Hybridisation
Hybridisation is the interbreeding of individuals from genetically distinct populations, subspecies, or species, producing offspring known as hybrids that inherit a combination of genetic material from both parental lineages.[1][2] This process occurs naturally in various taxa, including plants and animals, and can result in outcomes ranging from enhanced fitness to reduced viability, depending on genetic compatibility and environmental factors.[3] In agriculture and breeding, hybridisation is deliberately employed to exploit heterosis, or hybrid vigor, wherein hybrids often surpass parental lines in traits such as growth rate, yield, and resistance to stressors, as evidenced by superior performance in crops like maize and livestock.[4][5] Evolutionarily, hybridisation facilitates gene flow through introgression, potentially driving adaptive evolution, novel speciation, or biodiversity loss when it erodes distinct genetic identities in endangered taxa.[3][6] While beneficial in controlled settings, uncontrolled hybridisation poses conservation challenges, as seen in cases where invasive hybrids outcompete native species or dilute adaptive gene pools.[7]Biological Hybridisation
Definition and Mechanisms
Hybridisation in biology is the interbreeding of individuals from two genetically distinct populations, typically of different species, subspecies, or varieties, resulting in offspring that possess a combination of genetic material from both parents. This process occurs when reproductive barriers are incomplete or absent, allowing gametes from divergent lineages to fuse and form viable zygotes. Unlike intraspecific mating, hybridisation often involves crossing taxonomic boundaries, leading to progeny with intermediate or novel traits, though fertility and viability vary widely. Empirical studies, such as those on Darwin's finches, demonstrate hybridisation's role in generating genetic diversity through admixture events dated to specific geological periods, like post-glacial expansions around 10,000 years ago. The primary mechanism initiating hybridisation is the overcoming of prezygotic barriers, which include temporal (e.g., differing flowering or breeding seasons), behavioral (e.g., mate choice via species-specific signals), mechanical (e.g., incompatible genitalia), and gametic (e.g., molecular recognition failures in sperm-egg interactions) isolations. When these fail, hybrid zygotes form via standard fertilisation processes: meiosis produces haploid gametes carrying recombinant chromosomes, followed by syngamy where paternal and maternal genomes merge. Postzygotic mechanisms determine hybrid success; genetic incompatibilities, such as Dobzhansky-Muller interactions—where alleles functional within species become deleterious in combination—can cause reduced fitness, sterility (e.g., Haldane's rule, where heterogametic sex is more affected), or developmental abnormalities. For instance, in Drosophila, hybrid males exhibit sperm dysfunction due to X-autosome imbalances, confirmed through controlled crosses yielding 0-5% viable offspring. In plants, mechanisms like polyploidy facilitate hybridisation by restoring fertility through chromosome doubling, as seen in wheat hybrids (Triticum aestivum) formed ~8,000 years ago via allopolyploid events. Causal drivers of hybridisation include ecological overlap, such as habitat fragmentation increasing contact zones—evidenced by genomic analyses showing admixture rates up to 20% in European house mice (Mus musculus) at hybrid zones. Anthropogenic factors, like translocations, amplify these, with data from IUCN reports indicating hybrid swarms in 15% of threatened species due to human-mediated gene flow. However, hybrid genomes often exhibit underdominance, where heterozygote disadvantage leads to purging of maladaptive alleles over generations, as modeled in simulations predicting hybrid zone stability only under balanced dispersal and selection pressures. Source credibility in such studies favors genomic datasets from repositories like NCBI, which provide raw sequence evidence over anecdotal field reports prone to observer bias.Historical Development
