Infrared and Ultraviolet spectra of the trivalent and tetravalent compounds of phosphorus and arsenic have been described. Among the trivalent compounds triphenyl, ortho, meta and para tri-tolyl phosphines and triphenyl, meta and para tolyl arsines have been studied. These phosphines have been reacted with alkyl halides with increasing chain from methyl to n-butyl and taking the halides as bromide or lodid, thus givin a series of onium compounds. Finally the tetrahalocobaltates of these onium compounds have been prepared.
It has been found from the study that a relation exists between vibrational spectra and the structure of these compounds and it is possible to find the oxidation state and arrangement of the various groups in these compounds through the study of their vibrational spectra. In most of the cases the findings of the present study are complimentary with the X-ray structure data. The results also find their confirmation from the ultraviolet spectra.
Lone pair interaction gives a band system in the infrared as well as ultraviolet region. Thusin the former fegion the (k) and (I) modes do not absorb separately, the out-of-plane C-H of the umbrella vibration mode (I), the out-of-plane ring deformation mode (Y) and the transition in the ultraviolet are all one single band. The formation of onium compounds brings in fine structure or spoitting of bands in these regions. These changes nave been related to the formal positive charge on the central atom which attracts the electronic density of the rings thereby creating polarizability, on them. Consequently all the vibrations are intensified and sometimes new bands apper and splittings are noted.
Emphasis has been placed on the band position as well as the intensity and for this purpose integrated intensity of each band has been calculated and various useful correlations have been obrained between the intensity and the effect of various groups attached to the central atom.
The spectra in certain regions are very sensitive to hybridization. Thus mode (O) occurring at 1305_+ 10 cm01 in the phosphines and arsines is shifted to higher wavelengths on forming quaternary compounds. The same is shifted further on forming the tetrahalocobaltates. This diagnostic band has been detected only through the determination of the integrated intensity.
The (n) mode similarly gains in intensity in onium compounds. This band is not sensitive to the charge and honce it has been suggested to be due to a certain amount of distortion or twisting of the rings with respect to one another.
The intensity of mode (m) is also quite high in the onium compounds. This is a charge sensitive mode and the gain in intensity has been related in terms of the indeuced polarizability in the ring. For similar reasons mode (k) in the p-tolyl onium compounds is quite intense. This enhancement in intensity can be safely used as a test of qua-termination of the p-tolyl phosphines or arsines.
Certain X-sensitive modes are also seen and two distinct classes have been identified one is the mass sensitive and the other is charge sensitive. The mass sensitive vibrations change their position from phosphorus to arsenic due to the effect of mass whereas the charge sensitive modes are noted by shift from phosphine or arsine to their onium compounds mode (q) absorbs in ph3p at 1090 cm-1 and in phosphonium it absorbs at 1112 cm1. The same band occurs in Ph3As at 1075 cm-1 and in arsonium compounds at 1087 cm-1.
In the tolyl derivatives of these elements it has been noted that the spectra usually resemble those of disubstituted benzenes having a methyl group attached in the ortho, meta and para position. Thus in the para position there is a slight enhancement in the intensity of the bands which are now well resolved. In the tri-m-tolyl derivatives steric over-crowiding starts playing it role since the intensity in the trivalent and the tetravalent states are not very much different. This is further magnified in the ortho tolyl phosphorus compounds.
In the infrared region the alkyl chain attached to the central atom effects the vibrations due to the benzene ring. It has been interpreted in terms of an increase of formal positive charge on the central atom through the positive inductive effect of the alkyls. Thus for quite a few modes very regular and gradual variation has been noted with an increase in the alkyl chain length.
A greater enhancement in the intensity of various absorption has been noted from the lodide to the bromide. It is suggested that as the electro negativity of the anion is increased, the formal positive and hance the polarizability of the aromatic ring is increased. This gives rise to higher values of the intensity and sometimes slight shifts in the band positions. The effect was further studied by extending the present studies to the bulky and more electronegative anions like the tetrahalocoblatate anion. It has been found that there is a further rise in intensity which is about five to eight times higher than that of the lodides.
Arsines as a class have lower intensities. This has been related to the greater ionic radius and lower electrone-gativity of the arsenic atom compared with phosphorus.
The ultraviolet spectra of these compounds have also been recorded in methanol solutions and a mixture of methanol and acetic acid. Two definite band systems have been noted one due to transition and the other due to charge transfer. The charge transfer band disappears in acetic acid solution. The fine structure which is due to transition of the benzene ring is noted only in the onium compounds where the formal positive charge on the central atom attracts the electronic charge density. In cases where there is a repulsion due to the lone pair the fine structure is absent. The cobaltite have the same spectra as those of their parent onium compounds and differ in having higher intensities and also in an additional band at 350mu . This last band is related to the cobaltate anion.