Download
1 / 4
40 likes | 259 Views
Explore the bonding, reactivity, and IR spectroscopy of transition metal carbonyl complexes, including MO diagrams and coordination effects. Learn about metal carbonyl ligands and their electron-donating and -accepting properties.
E N D
Lecture 31 Structure and reactivity of organometallic compounds 1) Bonding and reactivity of transition metal carbonyl complexes • Transition metal carbonyl complexes (metal carbonyls) are among the most well studies organometallic compounds (compounds with M-C bonds). Almost all transition metals form carbonyls. Many of them comply with the 18 electron rule. V(CO)6, Cr(CO)6, Mn2(CO)10, Fe(CO)5, Co2(CO)8, Ni(CO)4 • Carbonyl ligand is considered as a week 2-electron donor (s-donor) and very strong p-acceptor. • Because of the p-acceptor ability of CO a number of stable anionic metal carbonyls exists: Ti(CO)62-, V(CO)6-, Cr(CO)42-, Mn(CO)5-, Fe(CO)42-, Co(CO)4- • Some of them can be protonated to form metal carbonyl hydrides (HCo(CO)4, H2Fe(CO)4, HMn(CO)5 etc.) which often behave as strong protic acids in aqueous solutions. • Since CO is a weak s-donor, cationic metal carbonyls are rare and very electrophilic: Mn(CO)6+, Fe(CO)62+, Co(CO)5+, Ir(CO)63+, Hg(CO)22+ • There is a variety of polynuclear cluster carbonyl compounds containing M-M bonds. Analysis of the structure of CO helps understand the structure and reactivity of metal carbonyls
2) MO diagram of carbon(II) oxide CO (C∞v) HOMO LUMO 5s 4s
3) Bonding and IR spectroscopy of transition metal carbonyls • Electrons of HOMO of CO ligand are donated to an empty orbital of the metal (M-C s-bonding) • Electrons from one of the filled d-orbitals of the metal are back-donated to the LUMO of CO (M-C p-backbonding) • A good measure of the balance of the s-donation and p-back-donation is the IR stretching frequency of coordinated CO. • In free CO, n(CO)=2143 cm‑1 ; l(C-O) = 1.128Å • In terminal carbonylsn(CO) ranges from 1750 to 2278 cm-1 and l(C-O) from 1.12 to 1.18 Å C O C O
4) Coordination can make CO electron-poor or electron-rich • Donation to the metal or back-bonding: what dominates? The p-backbonding between CO and a very electron-rich metal center makes the ligand also electron-rich: • Vice versa, at electron-poor metal centers CO is electron-poor: or: Co2(CO)8 + NaOH NaCo(CO)4 + Na2CO3
