Exhibit 2.1
 Execution Version
 PURCHASE AGREEMENT
 BY AND AMONG
 CONSTELLATION ENERGY COMMODITIES GROUP, INC.,
CONSTELLATION ENERGY PARTNERS HOLDINGS, LLC,
AND
CONSTELLATION ENERGY PARTNERS MANAGEMENT, LLC
 AS SELLING PARTIES
 AND
 POSTROCK ENERGY CORPORATION
 AS BUYER




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TABLE OF CONTENTS
 PageARTICLE I DEFINITIONS
    
    
    1Section 1.01 Definitions
    
    
    1Section 1.02 Rules of Interpretation
    
    
    7ARTICLE II SALE AND PURCHASE
    
    
    8Section 2.01 Sale and Purchase
    
    
    8Section 2.02 Closing
    
    
    10Section 2.03 Post-Closing Payments for Distributions on Class D Subject Interests
    
    
    10ARTICLE III REPRESENTATIONS AND WARRANTIES REGARDING THE SELLING PARTIES AND THE SUBJECT INTERESTS
    
    
    10Section 3.01 Existence
    
    
    10Section 3.02 Validity of Agreement; Authorization
    
    
    10Section 3.03 Consents and Approvals
    
    
    11Section 3.04 No Breach
    
    
    11Section 3.05 Ownership, Due Authorization and Transfer of Subject Interests
    
    
    11Section 3.06 Litigation
    
    
    12Section 3.07 Financial Advisors
    
    
    12Section 3.08 Matters Relating to Acquisition of Buyer Common Stock
    
    
    12Section 3.09 Solvency
    
    
    12Section 3.10 No Other Representations and Warranties
    
    
    13ARTICLE IV REPRESENTATIONS AND WARRANTIES REGARDING THE COMPANY
    
    
    13Section 4.01 Existence
    
    
    13Section 4.02 Capitalization and Valid Issuance of Company Equity Interests
    
    
    13Section 4.03 Company SEC Documents
    
    
    14Section 4.04 No Material Adverse Change
    
    
    15Section 4.05 Litigation
    
    
    15Section 4.06 Environmental Laws
    
    
    15Section 4.07 Oil and Gas Properties
    
    
    15Section 4.08 Compliance with Laws
    
    
    16
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Table of Contents
 PageSection 4.09 Investment Company Status
    
    
    16Section 4.10 Related Party Agreements
    
    
    16Section 4.11 Employee Compensation Matters; No Termination
    
    
    16Section 4.12 Tax Matters
    
    
    16ARTICLE V REPRESENTATIONS AND WARRANTIES OF BUYER
    
    
    17Section 5.01 Existence
    
    
    17Section 5.02 Validity of Agreement; Authorization
    
    
    17Section 5.03 Consents and Approvals
    
    
    17Section 5.04 No Breach
    
    
    17Section 5.05 Capitalization; Valid Issuance of Buyer Common Stock
    
    
    18Section 5.06 Buyer SEC Documents
    
    
    18Section 5.07 No Material Adverse Change
    
    
    19Section 5.08 Litigation
    
    
    19Section 5.09 Environmental Laws
    
    
    19Section 5.10 Oil and Gas Properties
    
    
    20Section 5.11 Taxes
    
    
    20Section 5.12 Compliance with Laws
    
    
    20Section 5.13 Investment Company Status
    
    
    20Section 5.14 Matters Relating to Acquisition of the Subject Interests
    
    
    20Section 5.15 No Other Representations and Warranties
    
    
    21ARTICLE VI COVENANTS
    
    
    21Section 6.01 Conduct Pending the Closing
    
    
    21Section 6.02 Investigation of Business; Access to Properties and Records;
Notification of Certain Matters
    
    
    23Section 6.03 Cooperation
    
    
    24Section 6.04 Right to Update
    
    
    25Section 6.05 Listing of the Additional Shares
    
    
    25Section 6.06 Non-Disclosure; Interim Public Filings
    
    
    25Section 6.07 No Shop
    
    
    26Section 6.08 Marks
    
    
    26
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Table of Contents
 PageSection 6.09 Class A Managers
    
