I infrared spectroscopic studies the final assignments of the absorption bands of malonic malic and lactic acid made with the help of the data available from the literature and by the study of known structural changes such as the alkali-metal salt formation and deuteration are summarized in table (22) studies of metal complexes coordinated water molecules of the three series (malonate,malate and lactate) investigation ,only Cu(II),CO(II),Ni(II),Mn(II) (?),Zn(II) and Cd(II) malonates exhibit the bands of coordinated water molecules in the region 900-750 cm-1. these bands are ascribed to the wagging or rocking motion of OH2 ligand as a whole the absence of this type of bands in malates and lactates leads to the conclusion that the structure of the aquo malonates in differnent form those of the malates and lactates. coordination sites: spectra of the malates and lactates do not have any absorption bands due to the lacoholic group, CHOH, in its usual range 3450-3300 cm-1. its absence indicates that the oxygen of this group is covalently bounds to the metal atom in the bidentate lactate complexes both the oxygens of two alcoholic groups are coordinated.
Table-22 The final assignments of malonic malic and lactic acids.
frequency cm -1. Malonic acid Lactic acid Malic acid 3333 s b 3448 S S Alcoholic O-H stretching C =O (acid) overtone C-H stertching, cerboxylic acid (dimer) C-H stretching 3300- 3100 300 2900-8000 m b 2800 2800 summation band of coupled vibrations of C-O stretching 0-H deformat ion of carboxylie acid group (satellite bands). 2600 2500 1746 vs sh 1706 vs b 1714 vs s 1709 VS b Acid C = O stretching. 1440 s' s 1300 m s 1400 s b s s Compile vibrations of C-O stretching a ad 0- H deformation of cerboxylic group. 1280 s b 1272 s sh Alcoholic O-H deformation coupled vibrations- of C = O stretching and O=K deformation of carboxylic group. 1210 s s 1170 s s 1220 s s 1178 s s OH2-COOH group. 1125 s s 1100 s S Alcoholic C-O stretching. 918 m b 926 m b 948 m b Acid O-H out-of-plane de formation.
relative stabilities of complexes: in the case of malonates of Mn(II),CO(II),Ni(II),Cn(II),Zn(II) and Cd(II),the antisymmertric stretching frequency of the COO- group is found to be most sensitive to the changes in the metal ion the value of this frequency incresses with the increaseing stability of the complex i.e. highest value of 1590 cm-1 is observed in the Cu(II) compounds and the lowest in Mn(II) complex. in the lactate complexes however the asymmetric COO- streching frequency values do not alter markedly all the metal complexes studied by us have this vibration near 1585 cm-1.lithium lactate which is probably a weak chelate has also the same value it is therefore concluded that the extent of covalent character of the bond between the metal ion and the oxygens of carboxyl groups is identical in all the complexes. in malate complexes the corboxyl group frequencies behave differently CU(II) and Zn(II) show a doublet, CO(II) and NI(II) a very broad band (the doublet is not observed because of complication s of O_H deformation of OH2).Mn(II) on the other hand shows a relatively sharp single peak this appearance of doublet in some cases and a single peak in the other may be attributed to the influence of steric environments. C copper (II) complexes of malonic malic and lactic acid in copper (II) malonate trihydrate the possible coordination sites are the oxygen's of two carboxyl groups and those of the water molecules. the structure of these compounds cannot be similar to that of Cc(II) oxalate dihydrate which comprises infinitely extending two dimensional net-work involving coorfination links, rather than the separate molecules as the turning of the chains at the central carbon atom renders such net-work structures improbable the structure of Cu(II) malonate trihydrate may be postulated to consist of a single molecule in which the six coordination sites are comprised of :two to teo carboxyl groups three by three water molecules and the remaining by the non-coordinated oxygen of the neighbouring molecule in the case of CU(II) malate trihydrate three coordination sites are filled by three water molecules one by the alcohol groups oxygen and ramaining two by oxygen of two carboxyl groups bidentate Cu(II) lactate must be having its octahedron filled by one oxygen each from two carboxyl groups one each from two alcoholic oxygens and remaining two by water molecules. infrared spectra of these compounds support the proposed structures as firstly no free COOH group is detected and secondly in the malate C-OH group is established Ni(II) and Co(II) complexes: spectra of these two compounds of each series are comparable assuming an octahedral configuration the coordination atoms may be said to be identical to those in the corresponding copper (II) compounds. Mn(II) and Zn(II) complexes: spctra of the two are comparable in the malonates. two bands of O-H deformation of water are found in both these compounds, while all other compounds of this series have only one band indicating the difference in structures spectrum of Mn (II) malate trihydrate is also different from the Cu(II),Co(II) and Ni(II) malates. II thermogravimetric analysis the decomposition of a compound is found to be sluggish in carbon dioxide and in nitrogen atmospheres:the dehydration temperatures being very little affected. the order of thermal stability which obviously corresponds to the decomposition temperatures is found to be Ni>Cd>Co>Mn>Zn>Cu, i the case of malonate c complexes this oder is comparable with that of other workers' results. however due to several uncertainties determination of such order has not been attempted at thisstage in the case of malates and lactates. the activation energies for the dehydration are found to be 30(or more) Kcal/mole for Co(II),Ni(II), Zn(II),MN(II) and CD(II) malonates and 15.5 kcal/mole for Cu(II) malonate. it is interesting to note that in CU(II) where the activation energy for dehydration is found to the lowest the corresponding OH2 group frequency (804cm-1)is also the lowest whereas the order of relative stability found from the metal sensitive band frequencies is in agreement with that of the irving and williams series (Viz.Mn