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The 18-electron rule and its exceptions (1) Octahedral complexes à can be split into 3 categories: category A B C number of electrons 18 12-18 12-22 description 18-electron rule obeyed 18-electrons not exceeded 18-electron rule disobeyed A. Those that obey the 18 electron rule Complexes with strong π-acceptor ligands (e.g. [V(CO)6]-, [Cr(CO)6], [W(CO)6], [Mn(CO)6]+) - t2g strongly bonding ∴filled - eg strongly antibonding (due to synergic bonding) ∴empty t 1u* 4p (t1u ) a1g* 4s (a1g ) strongly anti-bonding (empty) t2g* π* orbitals of CO (t2g) e g* large ∆O 3d (e g + t2g) strongly bonding (filled) t2g (a1g + t1u + e g) eg Cr 6 CO t1u a1g - Complexes of strong π-acceptor ligands à tend to obey the 18-electron rule irrespective of their coordination number (e.g. [Fe(CO)5], [Fe(CO)4]2-, [Ni(PPh3)4]). - Note: d8 and d10 configurations are exceptions to this rule (see later). B. Octahedral complexes with 12-18 electrons 2nd and 3 rd row TM complexes (high in the spectrochemical series of metal ions) with σ-donor or π-donor ligands (low to medium in the spectrochemical series) - t2g non-bonding or weakly anti-bonding (because the ligands are either σ-donors or π-donors) ∴ t2g can contain from 0 to 6 electrons - eg fairly strongly antibonding (because 2nd/3rd row TMs bond more effectively to the ligands)∴eg empty e.g. [ZrF6]2- (Zr4+, d0, 12 e), [PtF 6]2- (Pt4+, d6, 18 e), [OsCl6]2- (Os4+, d4, 16 e), [WMe6] (W6+, d0, 12 e), [Zr(OH2)6]3+ (Zr3+, d1, 13 e) Octahedral, σ-donor ligands (e.g. [WMe6]) t1u* 4p (t1u) a1g* eg* is reasonably antibonding for a 2nd or 3rd row transition metal 4s (a1g ) eg * moderate ∆o for 2nd or 3rd row transition metal ∆O t 2g non-bonding 3d (e g + t2g) t2g (a 1g + t1u + eg) eg W t 1u a 1g 6 Me C. Octahedral complexes with 12-22 electrons 1st row TM complexes (low in the spectrochemical series of metal ions) with σ-donor or π-donor ligands (low to medium in the spectrochemical series) = 1st row transition metal coordination complexes {e.g. [TiF 6]2- (Ti4+, d0, 12 e), [Co(NH3)6]3+ (Co3+, d6, 18 e), [Cu(OH2)6]2+ (Cu2+, d9, 21 e)} - t2g non-bonding or weakly anti-bonding (because the ligands are either σ-donors or π-donors) ∴t2g can contain from 0 to 6 electrons - eg only weakly antibonding (because 1st row TMs don’t bond as effectively to the ligands) ∴eg can contain from 0 to 4 electrons Octahedral, π-donor ligands (e.g. [Cu(OH2)6]2+) t1u* 4p (t1u ) a1g * eg* is only moderately antibonding for a 1st row transition metal 4s (a1g) eg* small ∆o t2g only weakly anti-bonding 3d (eg + t 2g ) t2g * π orbitals (t2g ) t2g (a1g + t1u + eg) eg Cu t1u a1g 6 OH2 Tetrahedral Complexes • Tetrahedral complexes cannot exceed 18 electrons because there are no low lying MOs that can be filled to obtain tetrahedral complexes with >18 electrons. In addition, a transition metal complex with the maximum of 10 d-electrons, will receive 8 electrons from the ligands à a total of 18 electrons. • ∆t is small (~4/9 ∆o), so there is no particular preference for the e or t2 orbitals to be filled (can have 8-18 electrons) – similar to class C octahedral complexes. Tetrahedral - e.g. [Ni(PPh3)4] (Ni0, d10, 18-electron complex) t2* 4p (t1u) a1* 4s (a1g) t2* ∆t ∆t ~ 4/9 ∆o 3d (eg + t2g) e (a1g + t 1u + eg) a1 4 PPh3 Ni t2 (2) Square Planar complexes (d8, 16 electrons) d x2-y2 • d8-metals with 4 ligands, so 16-electron complexes large ∆E • common with metals and ligands high in the spectrochemical series d xy d z2 I I II II III - Rh , Ir , Pd , Pt , Au