The Pt complex [Pt(PPh )(Eind -BPEP)] with a pyridine-based PNP-pincer-type phosphaalkene ligand (Eind -BPEP) has a highly planar geometry around Pt with ∑(Pt)=358.6°. This coordination geometry is very uncommon for formal d complexes, and the Pd and Ni homologues with the same ligands adopt distorted tetrahedral geometries. DFT calculations reveal that both the Pt and Pd complexes are M species with nearly ten valence electrons on the metals whereas their atomic orbital occupancies are evidently different from one another. The Pt complex has a higher occupancy of the atomic 6s orbital because of strong s-d hybridization due to relativistic effects, thereby adopting a highly planar geometry reflecting the shape and orientation of the partially unoccupied dx2-y2 orbital.

The reaction of the bulky diphosphenes (Rind)P=P(Rind) (1; Rind=1,1,3,3,5,5,7,7‐octa‐R‐substituted s‐hydrindacen‐4‐yl) with two molecules of N‐heterocyclic carbene (NHC; 1,3,4,5‐tetramethylimidazol‐2‐ylidene) resulted in the quantitative formation of the NHC‐bound phosphinidenes NHC→P(Rind) (2), along with the cleavage of the P=P double bond. The reaction times are dependent on the steric size of the Rind groups (11 days for 2 a (R=Et) and 2 h for 2 b (R=Et, Me) at room temperature). The mechanism for the double bond‐breaking is proposed to proceed via the formation of the NHC‐coordinated, highly polarized diphospehenes 3 as an intermediate. Approach of a second NHC to 3 induces P−P bond cleavage and P−C bond formation, which proceeds through a transition state with a large negative Gibbs energy change to afford the two molecules of 2, thus being the rate‐determining step of the overall reaction with the activation barriers of 80.4 for 2 a and 29.1 kJ mol−1 for 2 b.

Diphosphenes stabilized by the fused-ring bulky 1,1,3,3,5,5,7,

The reaction of the bulky diphosphenes (Rind)P=P(Rind) (1; Rind=1,1,3,3,5,5,7,7-octa-R-substituted s-hydrindacen-4-yl) with two molecules of N-heterocyclic carbene (NHC; 1,3,4,5-tetramethylimidazol-2-ylidene) resulted in the quantitative formation of the NHC-bound phosphinidenes NHC→P(Rind) (2), along with the cleavage of the P=P double bond. The reaction times are dependent on the steric size of the Rind groups (11 days for 2 a (R=Et) and 2 h for 2 b (R=Et, Me) at room temperature). The mechanism for the double bond-breaking is proposed to proceed via the formation of the NHC-coordinated, highly polarized diphospehenes 3 as an intermediate. Approach of a second NHC to 3 induces P-P bond cleavage and P-C bond formation, which proceeds through a transition state with a large negative Gibbs energy change to afford the two molecules of 2, thus being the rate-determining step of the overall reaction with the activation barriers of 80.4 for 2 a and 29.1 kJ mol for 2 b.

Figure 2. Dependence of the potential energy (DE [kcal mol À1 ]) on the N-M-PH 3 angle [8 8]f or 1 sm (Pt) and 2 sm (Pd).Figure 3. Frontier molecular orbitals of 1 sp (left) and 2 dt (right).