Figure 1 M-O interaction modes. triple bond p x p y. resonance structures. double bond and lone pair. M d 0 -O 2- p z. σ-donor interaction

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Ann Randall
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1 Slf-Stud Problms / Eam Prparation In what situations is th valu of Δ oct difficult to idntif? o whn thr ar multipl lctronic transitions which giv ris to multipl paks in th uv-vis spctrum, for ampl [n(h 2 ) 6 ] 2+ has a d 5 ground stat configuration and has svn d-d bands in its UV-vis spctrum o whn thr is a mitur of diffrnt tps of ligands, and/or and/or π- accptor, this will also giv ris to multipl nrg lvls and transitions, w can diffrntiat btwn th Δ oct of individual ligand tps and for th whol compl o Δ oct can b takn as th maimium splitting of th da manifold o dtrmining Δ oct is difficult primntall whn th T-compl will not dissolv (is insolubl) prcluding masuring a uv-vis spctrum, or whn thr ar solvnt or contamination bands which obscur th d-d paks in th primntal spctrum, th transition can also b vr broad, in this cas taking th pak maimium dos not wll rprsnt th rang of transitions possibl, altrnativl thr ma b two transitions which combin to form a singl broad fatur, with th pak maimium now btwn th two transition maima, in ths cass carring out a calculation and computing th spctrum aids intrprtation Wh is it as for th oo ligand 2- to convrt btwn a doubl and tripl bond with a mtal? o Th diagram for an oo ligand shows that thr is and Fs. has th configuration 1s 2 2s 2 2p 4. If w considr onl th pas, 2 ntr into th thr ar 2 lft to shar ovr th two orbitals, if on lctron is shard with mtal for ach thn w hav a vr covalnt tripl bond, Figur 1. o Howvr if th 2 pair up in on of th Fs thn w hav a doubl bond with strong donating charactr from th oo ligand. Tchnicall in this latr cas th mpt π-orbital on 2- could act as an lctron accptor via back-donation, Figur 1. o If th ligand is chargd such as 2 thn all of th pas on th ar filld and ar unabl to shar with th mtal, th oo ligand is thn a pur donor ligand, this would b favourd b mtals in high oidation stats and low numbrs of d lctrons, Figur 1. o Th ralit of an bonding situation will b a rsonanc btwn all th options shown in Figur 1, som of which ar favourd mor than othrs. Somtims th lctrons shard b th mtal ar assignd to th ligand, and sharing maks th oo ligand formall 2. In ths s th das ar highr in nrg and so th shard lctrons will spnd mor tim on th ligand. It is important to b clar about whr th tr com from! tripl bond rsonanc structurs doubl bond and lon pair π-accptor d 0-2- Figur 1 - mods 1

2 2 can coordinat to th th T nd on or sid on, what tp of ligand is 2 in ach orintation? o nd on 2 is lik C and is a π-accptor ligand, howvr bcaus th π* Fs ar partiall occupid 2 is a wak π-accptor. Also bcaus ths orbitals ar partiall occupid 2 could potntiall act as a wak. Howvr, is lctrongativ so this is lss favourabl. o Considring th s alon w might think nd on 2 could b a, through th π-s of 2, howvr as is vr lctrongativ compard to a mtal th π-s on 2 ar dpr in nrg and with th das is not favourd. o sid on 2 is lik an alkn and is a δ-accptor ligand, howvr bcaus th π* Fs ar partiall occupid 2 is a wak δ-accptor. Also bcaus ths orbitals ar partiall occupid 2 could potntiall act as a wak δ-donor. Howvr, is lctrongativ so this is lss favourabl. Draw out th possibl da- v ligand s and labl ach componnt, Figur 2 ligand F -L π ligand π -accptor F -L π ligand π -accptor F -L δ ligand F -L π ligand π -accptor F -L π ligand π -accptor F -L δ ligand F -L σ ligand F -L σ Figur 2 th various ligand F and th -L tps Th following is information for our intrst o narl all T bind diogn. Diogn can bind as a nutral molcul, as an anion (suproid) or as th dianion (proid). Adding lctrons to th diagram of 2 is as bcaus in nutral 2 th H is a dgnrat st of π* orbitals with onl two lctrons. Adding 1 or 2 into an antibonding π* orbital rducs th bond ordr for ampl bond distancs (-) of 2, 2 and 2 2 ar 1.21, 1.33 and 1.49 Å rspctivl. o isolatd diogn has triplt charactr, this is changd to singlt on coordination to a T, whn 2 formall accpts 1 ( 2 1 ) and hibits - doubl bond charactr it is calld a suproo ligand. Whn 2 formall accpts 2 ( 2 2 ) and hibits - singl bond charactr it is calld a proo ligand. In both cass ogn is rducd whil th T is oidisd. o th binding of ogn is particularl intrsting in th contt of biolog, as moglobin and hmoglobin. In addition 2 has widsprad application in oidation catalsts, and is of particular intrst bcaus diogn is grn as it is frl availabl in th atmosphr. Howvr, thr ar problms as th ractivit is difficult to control (combustion occurs!), ractions can b difficult to initiat du to th triplt charactr of 2 and oftn an atom of ogn is rtaind following oidation. o X-ra diffraction can b problmatic bcaus th diogn is closl bound to th mtal th act location of th atoms ar difficult to locat du to th larg lctron dnsit of th mtal. ftn othr mthods ar mplod to confirm th structur, including IR and Raman studis of th - bond strtching frquncis and thortical calculations. In a compl th suproid is cm -1 and th proid 2 2 is cm -1. o 2 is oftn found as a bridging ligand, Figur 3 2

