Linkage Isomers

transcript

Maricris Monica N.Cabreros

In 1893, at the age of 26, Alfred Werner "stepped outside the box" and proposed his coordination theory that revolutionized the field of inorganic chemistry.

Werner synthesized the green trans and violet cis isomer of [Co(NH3)4Cl2]2+

StereoisomersCompounds having the same atom to atom

bonding but differ only in spatial arrangement of atoms or groups around central metal atom.

Structural IsomerismA type of isomerism that arises due to

different arrangement of coordinating groups around the central metal atom in coordination sphere

Ionization Isomerismexists in coordination compounds having the

same chemical composition but gives different ions if dissolved in a solution.

For this kind of isomerism, an interchange of ligand between the coordination sphere of metal ion and ions inside the sphere occurs.

Hydration Isomerismoccurs when there is an interchange of water

molecule and another ligand between the first coordination sphere and the other ligand outside it.

[Cr(H2O)4Cl2]Cl 2H2O-green;

[Cr(H2O)5Cl]Cl2 H2O – blue-green; and

[Cr(H2O)6]Cl3-violet

Coordination Isomerismisomerism which is observed in coordination

compounds having both cationic and anionic parts as coordination ions.

In this category, isomers are formed if ligands between two metal centers are interchanged

[Co(NH3)6][Cr(Cn)6] and [Cr(NH3)6][Co(CN)6]

Linkage Isomersisomerism takes place when the same ligand

has two possible coordination sitesthis means the ligand has two atoms which

can donate electron pairs to metal ionsthese ligands are commonly called as

ambidentate ligands

Ambidentate ligandsCN- (cyano) links

to CNC- (isocyano)

links to N OCN- (cyanato)

links to O NCO-

(isocyanato) links to N

SCN- (thiocyanato) links to S

NCS- (isothiocyanato) links to N atoms

Nitrate Ion

two different resonance structures, each with the negative charge on a different oxygen atom.

binds through the lone pair of the central atom: nitrogen for the nitro form and oxygen for the nitrito form.

Goalto synthesize the linkage isomers nitro- and

nitrito- of pentaamminecobalt(III) chloride understand the knowledge behind linkage

isomerismto characterize the products using Infrared

spectroscopy and their reactions in different conditions (presence/absence of light and heat).

Nitrito and Nitro isomer

Preparation of [CoCl(NH3)5]Cl215 grams of ammonium carbonate in 250mL E. flask

75 mL water

40 mL concentrated ammonia9.25 grams cobalt(II) chloride hexahydrate in 25 mL waterPlace in a 500 mL suction flask attached with a rubber stopper connected to an aeratorGently draw air for about two hours

In the preparationcobalt chloride is the source of the Co2+ ion ammonium carbonate is the source of

carbonate (CO32-) and ammonia (NH3) ligands.

The long aeration process was done to oxidize the metal Co2+ to Co3+ by reacting with the oxygen brought by the aeration process.

The following steps can be summarized in a reaction equation written as follows

Co2+ + 4NH3 + HCO3- + 1/2O2 [Co(NH3)4CO3]+ + 1/2

Oxidation of the Co2+ to Co3 is done in aqueous medium liberate the HCO3- from ammonium

carbonateCo in pure aqueous medium is predominantly

controlled by oxidation Co2+ at ambient conditions, Eo = 1.81 + 0.06 log Co2+/Co3+

Co3+ has a extremely low solubility so it will only be observed in aqueous solutions with strong chelating molecule

Crystal Field Stabilization EnergyIn the ligand field, d-orbitals are split and

some of them become lower in energy than before with respect to a spherical field known as the barycenter, where all five d-orbitals are degenerate. In an octahedral case, the t2g becomes lower in energy than the orbitals in the barycenter.

If electrons occupy these orbitals, the metal ion is more stable in the ligand field relative to the barycenter by an amount known as the CFSE.

CFSE= (6Dq x no. of e-s in eg) – (4Dq x no. of e-s in t2g)

In the spectrochemical series, the value of CFSE increases along the series so having water in the weak field stabilizes the Co2+ more since a more negative CFSE value indicates a more stable configuration.

transferred to a beaker

1.5 grams of ammonium chloridewas gently heated until crystals started to appear at the surfaceConcentrated HCl, until no further evolution of gas observed

concentrated HCl was added dropwise to expel CO2 evident as evolution of gas.

