequal occupation of orbitals w/ identical E. molecules distorted no longer degenerate orbitals!! ex) Cu(Ⅱ), d 9, O h - Fig e - in the two e g w/o J-T effect J-T effect slight shape change of a complex change in E of the orbitals distortion 1) elongation (along one axis) or 2) compression (along one axis) in O h,, e g *: direct toward ligands larger effect on E level t 2g : No direct overlap small effect 8

9 10.5 The John-Teller Effect - strong J-T effect: when e g * are unequally occupied weak J-T effect: when t 2g are unequally occupied - Expected J-T effect - ex) significant J-T effects, Cr(Ⅱ) (d 4 ) high-spin Mn(Ⅲ) (d 4 ) Cu(Ⅱ) (d 9 ) Ni(Ⅲ) )(d 7 ) low-spin Co(Ⅱ) (d 7 ) -low-spin Cr(Ⅱ): distorted from O h to D 4h : 2 absorption bands one in Vis one in near IR : if pure O h only one d-d transition (Chapter 11) 10.5 The John-Teller Effect -ex)-cu(Ⅱ): significant J-T effects elongation : affect equilibrium constant : [Cu(NH 3 ) 4 ] 2+ distorted O h w/ 2 H 2 O at greater distance putting the 5 th & 6 th NH 3 is difficult see the formation constant the bond distance of two axial position: longer!! smaller equilibrium constant for these positions!! Cu(Ⅱ) complexes prefer square-planar geometry!! [Cu(H 2 O) 6 ] 2+ + NH 3 [Cu(NH 3 )(H 2 O) 5 ] 2+ + H 2 O K 1 = 20,000 [Cu(NH 3 )(H 2 O) 5 ] 2+ + NH 3 [Cu(NH 3 ) 2 (H 2 O) 4 ] 2+ + H 2 O K 2 = 4,000 [Cu(NH 3 ) 2 (H 2 O) 4 ] 2+ + NH 3 [Cu(NH 3 ) 3 (H 2 O) 3 ] 2+ + H 2 O K 3 = 1,000 [Cu(NH 3 ) 3 (H 2 O) 3 ] 2+ + NH 3 [Cu(NH 3 ) 4 (H 2 O) 2 ] 2+ + H 2 O K 4 = 200 [Cu(NH 3 ) 4 (H 2 O) 2 ] 2+ + NH 3 [Cu(NH 3 ) 5 (H 2 O)] 2+ + H 2 O K 5 = 0.3 [Cu(NH 3 )(H 2 O)] 2+ + NH 3 [Cu(NH 3 ) 6 ] 2+ + H 2 O K 6 = very small 9