The phenomenon of biological hybridization, involving the interbreeding of distinct species or varieties to produce offspring, was informally observed in antiquity through examples such as the mule, a hybrid between horse and donkey, documented in Assyrian records dating back to around 3000 BCE.[8] Systematic scientific investigation began in the early 18th century with artificial plant hybrids; Thomas Fairchild produced the first documented artificial hybrid in 1716 by crossing Dianthus species, yielding a sterile offspring with intermediate traits.[9] Concurrently, Cotton Mather reported natural hybrids between maize (Zea mays) and squash (Cucurbita spp.) in 1716, marking one of the earliest scientific identifications of interspecific plant hybrids.[10] In the mid-18th century, Joseph Gottlieb Koelreuter conducted the first large-scale systematic experiments on plant hybridization between 1760 and 1766, performing over 500 crosses primarily in tobacco (Nicotiana spp.) and observing consistent intermediate phenotypes, occasional hybrid vigor, and frequent sterility or reduced fertility in offspring.[11] Koelreuter's work demonstrated that hybrids could revert toward parental forms in subsequent generations and highlighted barriers to fertility, laying foundational empirical data for understanding hybridization mechanisms. Carl Linnaeus, in parallel, examined numerous plant and animal hybrids during the 1750s and 1760s, classifying them within his binomial system while initially upholding species fixity; later reflections suggested hybrids might contribute to new species formation, influencing early evolutionary thought.[12] The 19th century advanced hybridization studies through Charles Darwin's extensive research, detailed in On the Origin of Species (1859) and The Variation of Animals and Plants under Domestication (1868), where he analyzed over 100 hybrid cases across plants and animals, emphasizing sterility as an evolved reproductive isolation mechanism rather than a direct adaptive trait.[13] Independently, Gregor Mendel performed controlled crosses on pea (Pisum sativum) hybrids from 1856 to 1863, publishing in 1866 his laws of segregation and dominance, which explained trait inheritance patterns in hybrids—though overlooked until rediscovery in 1900. These findings shifted focus from descriptive morphology to quantitative genetics. Early 20th-century developments integrated Mendelian principles with practical breeding; George Shull and Edward East independently demonstrated hybrid vigor (heterosis) in maize (Zea mays) in 1908, showing inbred lines crossed to produce hybrids with 20-50% yield increases due to heterozygote advantage.[14] This spurred commercial hybrid crop production, with the first hybrid corn varieties commercialized in the 1920s, transforming agriculture while revealing outbreeding depression risks in some contexts.[15] Modern genetic tools from the mid-20th century onward, including cytogenetics and molecular markers, further elucidated hybridization's role in speciation and gene flow, confirming Koelreuter and Darwin's observations at the chromosomal and genomic levels.[16]Types and Examples
Interspecific hybridization occurs between individuals of different species within the same genus and is documented across both animals and plants, though it produces fertile offspring more reliably in plants due to mechanisms like chromosome doubling. In animals, such hybrids frequently exhibit sterility from meiotic irregularities, as seen in the mule (Equus caballus × E. asinus), which possesses 63 chromosomes and cannot produce viable gametes.[17] Approximately 9% of bird species engage in interspecific hybridization, often in hybrid zones where parental ranges overlap, though most resulting offspring have reduced fitness.[18] Intergeneric hybridization, involving parents from different genera, is rarer and typically requires human intervention or specific ecological conditions, yielding hybrids with greater genetic divergence and challenges in viability. In plants, the cereal triticale (× Triticosecale), derived from wheat (Triticum spp.) and rye (Secale cereale), exemplifies a fertile intergeneric hybrid cultivated since the late 19th century for improved yield and disease resistance in agriculture.[19] Another plant example includes hybrids between Raphanus (radish) and Brassica (cabbage), which have been produced experimentally to transfer traits like disease resistance, though fertility often requires embryo rescue techniques.[20] Hybridization can also contribute to speciation, categorized as homoploid (same ploidy level as parents) or polyploid (involving genome duplication). Homoploid hybrid speciation is rarer in animals but evident in the Italian sparrow (Passer italiae), which arose around 4,000–10,000 years ago from hybridization between the house sparrow (P. domesticus) and Spanish sparrow (P. hispaniolensis), stabilized by ecological divergence in the Italian peninsula.[21] In plants, polyploid hybrid speciation is common, as in certain Brassica crops resulting from interspecific crosses followed by chromosome doubling, enabling reproductive isolation from parents.[22] These processes highlight hybridization's role in generating novel genetic combinations, though success depends on overcoming post-zygotic barriers like endosperm failure in plants or Haldane's rule in animals, where heterogametic sex (e.g., XY males) is more often sterile.[10]Genetic and Evolutionary Implications