3 supr-oo Figur 3 diogn bound nd on or sid-on pr-oo Draw th rlvant s for 2 coordinatd to a T nd on (linar), clarl show what tp of ligand 2 is for this orintation, Figur 4 d 2-2 d 2 d d d 2 is a π-accptor ligand bcaus th π* Fs on th ligand ar intracting with th das C 4v part of th σ-framwork 1s and 2s not shown Figur 4 Th k s for 2- nd on Etra for prts! What happns to th diagram whn th 2 bnds awa from linar? o Diogn can bind nd-on, howvr th nd-on compl is not normall linar but is bnt, th nd on bnt conformation is gnrall favourd. o Whn a ligand of th tp X 2 intracts with a mtal, thr is a da-lπ*. If this is strong th X-X bond is rducd whil strngthning th -X, Figur 5a Thus thr is a subtl intrpla btwn th stabilit of ths two bonds and th gomtr favourd. o th nitrosl ligand (N) has on mor lctron than C and th 2 ligand which has two mor lctrons than C, can form linar or bnt bonds, dpnding on th tnt of with a T. C and N 2 which hav no lctrons in th π*fs prfr a linar nd on coordination,, Figur 5b o Whn th -X 2 bond bnds thr is a drop in smmtr, causing th s to no-longr b dgnrat and allowing miing. In particular th unoccupid a of th d 2 -Lσ* will b 3

4 abl to mi with th partiall occupid d -Lπ* dropping th nrg of th d - Lπ* slightl, Figur 5c. wakn - bond wak backbonding strngthn - bond strong backbonding (a) (b) C=both mpt N=on lctron 2 =two lctrons (c) C 4v C s + d 2-2 d 2 d d d a' a" Figur 5 bnt vs linar nd-on coordination dciding on an oidation stat can b compl onc ou undrstand th s, as th two approachs do not alwas match up! o N has 1 in th π*-fs (and th σ-f is countd as 2 donation) o whn N is linar, alignmnt with th π*-f mans back-donation can b maimisd, howvr in this cas N is assumd to b N + (isolctronic with C) and formall forms -N + (π*-f 0) whr N has donatd 1 to th (thus th mtal is rducd). o whn N is bnt, miing occurs, bcaus th orbitals miing ar dominatd b th das this miing is mphasisd, howvr th ligand N Fs mi as wll (forming a σ /π*-mid orbital). Th situation is compl and a calculation is gnrall th bst wa to procd. Howvr, formall in trms of Lwis structurs th N is assumd to b N (isolctronic with 2 ) forming + -N (σ /π*-mid orbital has 2) whr N has accptd 1 from th (thus th mtal is oidisd) o this illustrats that formal oidation stats ar just that a formalism usd b chmists bfor th undrstood thor. thor is compl and chmists nd a quick and as wa to think about T-ligand s, formal oidations stats hlp do this. I find it asist to not ask on thor to justif or inform on th othr. s hav littl to do with formal oidation stats and formal oidation stats hav littl to do with thor. Ths ar two was of approaching th sam problm, th mor information w hav, and th mor was w hav of thinking about a problm th bttr. 4

5 Draw th rlvant s for 2 coordinatd sid-on to a T, clarl show what tp of ligand 2 is for this orintation, Figur 6 d is a δ-ligand bcaus th π Fs on th ligand ar intracting with th das in a δ-pattrn d 2 d d d π*-accptor F ligand basd s C 2v Figur 6 Th k s for 2- sid-on What tp of ligand is H 2? Draw a diagram showing th -H 2 orbitals that will intract. Eplain th dichotom of H 2 bing a strongr fild ligand than 2. o H 2 is strong and wak ligand, hnc it is oftn classd as a ligand. o th polarisation of th 3 mans thr is a strong σ-ovrlap and maks H 2 a good σ- donor o in th following I will kp th C 2v notation for an isolatd watr molcul to nabl us to discuss th nrg lvls and orbital s rlativ to th watr molcul. In th compl th -ais changs and so (in trms of th compl) th orbital labls will chang. o th 1 has th corrct smmtr to bhav as F. Howvr, this F lis vn dpr in nrg than th 3 and has lctrons dlocalisd ovr th whol, thus th nrg match with th das is poor, and ovrlap dnsit with th mtal is rducd, this onl forms a wak. o th 1 (H) is a non-bonding pa and vr similar to th p in 2 and forms a strongr. o compard to 2 which has 2 strong s, H 2 has on poor F ( ) and on strongr F ( ), thus compard to 2- H 2 is a wakr, Figur 7. o crstal fild thor would suggst that as 2- is ngativl chargd and H 2 is nutral that 2- should b th strongr fild ligand, howvr mpirical vidnc of th spctrochmical sris shows th rvrs. 5