Also this makes the reaction forward to attain equilibrium, stated by the Le Chatelier’s principle

[Co(NH3)4CO3]+ + 2H3O+ [Co(NH3)4(H2O)2]3+

+ CO2 + H2O

CO32- was just

replaced by water through ligand substitution reaction

Concentrated ammonia, until neutralized

5 mL more concentrated aqmmonia

steam bath for 45 minutes

concentrated NH3 was added to neutralize the mixture and serve as the source of the NH3 ligand.

The solution was heated to drive the reaction forward by evaporation of water through heating the mixture in a steam bath.

[Co(NH3)4(H2O)2]3+ + NH3 [Co(NH3)5(H2O)]3+ +

40 mL concentrated HCl

Continued to heat for 30 minutesCooled to room temperature

Cool in an ice bath until no more significant changes observed

. Water ligands are displace due to addition on concentrated HCl which contributed large amounts of reacting chloride ions and also combined with heating the mixture in steam bath which allowed the evaporation of water.

[Co(NH3)5(H2O)]3+ + 3Cl- [Co(NH3)5Cl]Cl2 + H2O

Filter

Residue Filtrate

Acid waste binWash with min.

amounts cold water and cold ethanol

Air dry

[CoCl(NH3)5]Cl2

overall reaction for the synthesis of pentaamminechlorocobalt(III) chloride

Co2+ + 5 NH3 + HCO3- + 1/2O2 + 2H3O+ + 3Cl-

[Co(NH3)5Cl]Cl2 + 7/2H2O + CO2

Synthesis of [CoCl(NH3)5]Cl2

Reagent/Action taken Observationammonium carbonate white crystalsaddition of water clear colorless solutionaddition of concentrated ammonia clear colorless solutionaddition of cobalt chloride solution dark purpleaeration of solution dark purple with distinct ammonia odor

addition of ammonium chloride dark purpleaddition of concentrated HCl evolution of gasaddition of concentrated ammonia violet mixtureheating formation of light violet layer at the

bottomaddition of concentrated HCl evolution of gas, formation of light violet

precipitate at the bottom and surface

filtrate purple mixtureresidue purple solid

Preparation of [Co(NH3)5ONO]Cl2 5 grams [CoCl(NH3)5]Cl2

1.5 mL concentrated ammonia in 80 mL waterGently heat

FilterResidue Filtrate

Cool in ice bath

Solid wast ebin

Filtrate

titrated with 2M HCl until the solution is neutral

5 mL 6M HClStand for one hour

FilterFiltrate

Acid waste bin

Wash with 2mL cold water and 25mL cold ethanol, dry

Residue

[Co(NH3)5ONO]Cl2

5 g. NaNO2

Reaction with water makes the reaction forward

[Co(NH3)5Cl]Cl2 + H2O [Co(NH3)5 H2O]3+ + 3Cl-

Filtrate was then treated with HCl to provide chloride ions and NaNO2 to provide the nitrite ion.

Theoretically :red-orange in color

In the experiment: light-yellow brown precipitate.

[Co(NH3)5 H2O]3+ + Cl- + NO2- H2O +

[Co(NH3)5ONO]Cl2 + H2O

Due to the doubtful bulk product formed, the synthesis of [Co(NH3)5NO2]Cl2 was not made

Reagent/Action taken Observation[Co(NH3)5Cl]Cl2 purple solid

addition of concentrated ammonia purple mixture with purple solidsaddition of water purple mixture with purple solidsheating brownish purple mixtureresidue light purple solidsfiltrate brownish purplecooling of filtrate brown mixture titration with 2M HCl brown solution (neutralized)addition of NaNO2 formation of few bubbles; brown mixture

addition of 6M HCl formation of bubbles on the surface of the brown mixture

ice bath formation of precipitate at the bottom of the brown mixture

filtrate brown liquid mixtureresidue brown solidsafter washing light yellow-brown solid[Co(NH3)5ONO]Cl2 light yellow-brown solid

Preparation of [Co(NH3)5ONO]Cl2

Oxygen-nitrosation reactive species is actually not [Co(NH3)5

H2O]3+ but the conjugate base of the complex, [Co(NH3)5(OH)]2+.

This reacts with N2O3 to form the nitrito isomer and nitrous acid, HNO2.

In here, the oxygen bonded to the Cobalt ion in the nitrito complex was originally from the water ligand, and not from the nitrite ion ligand.

