Intersystem crossing (ISC) in solid [(C4H9)(4)N](4)[Pt-2(mu-P2O5(BF2)(2))(4)], abbreviated Pt(pop-BF2), is remarkably slow for a third-row transition metal complex, ranging from tau(ISC) approximate to 0.9 ns at 310 K to tau(ISC) approximate to 29 ns below 100-K A-classical model-based on Boltzmann population of one temperature-independent and two thermally activated pathways was previously employed to account for the ISC rate behavior. An alternative we prefer is to treat Pt(pop-BF2) ISC quantum mechanically, using expressions for multiphonon radiationless transitions. Here we show that a two-channel model with physically plausible parameters can account for the observed ISC temperature dependence. In channel 1, (1)A(2u) intersystem crosses directly into (3)A(2u) using a high energy B-F or P-O vibration as accepting mode, resulting in a temperature-independent ISC rate. In channel 2, ISC occurs via a deactivating state of triplet character (which then rapidly decays to (3)A(2u)), using Pt-Pt stretching (160 cm(-1)) as a distorting mode to provide the energy needed. Fitting indicates that the deactivating state, X-3, is moderately displaced (S = 0.5-3) and blue-shifted (Delta E = 1420-2550 cm(-1)) from (1)A(2u). Our model accounts for the experimental observation that ISC in both temperature independent and thermally activated channels is faster for Pt(pop) than for Pt(pop-BF2): in the temperature independent channel because O-H modes in the former more effectively accept than B-F modes in the latter, and in the thermally activated pathway because the energy gap to X-3 is larger in the latter complex.