6 o in th spctrochmical sris 2- < H 2 this is bcaus 2- has strong π-donation via both pas raising th g * nrg and rducing Δ oct. H 2 is a wak this lowrs th g * nrg incrasing Δ oct compard to 2-. o th non-bonding natur of th 1 should man th top g * is shiftd a similar amount in 2. Howvr Δ oct is maimum span of th das, thus w ar concrnd with th smallst shift in th nrg of th g *, and so it is th 1 that is important for dtrmining Δ oct for H 2 (s th answr to th nt qustion for an nrg lvl diagram) H H Figur 7 2- and H 2 mods Etra for prts! Eplain using thor and diagrams wh cis-[cocl 2 (NH 3 ) 4 ] + is violt and trans-[cocl 2 (NH 3 ) 4 ] + is grn. o NH 3 is a ligand and Cl is a ligand o first work out th d lctron count and oidation stat of th mtal: Co is in group 9 which mans d 9 as nutral mtal, th 2Cl ligands ar -1 ach, th NH 3 is a 0 accptor, and thr is a positiv charg rmoving anothr lction thus this is Co(III) and th compl is d 6. o a violt colour of cis-[cocl 2 (NH 3 ) 4 ] + mans th compl absorbs in th llow rgion 570nm and a grn colour of trans-[cocl 2 (NH 3 ) 4 ] + mans that th compl absorbs in th rd rgion 680nm, thus trans-[cocl 2 (NH 3 ) 4 ] + has a smallr Δ oct than cis-[cocl 2 (NH 3 ) 4 ] + o bcaus ths ar a mid / compl thr will b multipl contributions to th spctra howvr th lowst nrg transition will b to das intracting with th ligands and this will dominat in dtrmining Δ oct o th rducs Δ oct b raising th g s, Figur 8 o howvr w nd to considr th combination of th two sts of ligand orbitals, rprsntativ diagrams ar givn blow, b carful about choosing th as, Figur 8 o as th ligand Fs ar far apart thir will shift th nrg lvls onl slightl from th p and p, howvr th π -ligand combination will li slightl highr than th π-ligand combination allowing it to intract slightl bttr with th mtal bcaus th nrg diffrnc btwn th Fs is lss. o th cis-ligands ar actuall closr togthr and hav an unimpdd through spac, which mans th will intract mor pushing th π* ligand pair up vn highr and th π ligand pair down lowr. o howvr Δε is not th onl considration and th tnt of orbital ovrlap must also b considrd. In trans-[cocl 2 (NH 3 ) 4 ] + th π -ligand combination intracts vr favourabl with th mtal das, howvr in cis-[cocl 2 (NH 3 ) 4 ] + th Fs within th quatorial plan for cis-conformr do not ovrlap ffctivl with th availabl das. o in this cas th vr favourabl ovrlap of th trans-ligands could b considrd to dominat, thus in trans-[cocl 2 (NH 3 ) 4 ] + th π -ligand combination intracts bst with th mtal das, pushing th g s up b th gratst amount and thus rducing Δ oct th most. 6

7 4p u u D 4h 4s g g g Δ oct g g 3d g g g g u g g g g u u C 2v all ligand lvls ar occupid g using D 4h Figur 8 cis vs trans isomrs and th ffct on Δ oct o whil this planation provids a rationalisation for th colour diffrncs btwn trans- and cis-[cocl 2 (NH 3 ) 4 ] + th ralit is mor compl, non-bonding ligand orbitals li in this rgion, morovr th ffct of th diffring orbitals for NH 3 vs Cl mak a diffrnc to th antibonding g and g lvls. Calculation combind with primnt is ndd to bst undrstand th lctronic structur of vn simpl compls lik ths! Th important point is that w would not hav a handl on what is happning lctronicall if w did not undrstand th basic lctronic structur of octahdral and rlatd compls. o hav a look at th computd orbitals for th two conformrs using th *.fchk fils supplid on-lin. Form th nrg diagram for a compl L 4 X 2 whr L= σ-bonding ligand and X ar ligands positiond aiall. Includ diagrams of th k s. o plas s th hand-drawn answr on-lin as a pdf 7

Molecular Orbital Diagrams for TM Complexes with

Molecular Orbital Diagrams for TM Complexes with olcular Orbital Diagrams for T Complxs with p-intracting igands CO and othr iso-lctronic p-accptor s CO s ar vry usful for charactrising th amount of back donation in othr s o th vibrational IR spctrum