Hybridization introduces novel genetic combinations through recombination between divergent parental genomes, potentially leading to introgression of adaptive alleles across species boundaries. This process can enhance genetic diversity within hybrid populations, as evidenced by genomic analyses showing biased retention of ancestry blocks from one parent due to selection against incompatible regions. However, it often disrupts co-adapted gene complexes, resulting in Dobzhansky-Muller incompatibilities that cause hybrid sterility or inviability, particularly in animals where chromosomal mismatches are common.[6][23] At the genomic level, hybridization triggers dynamic evolutionary processes such as biased gene conversion, where recombination hotspots favor transmission of one parental allele over another, and the formation of hybrid zones where admixture gradients reveal selection pressures. In plants, polyploid hybridization frequently stabilizes genomes via chromosome doubling, restoring fertility and enabling allopolyploid speciation, as seen in approximately 15% of angiosperm speciation events. In contrast, homoploid hybrid speciation—without ploidy change—is rarer and requires ecological divergence to fix hybrid genotypes, with genomic evidence confirming its occurrence in systems like Helianthus sunflowers and marine snails.[6][24][25] Evolutionarily, hybridization facilitates adaptive introgression, transferring beneficial traits such as pesticide resistance or cold tolerance between species, thereby accelerating adaptation in changing environments. It can reshape fitness landscapes by generating transgressive segregants—hybrids exceeding parental trait extremes—potentially accessing novel adaptive peaks during radiation events. Yet, pervasive genomic instability, including aneuploidy and deleterious epistasis, often imposes fitness costs, limiting long-term persistence unless stabilized by selection or isolation; studies indicate that while short-term heterosis (hybrid vigor) occurs, later generations frequently suffer outbreeding depression. Hybridization thus acts as a double-edged sword: promoting biodiversity through speciation in some taxa (e.g., over 10% of plant species) while eroding species distinctions via gene swamping in others, particularly in fragmented habitats.[26][23][27]Hybrid Vigor and Outbreeding Depression
Hybrid vigor, also known as heterosis, refers to the superior performance of hybrid offspring compared to their parental lines in traits such as biomass, yield, height, and stress resistance.[28] This phenomenon arises primarily in the first filial generation (F1) from crosses between inbred or divergent parental genotypes.[4] The genetic mechanisms underlying hybrid vigor include dominance effects, where deleterious recessive alleles from one parent are masked by dominant alleles from the other, leading to complementation; overdominance, in which heterozygotes exhibit enhanced fitness beyond either homozygote; and epistasis, involving favorable interactions between non-allelic genes that are disrupted in inbred lines but restored or amplified in hybrids.[28] [4] For instance, in maize, epistatic quantitative trait loci (QTLs) contribute to yield heterosis by activating paternal alleles that counteract maternal repression of genes like ubi3, resulting in increased plant height and ear weight across 42,840 analyzed F1 hybrids.[28] Transcriptomic studies further reveal non-additive gene expression, such as upregulation of photosynthesis-related genes, enhancing carbon fixation and leaf area in hybrid rice and Arabidopsis.[4] Empirical examples abound in agriculture, where hybrid maize yields rose from approximately 1 ton per hectare in 1930 to 12 tons per hectare by 2017, attributed to heterotic effects in commercial breeding programs.[4] Similarly, hybrid rice varieties yield 10–20% more than inbred lines due to overdominant epistatic loci influencing biomass and grain production.[4] In natural populations, such as crosses between closely related Arabidopsis thaliana accessions from Italy and Sweden, F1 hybrids exhibited 10–23% higher fitness through dominance complementation, primarily increasing fruit number.[29] Outbreeding depression represents the converse outcome, where crosses between genetically distant populations yield offspring with reduced fitness relative to parents, often due to the breakdown of locally adapted gene complexes or emergent incompatibilities.[29] This occurs when hybridization disrupts coadapted allelic interactions honed by selection in specific environments or introduces negative epistasis, such as Dobzhansky-Muller incompatibilities between diverged genomes.