Lecture 5 Part 1 CH202 CH324 PDF

LECTURE 5 PART 1 TOPIC 5: Introduction to other important concepts and new types of complex continued What is the rationale behind the trans effect? ligands trans to each other share a dx2-y2 orbital; hence the donor-acceptor ability of one ligand will affect the...

LECTURE 5 PART 1 TOPIC 5: Introduction to other important concepts and new types of complex continued What is the rationale behind the trans effect? ligands trans to each other share a dx2-y2 orbital; hence the donor-acceptor ability of one ligand will affect the other L* If ligand D (donor) is a strong electron σ-donor then due to inter-electron repulsion this will assist the dissociation of the other L* ligand. D This is called a dissociative mechanism (dx2-y2) L* that is, D-M bond is strong therefore M-L* bond is weak so L* dissociates (breaks away) from the metal D  when L = a strong s donor, substitution occurs through a dissociative mechanism But there is an alternative mechanism the presence of a strong p acceptor (A) will remove electron density from the orbital lobe trans to it, making it prone to attack by other ligands at this site this is termed an associative mechanism (most common) d+ L* Nu - C N and H2C CH2 (ethene) are strong p - acceptors A A Nu = nucleophile Key points of the Trans Effect: (1) it is likely that both s and p effects contribute to the order of ligands in trans effect series (2) trans effect is predominately a kinetic phenomenon, that is, one isomer preferentially forms before another, irrespective of whether isomer is the thermodynamic (energetically preferred) product or not Note cis-platin is less stable than trans-platin but it forms first in reactions starting from PtCl42- p-Acceptor complexes: a basis for organometallic complexes To date we have considered the bonding in transition metal complexes to be governed by electrostatic interactions between a metal cation and ligand (L) electrons Represent simply as Metaln+ : Ligand if L = Cl ; the bond would be Mn+ Cl Note: remember ligands can be neutral (no charge) or anionic (with a negative charge) Good orbital overlap – efficient bonding a strong bond is one with efficient overlap between s (sigma) orbitals, for example, between a dz2 (metal) and an sp3- hybrid orbital from a ligand (such as the lone pair in ammonia NH3) carbon monoxide (CO) is only a weak s donor, that is, it is a poor Lewis base (compared to such ligands as halides, X- ) demonstrated in reactions with Lewis acid, BF3 memory aid: Lewis acids have an empty orbital to accept electrons so A for acid; A for acceptor  - - an additional complication with CO trying to act as a ligand: in the octahedral complex Mo(CO)6 [Mo(0)] the metal has 12 extra electrons above its normal electronic configuration of 5s14d5 It accommodates these extra electrons through ‘backbonding’ The important concept of Backbonding (p.81) The bonding in a transition metal carbonyl (M-CO) complex involves two components: 1. s (sigma) component Donation of a lone pair on C to a vacant metal s orbital M + C O: M C O: dz2  2. p component Back donation from a filled metal d orbital to a p* C-O antibonding orbital M + :C O: M :C O: dxz (or dxy) px-p*(or py- p*) s and p bonding components act in unison with each other, and collectively the bonding is termed synergic excellent overlap occurs between the metal and carbon monoxide p* = an antibonding orbital Empirically can represent bonding as below: complexes containing this type of bonding are called p- acid (as in Lewis acid; but here accepts electrons via a p interaction) or p-acceptor complexes CO stretching frequencies in Infra-red spectra confirm a resonance hybrid intermediate between a triple bond and double bond for the metal bound CO ligand the frequency is a measure of the relative bond length, that is, the higher the frequency the shorter and stronger the bond Typical Infra-red CO Stretching Frequencies as predicted the C-O bonds are longer and weaker in the metal carbonyl complex due to the population of electrons in an antibonding (p*) orbital (via backbonding) Electron counting and the 18-electron rule in Organo- Transition Metal Chemistry p.91 many low oxidation state organotransition metal complexes (in particular metal carbonyls) have 18 electrons in the valence shell of metal (noble gas stability, for example, Ar 1s2 2s2 2p6 3s2 3p6) compare with octet (8) rule in main group chemistry, that is the quest to have a full valence shell of electrons in organotransition metal complexes these 9 valence atomic bonding orbitals are available on the metal 1 x s; 5 x d; 3 x p each