almost always square planar All are very common oxidation states except AuIII which is quite oxidising. d xz, d yz - NiII can be square planar, but only with strong π-acceptor ligands (because 1st row TMs are lower in the spectrochemical series than 2nd or 3rd row TMs). - CoI is almost never square planar because it is a first row transition metal in a low oxidation state (very low in the spectrochemical series for metals). - d8 CuIII and AgIII complexes are extremely rare and highly oxidising. - d8 Ru0 and Os 0 are not square planar (2nd or 3rd row TMs but low in the spectrochemical series of metal ions because their oxidation state is zero). - d9 complexes are sometimes square planar (e.g. [CuII(py)4]2+) – this may be considered an extreme form of Jahn-Teller distortion. EXAMPLES: Ph3P Ph3P Ph3P OC RhI Ir I PPh3 Cl CO Cl Ph 2 P P Ph 2 Cl Cl PtII PtII NC NC Cl Cl Cl Cl 2- CN CN NiII Cl Cl 2- 2- NiII Cl Cl CO - CO CN OC Fe CO CO CO OC Mn CO CO NC CO OC Co I CN 2- (3) Linear complexes (d10, 14 electrons) p small ∆E • d -metals with 2 ligands, so 14-electron complexes 10 s • Common for AgI, AuI and HgII small ∆E • Less common for Cu , Zn and Cd I II II d Explanation: • For d 10 complexes, there is a relatively small energy difference between the d, s and p orbitals (e.g. 5d, 6s and 6p for AuI). • This permits extensive hybridization between the dz2, s and p z orbitals as shown below: Step 1 Step 2 + filled d z2 + + empty s filled sd-hybrid empty sd-hybrid + Overall: filled dz2 empty sd-hybrid + empty p filled sd-hybrid empty + + + empty s empty empty p empty empty orbitals suitable for forming a pair of linear covalent bonds • More common for group 11 (Cu, Ag, Au) than group 12 (Zn, Cd, Hg) because the energy difference between the d, s and p-orbitals is smaller for group 11. • More common for the heavier elements (AgI, AuI, HgII). • Examples: [Ag(CN)2]-, [Ag(NH3)2]+, [Cu(NH3 )2]+, [(R3P)AuCl], [HgMe2 ], [CdMe2], [ZnMe2 ] • However, there are also lots of tetrahedral complexes of AgI, AuI, CuI, ZnII, Cd II and HgII (e.g. 14 e- linear [(R3P)AuCl] + 2 PR3 ßà 18 e- tetrahedral [(R3P)3AuCl]). Distribution of coordination numbers for crystallographically characterized CuI, AgI, and AuI compounds as found in the Cambridge Structural Database (2003). S. Alvarez et al., JACS, 2004, 1465-1477 (4) Steric Effects and Early Transition Metal Compounds • Steric effects can produce low-coordinate (not many ligands) complexes which often have <18 electrons. Me3Si SiMe 3 Si Me3Si ScIII N N Me3Si SiMe3 N Cl SiMe 3 MII SiMe3 Si SiMe3 d0 M = Cr2+ M = Mn 2+ M = Fe2+ 6 electrons SiMe3 SiMe 3 Me3Si M = Sc3+ SiMe3 d4 d5 d6 10 electrons 11 electrons 12 electrons • For early transition metals (e.g. with d0 metals) it is often not possible to fit the number of ligands necessary to reach 18 electrons around the metal. CO Me Me Me WVI Me OC Me OC V0 CO CO Me CO W6+ d0 12 electrons V0 d5 17 electrons Sc III Cl PMe3 Sc3+ d 0 16 electrons (5) Strong oxidants or reductants • Many 18 electron complexes can be reduced or oxidised to give 17 or 19 electron complexes. Such compounds are often good oxidising or reducing agents (i.e. they want to get back to being 18-electron compounds). - eFeII FeII, FeIII + e- Ferrocene d6, 18 electrons Ferrocenium cation FeIII, d5, 17 electrons Orange, diamagnetic Perfectly Happy ! Dark blue, paramagnetic Oxidant + eCoIII CoII - e- Cobaltocenium cation CoIII , d6, 18 electrons Co , d7, 19 electrons Yellow, diamagnetic Dark purple, paramagnetic Perfectly Happy ! Strong Reductant Cobaltocene II