[29] Mechanistically, outbreeding depression can stem from underdominance at key loci or pseudo-underdominance from linked chromosomal rearrangements, leading to maladaptive phenotypes; it may also arise from the loss of extrinsic adaptations, where hybrid genotypes fail to match parental environmental optima.[29] In Arabidopsis thaliana, crosses between more divergent Italian and Swedish populations resulted in F1 fitness reductions of 15–44%, characterized by stunted growth and decreased seed and fruit production, contrasting with heterosis in closer crosses.[29] Such effects highlight a fitness optimum at intermediate genetic distances, where excessive divergence shifts from vigor to depression via disrupted physiological compatibility.[29] In conservation biology, outbreeding depression poses risks in managed populations, as artificial admixture of distant stocks can erode local adaptations, exemplified by reduced hybrid viability in fragmented plant species; however, it is less common than inbreeding depression and predictable based on genetic distance and environmental dissimilarity.[29] These dynamics underscore the balance in hybridization: moderate outcrossing promotes vigor by alleviating inbreeding, while extreme distances incur costs through incompatibility.[29]Conservation and Ecological Controversies
Biological hybridization poses significant challenges in conservation biology, particularly when it leads to genetic introgression that erodes the distinctiveness of endangered taxa. Empirical reviews indicate that hybridization contributes to extinction risk through mechanisms such as genetic swamping, where alleles from more abundant populations overwhelm rare ones, and outbreeding depression, reducing hybrid fitness due to disrupted local adaptations.[30] In a synthesis of 143 case studies, hybridization was identified as an extinction threat in 69 instances, predominantly via genetic swamping (87% of cases), with human-mediated factors elevating risk in 72% of those scenarios.[30] For instance, invasive rainbow trout (Oncorhynchus mykiss) have hybridized with native cutthroat trout, compromising the latter's genetic identity and fitness by up to 50% in affected streams.[30] Ecological controversies arise from hybridization's variable outcomes, including altered phenotypes that can disrupt community dynamics or enhance invasiveness. In mammals, systematic reviews of 140 studies reveal negative consequences in 21% of cases, such as genetic swamping threatening species integrity, with high extinction risk for rare taxa interbreeding with common relatives.[31] Examples include mallard ducks (Anas platyrhynchos) hybridizing with endangered Hawaiian koloa (Anas wyvilliana), leading to demographic swamping and population decline.[30] However, evidence challenging blanket threat perceptions shows hybridization rarely drives outright extinction; analysis of the IUCN Global Invasive Species Database found direct fitness reduction in hybrids from only 9 of 870 invasive species cases.[32] This underscores that while hybridization can cause local adaptations to break down, presuming universal harm overlooks cases where hybrids exhibit hybrid vigor or novel adaptive traits.[31] Anthropogenic drivers, including habitat fragmentation, species translocations, and invasive introductions, exacerbate hybridization, often blurring distinctions between natural and human-induced events. Climate change and pollution further erode reproductive barriers, potentially increasing introgression rates.[33] Conservation policies reflect these tensions: many exclude hybrids from protection under frameworks like the U.S. Endangered Species Act or IUCN Red List, prioritizing "pure" lineages despite limited empirical support for such purity as a proxy for viability.[32] Debates intensify over interventions like culling hybrids to preserve native gene pools versus deliberate genetic rescue, as in the Florida panther (Puma concolor coryi), where introgression from Texas cougars boosted population numbers from 20-30 in 1995 to over 200 by 2020 without evident long-term fitness costs.[33] Such cases advocate case-by-case assessments over ideological aversion to hybrids, emphasizing empirical monitoring of fitness and ecological roles.[32]Chemical Hybridisation
Orbital Hybridisation Theory
