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A comparative study of the integrated extinction coefficients (A) of the C~C stretching bands in the I R spectra of acetylene derivatives Me~SiC~CR, HC~CR. and Me3CC-~CR was carried out. The resonance interactions of substituents with a triple bond are the main cause of the changes in the values of A. The total resonance effect of the Me3Si fragment involves both acceptor (d,~t-conjugation) and donor (o,r~-cot~jugation) components; d,z:-conjugation dominates in the silylacetylenes studied. The c~R ~ resonance constant of the MejSi substituent in compounds Me3SiC-M2R is 0.17+0.02.

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The integrated extinction coefficients (A) of the C~ stretching modes in the IR spectra of 12 germylacetylenes Me3Ge~R are determined by the resonance interactions of substituents with the triple bond. The A I/2 values change linearly with change in the difference between the effective x-electron charges on the atoms at the triple bond and ~R constants of organic substituents R. The average value of the ~R constant of the Me3Ge substituent in the compounds studied is +0.06. The resonance acceptor effect of the Me3Ge substituent toward the triple bond (d,n-conjugation) is stronger tl-.an the donor effect to,x-conjugation).Key words: germylacetylenes, integrated extinction coefficients, effective charge, d,g-coltjugation, cr,n-conjugation.Generally, the conjugation in compounds R~--X is determined not only by the properties of organic (R) and organoelement (MR 3, M = Si, Ge, Sn, Pb) substituents X, but also by those of the reaction (indicator) center IL s (Ph--, H2C=CH -, HC---C--, etc.). !-3 Therefore, it is impossible to characterize the capability of substituents X for conjugation by universal parameters, which are invariant toward R~. The charge on the R~ fragment and its type (in the case of organoelement compounds with X = MR 3) am of great importance. 1-5 The o~R-parameters of the substituents are a measure of resonance interactions in the ground electron state of isolated I~--X molecules for any X (R and MR3). 6 [fa small (--O.0I e) or large (--0.1 e) positive charge is induced on the R~ fragment as a result of chemical or physical perturbations (e.g., upon complexation or ionization of R~--X molecules), then the conjugation is characterized t-5 by c~ R and OR + parameters, respectively. In addition, the values of resonance parameters of a fixed substituent MR 3 (for instance, GeMe 3 in compounds R~--MR 3) change when varying the type of R~ (in contrast to X = R). t.z,5,7,8 The reasons for the lesser universality of the parameters of organoelement substituents as compared to those of organic substituents remain unclear, which is, in particular, due to fragmentary experimental data on the dependence of a-parameters of MR3 substituents on the R~ type in compounds R~--MR 3. The solution of this problem, which is critical for physical orgapometallic chemistry, suggests a progressive increase in the number of studied series of R.n--M R 3 compounds.The aim of this work is to study the resonance effects in trimethylgermylacctylene derivatives Me3GeC~-CR by analyzing the integrated extinction coefficients of the v(C.~=C) IR bands, to estimate changes in the effective charge on the indicator center in the case of conjugation with the Me3Ge substituent, and to calculate the ~~ R parameter of this substituent. ExperimentalThe studied compounds were synthesized following the known procedures. 9.1~ The purity of the compounds was monitored by GLC.The purity of freshly prepared solvent CCI 4 was monitored by UV and IR spectroscopy.The integrated extinction coefficients (A) of the C---C stretching bands in the IR spectra ...

The dependence was analyzed of the firsl ionization potential l(Si--Si) corresponding to detachment of an electron from the ~(Si--Si) highest occupied molecular orbital on the parameters of organic (X = *%, Et. Bu t . Ph, CH=CH~), inorganic (X = F, CI, Br), and organosilicon (X = SIR.:; R is organic radical) substit~ents in eli-, tri-, and tetrasilanes X:~SiSiX 3. It was fotmd by correlation analysis that out of the tl~ree possible effects of substiments X (the inductive, polarizability, and resonance effects), only the first two of them affect the /(Si--Si) values. This means that no conjugation between the substituent X and the radical cation center occurs in X3Si "e-SiX 3.Key words: linear oligosilanes, ionization potential, radical cations, substituent effi:cts.The electronic structure of linear polysilanes R--(SIR2) --R (n > 2) that currently find a wide field of application has been studied in detail, t-6 Theoretical studies of these compounds are of great importance for re~ealing the peculiarities of the Si--Si chemical bond and tbr sol~ing actual problems of the chemistry, of organoelement compounds (in particular, for development of the modem concept of conjugation in the compounds of the silicon subgroup eiemcntsL According to the current concept, 7 organoelement substituents M R 3 (M = Si, Ge, Sn, Pb: R is organic radical) in R.,MR 3 molecules exhibit simultaneously resonance donor and resonance acceptor properties toward the =-electron centers R= (C6H s, I-I.,C=CH, HC~:C. etc.). The resonance acceptor effect (d-,~-conjugation) is due to the interaction of the vacant nd-orbitals of the M atom and antibonding cs*-orbitals of the M--R bonds with the orbitals of the R, fragment. This effect decreases as the atomic number of the M atom increases, i.e., in the series Si > Ge > Sn > Pb_ The resonance donor effect (o.r~-conjugation), which is due to the interaction of the cs-orbitals of the M--R bonds with the orbitals of the R,. fragment, increases as the number of the M atom increases (Si < Ge < Sn < Pb). In the RxCH2MR 3 molecules, d,~-conjugation is absent, while ~,=-conjugation with R involves the r of the C--M bond of the CHuM fragment. Both resonance effects are present in R SiR2SiR 3 compounds, namely, d,a-conjugation with the ~-electron system <3t" R~, involves the orbitals of the SiR, fragment, while ~,rt-conjugation occurs involving the r of the Si--Si bond. These regularities are characteristic of the abovementioned neutral systems in the ground electron state.At small perturbations, e.g., in the case of the fommtion of a H-complex of the ~-donor center R with an acceptor molecule ('phenol, pyrrole, etc_), the effective negative charge of the R fragment is somewhat decreased (by n = 0.01e), which increases o,=-conjugation. Appreciable decrease (n = O. le) in the negative charge on the R~ substituent (e.g., in tile excited state of rt,r,-type charge-transfer complexes of R~MR 3 and R,CH2M R 3 molecules with tetracyanoethylene) leads to a sharp increase in ~.a-conjugation. v-t~We have developed a ne...