Theoretically if the nitro isomer was to be prepared, after the [Co(NH3)5ONO]Cl2 crystals were filtered and dried, they will be dissolved in hot water. A few drops of aqueous ammonia will be added then a red orange solution will appeare. While cooling the mixture, concentrated HCl will be added and a yellow-brown product is expected as the nitro-isomer.

yellow-brown solid already appearedforwarded and skipped the formation of the

nitrito product

Formation of the nitro product is more favourable since the product is more resonance stabilized.

Co-N bond is stronger than Co-O bond because oxygen is more electronegative than nitrogen making the bond with cobalt less strong

CharacterizationObservations Nitrito Niitro

exposure to UV light light pink to light pink yellow brown to light pink to yellow brown

heating to >100o C light pink to yellow brown

yellow brown to yellow brown

aging in dark light pink to yellow brown

yellow brown to yellow brown

The nitrito isomer is the kinetically labile product since it forms immediately due to a low activation energy.

Activation energy is defined as the energy that must be overcome in order for a chemical reaction to occur.

Heating the nitrito isomer increases the kinetic energy of molecules and therefore more nitrito- molecules have enough energy to overcome the activation E barrier to form the thermodynamically stable nitro isomer

. upon standing, isomerization of nitrito to nitro isomer can also occur since there is enough time for the equilibrium to be reached.

Heating simply speeds up the rate for reaching equilibrium of a reaction.

Exposing the nitro isomer on UV light resulted to the formation of nitrito isomer. This is because nitrito isomer requires lower energy to form and is kinetically favored.

Therefore, the nitro isomer is considered as the more stable isomer at room conditions, upon aging and heating.

- nitropentaamminenitritocobalt(III) chloride, [Co(NH3)5NO2]Cl2

--- nitritopentaamminenitritocobalt(III) chloride, [Co(NH3)5ONO]Cl2, freshly prepared

……. nitritopentaamminenitritocobalt(III) chloride, [Co(NH3)5ONO]Cl2, aged

IR spectroscopyNitro Isomer

(Fresh Sample)Nitrito Isomer

(Aged Sample)Mode of Vibration

Wavenumber (cm-1)

Mode of Vibration

Wavenumber (cm-1)

Mode of Vibration

Wavenumber (cm-1)

NO2 scissoring,

NH3 rocking

845 ONO

scissoring, NH3

rocking

850 NO2 scissoring,

NH3 rocking

H-N-H asymmetric scissoring

1600 H-N-H symmetric scissoring

1316 H-N-H asymmetric scissoring

asymmetric stretching

1430 H-N-H asymmetric scissoring

1618 NO2

asymmetric stretching

NH3 rocking 850 N-O stretching 10601460

NH3 rocking 850

NO2 symmetric

stretching, H-N-H symmetric

scissoring

1300 NO2 symmetric

stretching, H-N-H symmetric

scissoring

1600 cm-1, 1316 cm-1 and 850 cm-1 and the said bands can be assigned as degenerate deformation vibrations, symmetric deformation vibrations and rocking vibrations of NH3 ligands

The vibrations of the NH3 ligands of the nitrito isomer were observed at 1618 cm-1, 1316 cm-1, and 1600 cm-1

the vibrations of the NH3 ligands of the nitro isomer were found at 850 cm-1and 1600 cm-1

The spectrum of the aged nitrito isomer is almost identical to the spectrum of the nitro isomer. This proves that as the nitrito isomer aged it becomes the nitro isomer.

On aging nitrito ligand, the spectra showed bands which can be assigned to the nitro ligand. Therefore, the strong bands that decrease in intensity can be assigned to the nitrito isomer which shows the rearrangement of the nitrito isomer to the more stable nitro isomer.

RecommendationIt is recommended that a much less time be

allowed in heating the nitrito mixture since this may have resulted to the conversion of the nitrito isomer to the nitro form. Also a comparison between concentrated and dilute system can be made.

Summary and ConclusionLinkage isomers arise when ligands bind to a

central metal ion through either one of the two possible donor atoms.

linkage isomers [Co(NH3)5ONO]Cl2 and [Co(NH3)5NO2]Cl2 were synthesized using the intermediate [Co(NH3)5Cl]Cl2.

The nitro isomer is more stable upon heating and ageing than the nitrito isomer. Also the nitrito isomer is converted to nitro isomer when exposed to heat or upon standing for an extended period of time.

Upon exposure to UV light, the nitro isomer turned into the nitrito isomer.

The nitrito isomer is kinetically favored while the nitro isomer is the one which is thermodynamically stable.

Linkage Isomers

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