Orbital hybridisation theory describes the mixing of atomic orbitals within an atom's valence shell to produce a set of new, equivalent hybrid orbitals that possess enhanced directional characteristics for optimal overlap in forming covalent sigma bonds. This process occurs when atomic orbitals of similar energy levels, such as s and p orbitals, combine linearly, resulting in hybrid orbitals that better explain the geometry and bond strengths observed in molecules compared to using pure atomic orbitals alone.[34][35] The theory integrates with valence bond theory by positing that covalent bonds form through the end-to-end overlap of these hybrid orbitals with those from bonding partners, concentrating electron density between nuclei to minimize energy. Hybridisation adjusts the spatial orientation of orbitals to match experimental bond angles, such as the tetrahedral arrangement in methane (CH₄) or trigonal planar in boron trifluoride (BF₃), which pure s and p orbitals cannot accommodate due to their spherical (s) or mutually perpendicular (p) shapes.[36][37] Mathematically, hybrid orbitals are constructed as linear combinations of atomic orbitals (LCAO), where the wave function of a hybrid orbital is expressed as ψ_hybrid = ∑ c_i ψ_i, with coefficients c_i ensuring normalization (∑ c_i² = 1) and equivalence among the set. For example, in sp³ hybridisation, the four hybrid orbitals derive from one 2s and three 2p orbitals of carbon, each hybrid containing 25% s-character and 75% p-character, directed toward the vertices of a tetrahedron with bond angles of approximately 109.5°. The increased s-character in hybrids with fewer orbitals (e.g., sp hybrids at 50% s) pulls bonding electrons closer to the nucleus, yielding shorter and stronger bonds, as evidenced by bond length trends in hydrocarbons.[38][39] This model predicts that the number of hybrid orbitals equals the number of sigma bonds plus lone pairs on the central atom, dictating electron pair geometries via VSEPR principles while specifying orbital overlap for bonding. Although an approximation within valence bond framework, it provides intuitive explanations for molecular shapes without invoking delocalized electrons, aligning with spectroscopic and diffraction data for simple molecules like water (H₂O, bent sp³ hybrids) and acetylene (HC≡CH, linear sp hybrids).[40][41]Historical Introduction and Key Contributors
Orbital hybridisation theory developed as an extension of valence bond (VB) theory in the early 1930s, amid efforts to reconcile quantum mechanical principles with observed molecular geometries and bond directions that pure atomic orbitals could not adequately explain. VB theory originated with the 1927 work of Walter Heitler and Fritz London, who applied Schrödinger's wave mechanics to describe the covalent bond in the diatomic hydrogen molecule (H₂) as resulting from the overlap of atomic orbitals and exchange of electrons.[42] This framework emphasized localized electron-pair bonds but initially struggled with predicting specific bond angles, such as the tetrahedral arrangement in methane (CH₄).[43] Linus Pauling, building on these foundations and his own analyses of X-ray diffraction data, introduced hybridisation in 1931 to address these limitations by proposing that atomic s and p orbitals could mathematically combine (or "hybridize") into equivalent hybrid orbitals with directional properties matching experimental geometries.[44] For carbon in CH₄, Pauling described four sp³ hybrid orbitals formed from one 2s and three 2p orbitals, arranged tetrahedrally at 109.5° angles to maximize overlap with hydrogen 1s orbitals, thus explaining the molecule's structure without invoking resonance for this case.[45] Pauling further elaborated these ideas in subsequent publications and his seminal 1939 book The Nature of the Chemical Bond, where hybridisation complemented concepts like resonance to model complex bonding.[46] Other contributors included John C. Slater, who collaborated with Pauling on VB applications and emphasized equivalent orbitals in polyatomic molecules around 1930–1931, and Robert S. Mulliken and J. H. Van Vleck, who independently coined the terms "hybrid atomic orbitals" and "hybridization" in papers from 1932–1935 to describe linear combinations of atomic orbitals for bonding purposes.[47] These developments marked a shift toward intuitive, semi-quantitative models that influenced organic chemistry education and structure prediction, though later quantum computations revealed nuances in orbital mixing that Pauling's qualitative approach approximated rather than precisely calculated.[45]Types of Hybrid Orbitals