of contains 2 electrons = 18 valence electrons in total Therefore the formation of 18 electron complexes can fill up all the available orbitals, to become stable like a noble gas  Covalent Orbital Picture in p-Acceptor Complexes t1u 6 anti-bonding a1g orbitals eg oct t2g 9 bonding eg orbitals t1u a1g Orbitals are either strongly bonding (bottom set) or strongly antibonding (top set) 18-electron configuration is thus very stable for strong crystal field ligands so !8-electron rule applies in particular to these p-acceptor complexes symmetry label a1g means singly degenerate with centre of symmetry t1u means triply degenerate and unsymmetrical to inversion  END OF LECTURE 5 PART 1 Essential Learning: The concept of backbonding LECTURE 5 PART 2 TOPIC 5 Continued Electron Counting Rules Always consider the metal is in zero oxidation state (that is, like the element, retaining its full complement of valence electrons) also consider the electrons formally donated by the ligand For carbon monoxide :CºO donates 2 electrons when terminally attached to the metal For example: consider Mo(CO)6 Mo electrons (5s14d5) = 1 + 5 = 6 CO OC CO CO electrons = 6x2 = 12 Mo total = 18 OC CO CO Mo(CO)6 obeys the 18-electron rule  Consider part of the 3d series of binary carbonyl complexes Transition metal V Cr Mn Fe Co Ni electronic configuration s2d3 s1d5 s2d5 s2d6 s2d7 s2d8 no. of metal electrons 5 6 7 8 9 10 electrons required 13 12 11 10 9 8 compound/fragment formed V(CO)6 Cr(CO)6 Mn(CO)5 Fe(CO)5 Co(CO)4 Ni(CO)4 Ni(CO)4, Fe(CO)5 and Cr(CO)6 are straightforward having the correct number of CO ligands to completely fill the valence orbitals these three carbonyl complexes obey 18-electron rule Vanadium hexacarbonyl V(CO)6 is the odd one out it cannot dimerise for steric reasons each V atom would have 6 CO ligands around it, therefore dimerisation would lead to a coordination number of 7 (too sterically crowded) thus V(CO)6 stays as a 17-electron radical species in violation of the 18-electron rule V(CO)6 is less stable than the carbonyl complexes, which obey the 18-electron rule, decomposing at 70°C. However, it will readily accept an electron to generate an 18-electron anion: Na + V(CO)6 ¾® Na+ + [V(CO)6] Note : Sodium also obeys the octet rule in the product of this reaction Manganese and Cobalt Carbonyls Mn(CO)5 and Co(CO)4 due to the odd number of metal electrons available (7 for Mn; 9 for Co), the monomers each contain only 17 valence electrons to attain the desired 18 electron configuration these monomeric fragments dimerise so that each metal can share the 2 electrons in the metal-metal bond their respective formulae are Mn2(CO)10 and Co2(CO)8  stream of CO Nipowder Ni(CO)4 Δ Ni is still in the zero Ni(0) oxidation state in this compound like the pure metal This was the first recognised metal carbonyl complex 1884 Ludwig Mond (never got a degree by exam!)  “Mond gave wings to Ni!” Lord Kelvin (Glasgow University) started university aged 10! Original name William Thomson Preparation of metal carbonyls 1. by direct action on metal element: but only nickel tetracarbonyl and iron pentacarbonyl can be prepared in this way M + x CO → M(CO)x Ni complex Ni(CO)4 can be prepared under mild conditions (30°C, 1 atmosphere pressure) potent carcinogen whereas the iron complex Fe(CO)5 requires harsher conditions (200°C, 200 atmospheres) 2. by reductive carbonylation: Carbon monoxide (CO) must be present in large excess Examples, Mo(CO)6, Ru3(CO)12, [Ti(CO)6]2-  Properties and structures of metal carbonyls generally these metal compounds show more organic characteristics than inorganic ones they are classified as organometallic compounds most metal carbonyls are volatile liquids or solids, and can be distilled or sublimed at modest temperatures – a standard purification method Ni(CO)4 liquid (r.t.) mp. –19°C b.p. 42°C Fe(CO)5 liquid (r.t.) mp. –20°C b.p. 103°C Mo(CO)6 solid (r.t.) mp. 150°C sublimes about 80°C  Mononuclear carbonyls tend to have simple structures axial ↓ ← equatorial Molecular structures with only weak intermolecular forces between them  Exercise Example Exercise Example 3 5 (i) Explain in detail the structure of, and bonding within, dimanganese decacarbonyl. Include in your answer, a description of the different interactions involved, the coordination number and oxidation state of the metal, a drawing of the structure and using electron counting, comment on whether this complex obeys the 18-electron rule. 8 marks  END OF LECTURE 5 PART 2 Essential Skills and Learning: To be competent in electron counting and knowledgeable on transition metal carbonyl structures