Transient absorption bands observed with organometals and the common electron acceptor, tetracyanoethylene (TCN E), are shown to arise from 1:1 charge-transfer complexes. The frequency of the charge-transfer band VCT. as well as the formation constant KCT and extinction coefficient εCT of weak (σ-π complexes derived from a series of homologous tetraalkyltin compounds R4Sn are sensitive measures of electronic and steric effects, as determined independently by the ionization potentials ID and the steric parameters of alkyl groups Es, respectively, of the donor. The disappearance of the charge-transfer (CT) complex follows overall second-order kinetics with rate constant kT, and it leads to the 1:1 insertion adduct R3Sn(TCNE)R. The same adduct can also be produced with unit quantum yield by the direct irradiation of the charge-transfer band at low temperatures where the thermal reaction does not occur. Steric effects in the formation of the complex (KCT) parallel those in the thermal insertion (kT). Steric effects are reflected in the photochemical insertion insofar as they influence the energy and oscillator strength of the CT transition as well as the formation constant of the complex. Thermal activation (log kT) and photochemical activation (hcCT) of insertion are both associated with an electron-transfer process which proceeds from the charge-transfer complex [R4Sn TCNE] to form the same paramagnetic ion pair [R4Sn+TCNE-]. Thermal and photochemical processes also share common intermediates subsequent to activation by electron transfer. Thus, in both, insertion follows from a scries of rapid, dark reactions involving the stepwise collapse of the ion pair [R4Sn+TCNE-] by (1) the spontaneous fragmentation of the R4Sn+ moiety akin to that observed in the gas phase upon electron impact, followed by (2) radical recombination and ion pairing all within the solvent cage. The nature of the ion pair is probed by examining selectivities in mixed methylethyltin compounds for Me-Sn and Et~Sn insertions. The alkyl and TCNE radical pair, that is, [R.R3Sn+ TCNE-], is shown to be the prime intermediate by quantum-yield measurements for their simultaneous formation during the irradiation of the charge-transfer band in a frozen matrix at -175°C. No CI DNP could be observed during thermal activation of insertion.