sp hybridization involves the linear combination of one s orbital and one p orbital, producing two equivalent sp hybrid orbitals oriented at 180° to each other, resulting in linear molecular geometry./Fundamentals/Hybrid_Orbitals) This type accommodates two sigma bonds and is exemplified in beryllium chloride (BeCl₂), where the central beryllium atom forms two linear bonds, and in acetylene (C₂H₂), where each carbon atom uses sp hybrids for the C–C triple bond's sigma component.[48] sp² hybridization arises from mixing one s orbital with two p orbitals, forming three sp² hybrid orbitals in a trigonal planar arrangement with 120° bond angles./Fundamentals/Hybrid_Orbitals) Each hybrid contains 33% s-character and 67% p-character, enabling three sigma bonds; a remaining p orbital is available for pi bonding.[37] Examples include boron trifluoride (BF₃), with trigonal planar geometry around boron, and ethene (C₂H₄), where sp² carbons facilitate the double bond./10%3A_Chemical_Bonding_II-_Valance_Bond_Theory_and_Molecular_Orbital_Theory/10.07%3A_Valence_Bond_Theory-_Hybridization_of_Atomic_Orbitals) sp³ hybridization combines one s and three p orbitals to yield four sp³ hybrid orbitals arranged tetrahedrally at 109.5° angles, each with 25% s-character and 75% p-character./Fundamentals/Hybrid_Orbitals) This configuration supports four sigma bonds or a mix with lone pairs, as in methane (CH₄) for tetrahedral bonding or ammonia (NH₃) where one hybrid holds a lone pair, slightly distorting angles to 107°.[48] For central atoms requiring five or more bonding pairs, valence bond theory incorporates d orbitals, though this extension is primarily descriptive for main-group hypervalent molecules. sp³d hybridization mixes one s, three p, and one d orbital to form five equivalent orbitals in trigonal bipyramidal geometry, with axial positions at 90° to equatorial ones at 120°.[49] Phosphorus pentafluoride (PF₅) exemplifies this, with phosphorus using sp³d hybrids for five P–F bonds.[50] sp³d² hybridization involves one s, three p, and two d orbitals, producing six octahedral sp³d² hybrids with 90° angles between adjacent orbitals.[49] Sulfur hexafluoride (SF₆) demonstrates this, where sulfur's expanded octet is accommodated by octahedral arrangement.[50] Higher types like sp³d³, forming seven pentagonal bipyramidal orbitals, apply to iodonium heptafluoride (IF₇), though such cases are rare and limited to heavier elements with accessible d orbitals.[50]| Hybrid Type | Orbitals Mixed | Number of Hybrids | Geometry | Bond Angle(s) | Example Molecule |
|---|---|---|---|---|---|
| sp | s + p | 2 | Linear | 180° | BeCl₂, C₂H₂ |
| sp² | s + 2p | 3 | Trigonal planar | 120° | BF₃, C₂H₄ |
| sp³ | s + 3p | 4 | Tetrahedral | 109.5° | CH₄, NH₃ |
| sp³d | s + 3p + d | 5 | Trigonal bipyramidal | 90°, 120° | PF₅ |
| sp³d² | s + 3p + 2d | 6 | Octahedral | 90° | SF₆ |
| sp³d³ | s + 3p + 3d | 7 | Pentagonal bipyramidal | 72°, 90° | IF₇ |
Applications in Molecular Structure
Orbital hybridisation theory is applied in molecular structure to rationalize the observed geometries of molecules by describing how atomic orbitals mix to form hybrid orbitals that maximize overlap and determine bond angles. In valence bond theory, this mixing accounts for the directional properties of bonds, enabling predictions of shapes such as tetrahedral, trigonal planar, and linear arrangements that align with experimental data from techniques like X-ray crystallography and electron diffraction.[51][52] For sp³ hybridisation, the central atom uses one s and three p orbitals to form four equivalent hybrid orbitals arranged tetrahedrally, as seen in methane (CH₄), where the C–H bond angle is 109.5°. This configuration explains the regular tetrahedral structure confirmed by spectroscopic measurements, and it extends to other saturated hydrocarbons like ethane (C₂H₆), where each carbon adopts sp³ hybridisation for sigma bonds.[53][54] In sp² hybridisation, one s and two p orbitals mix to produce three hybrid orbitals in a trigonal planar arrangement with 120° bond angles, exemplified by boron trifluoride (BF₃), where the boron atom's empty p orbital allows for planar geometry observed in its electron-deficient structure. Ethene (C₂H₄) demonstrates this with each carbon forming three sp² orbitals for sigma bonds in the plane, while the remaining p orbitals overlap sideways for the pi bond, yielding a planar molecule with measured C–C–H angles near 120°.[53][54] sp hybridisation involves one s and one p orbital forming two linear hybrid orbitals separated by 180°, as in acetylene (C₂H₂), where the carbon atoms' triple bond consists of one sigma and two pi bonds, resulting in a linear HC≡CH structure verified by rotational spectroscopy. Beryllium chloride (BeCl₂) in the gas phase similarly adopts sp hybridisation for its linear Cl–Be–Cl arrangement, reflecting the atom's two-electron valence shell.[53][54] These applications facilitate the interpretation of molecular reactivity and properties, such as the planarity influencing conjugation in unsaturated systems or tetrahedral carbons dictating stereochemistry in organic molecules, though hybridisation serves as an approximate model within valence bond theory rather than a literal atomic reconfiguration.[55][56]| Hybridisation Type | Geometry | Bond Angle | Example Molecule |
|---|---|---|---|
| sp³ | Tetrahedral | 109.5° | CH₄ |
| sp² | Trigonal planar | 120° | C₂H₄, BF₃ |
| sp | Linear | 180° | C₂H₂, BeCl₂ |