Theory at the ab initio level has described some of the characteristics of alkenes, silenes, and germenes.  As expected, ethene does not have a polarized double bond and the bond length is 15% shorter than that of a C-C single bond.  Silicon-carbon and germanium-carbon double bonds are polarized and the bond length gets increasingly longer as you go from carbon to silicon to germanium

There are many examples for the generation of silenes and germenes in the literature

In our lab, we have investigated the differences in reactivity of 1,1-diphenylsilene and 1,1-diphenylgermene which are produced photochemically from 1,1-diphenylmetallocyclobutane.  Below is a summary of the reactivity differences between the 2 compounds.

both of these transient species (silenes and germenes) have been studied by nanosecond laser flash photolysis techniques.  The insert shows the decay kinetics of the transient observed at 320 nm and it is evident that the lifetime of diphenylgermene is longer than that of diphenylsilene.  The spectrum below shows the absorbance plotted against wavelength in a point by point fashion with the absorption maximum for both diphenylsilene and diphenylgermene occurring at 320 nm.

below is a schematic representation of our nanosecond laser flash photolysis system.   The use of various gas mixtures in the laser can generate different wavelengths while the rest of the system is basically a large UV spectrometer.  The monochromator can be set to monitor the absorbance of a transient at a particular wavelength and this absorbance is measured over time.  The resulting decay trace is analyzed through the use of a computer and decay kinetics are then determined.

Nanosecond Laser Flash Photolysis

These are the absolute rate constants in hexane (230C) of diphenylsilene and diphenylgermene with a variety of chemical traps.

Irradiation of the disilane causes a formal [1,3-silyl] migration resulting in a transient 1,3,5-(1-sila)-hexatriene in >95% chemical yield.  The transient absorption spectra of this transient is also shown.

Trapping the 1,3,5-[1-sila]hexatriene with acetone affords 2 products - 1,2-siloxetane and a silyl ether and both of these compounds have been isolated and fully characterized.

It is reported in the literature that the irradiation of digermanes affords quantitative Ge-Ge bond homolysis

Germyl radicals are trapped with CCl4 yielding the corresponding chlorogermanes

The reactivity of triphenylgermyl radicals towards various quenchers has been reported

PhMe2GeSiMe3 forms 2 transients upon photolysis - dimethylgermylene and a 1,3,5-(1-germa)-hexatriene

Trapping experiments show that the ratio of germylene to germene is approx. 2:1

Laser flash photolysis studies on these germasilanes have been performed by both Gaspar (and coworkers) & Mochida (and coworkers).  Both groups get similar results and characterize the transient occuring at 420 nm as being dimethylgermylene.

	the transient absorption spectra of the germasilanes are shown below along with the spectrum of Ph3SiSiMe3 and one of the germasilanes in a polar solvent (acetonitrile).  The transient at 490 nm is the result of the formal [1,3-metallo] migration into the phenyl ring yielding a transient 1,3,5-(1-metallo)hexatriene.  The sharp absorption at 330 nm has been attributed to the Ge-Si bond homolysis affording germyl- or silyl- radicals.
	The photolysis of Ph3SiSiMe3 affords no signal that can be attributed to silyl radicals.
	The transient absorption spectra of the germasilanes in MeCN yields only peaks due to germyl- or silyl- radicals.  This is because MeCN is a polar solvent which enhances intersystem crossing to the excited triplet state of the germasilane.  It is known that Si-Si bond homolysis occurs from the triplet state of disilanes and therefore it is not surprising that similar results are seen in the germasilane case.

Absolute Rate Constants

this is a comparison of the absolute rate constants for the 3 hextriene transients in hexane.

absolute rate constants are determined by measuring the lifetime of a transient as a function of quencher concentration.  A plot of kdecay vs. quencher concentration gives a straight line (in most cases) and the slope of the line is the absolute rate constant for that particular quencher.

steady-state photolysis of the germasilanes affords a mixture of transient species

the extrusion of a germylene is the major pathways

products associated with a formal [1,3-metallo]- shift resulting in the hexatriene are also obvious

one transient (Me2M=CH2) is derived from a concerted dehydrosilylation mechanism

nanosecond laser flash photolysis affords only 2 transient absorptions: hexatriene (lmax = 490 nm) and radicals (lmax = 330 nm)

no germylene absorption could be detected

radical formation is suppressed in the presence of 2,3-dimethyl-1,3-butadiene

the dominant reaction pathway of Ph3SiGeMe3 & Ph3SiGeMe3 is the extrusion of a germylene

other excited singlet state pathways are a formal [1,3-metallo] shift and a concerted dehydrosilylation reaction yielding dimethylsilene/germene

radical formation is derived from the excited triplet state of the germasilane

the presence of germylenes is confirmed by product studies and could not be observed using NLFP techniques