    
    27Section 6.10 Trading and Manipulation
    
    
    27Section 6.11 Buyer Stockholders Meeting
    
    
    27Section 6.12 Confidential Information
    
    
    28ARTICLE VII CLOSING
    
    
    28Section 7.01 Conditions Precedent to Obligations of Buyer
    
    
    28Section 7.02 Conditions Precedent to Obligations of the Selling Parties
    
    
    29Section 7.03 Selling Party Deliveries
    
    
    30Section 7.04 Buyer Deliveries
    
    
    31ARTICLE VIII INDEMNIFICATION, COSTS AND EXPENSES
    
    
    32Section 8.01 Survival of Representations and Warranties
    
    
    32Section 8.02 Indemnification
    
    
    32Section 8.03 Indemnification Procedure
    
    
    33Section 8.04 Limitations
    
    
    34Section 8.05 Recovery
    
    
    35Section 8.06 Tax Treatment of Indemnity Payments
    
    
    35Section 8.07 Exclusive Remedy
    
    
    35ARTICLE IX TAX MATTERS
    
    
    36Section 9.01 Pre- and Post-Closing Actions; Post-Closing Assistance
    
    
    36Section 9.02 Transfer Taxes
    
    
    36ARTICLE X TERMINATION
    
    
    36Section 10.01 Termination of Agreement
    
    
    36Section 10.02 Procedure Upon Termination
    
    
    37Section 10.03 Effect of Termination
    
    
    37ARTICLE XI MISCELLANEOUS
    
    
    38Section 11.01 Expenses
    
    
    38Section 11.02 Specific Performance
    
    
    38Section 11.03 Arbitration
    
    
    38Section 11.04 Submission to Jurisdiction; Consent to Service of Process
    
    
    39
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Table of Contents
 PageSection 11.05 Entire Agreement; Amendments and Waivers
    
    
    39Section 11.06 Governing Law
    
    
    40Section 11.07 Notices
    
    
    40Section 11.08 Severability
    
    
    41Section 11.09 Binding Effect; Assignment
    
    
    41Section 11.10 Counterparts
    
    
    42
 EXHIBITS
 Exhibit A
    
    Form of Registration Rights AgreementExhibit B
    
    Form of Warrant
 SCHEDULES
 Schedule 3.03
    
    Selling Parties Consents and ApprovalsSchedule 5.03
    
    Buyers Consents and Approvals
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PURCHASE AGREEMENT
 Agreement CECG CEPH CEPM Selling Parties Buyer
   Company Class B Subject Interests Class A Subject Interests Class C Subject
Interests Class D Subject Interests Subject Interests
   ARTICLE I  DEFINITIONS
 Definitions
 AAA Section 11.03(a)
 Acquisition Transaction Section 6.07(a)
 Action
 Affiliate




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control
 Agreement
 Arbitration Rules Section 11.03(a)
 Arbitrator Section 11.03(d)
 Basic Documents
 Board of Managers
 Business Day
 Business Records
 Buyer
 Buyer/Affiliate Proxy and Voting Agreement
 Buyer Common Stock Section 2.01(a)
 Buyer Financial Statements Section
5.06(c)
 Buyer Indemnified Parties Section
8.02(a)
 Buyer Material Adverse Effect
 2




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Buyer SEC Documents Section 5.06(a)
 Buyer Stockholder Approval Section
7.01(g)
 Buyer Stockholders Meeting Section
6.11(b)
 Buyer Stockholder Proposal Section
6.11(b)
 Buyer Voting Stock
 Buyer Voting Preferred Stock Section
5.05(a)
 Cash Consideration Section 2.01(a)
 CECG
 CEPH
 CEPM
 Claim Deductible Section 8.04(a)
 Class A Managers
 Class A Subject Interests
 Class B Subject Interests
 Class C Subject Interests
 Class D Distribution Section 2.03(a)
 Class D Subject Interests
 Class A Units
 Class B Units
 Class C Units
 3




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Class D Units
 Closing Section 2.02
 Closing Date Section 2.02
 Code
 Commission
 Company
 Company Board Approval Section 7.01(f)
 Company Equity Interests
 Company Financial Statements Section
4.03(c)
 Company LLC Agreement
 Company Material Adverse Effect
 Company SEC Documents Section 4.03(a)
 DGCL
 DLLCA
 Encumbrances
 Environmental Law
 4




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Exchange Act
 GAAP
 Governmental Authority
 Indemnified Party
 Indemnity Notice Section 8.03(c)
 Issue Price
 Law
 LIBOR
 Loss Section 8.02(a)
 Market Price
 NASDAQ
 Organizational Document
 5




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Outside Date th Section 7.01(f) Section 7.02(e)
 Party
 Permitted Exceptions provided
 Person
 Purchase Price Section 2.01(a)
 Registration Rights Agreement Exhibit A
 Representatives
 Securities Act
 Seller Indemnified Parties Section
8.02(b)
 Seller Material Adverse Effect
 Selling Parties
 6




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Solvent Section 3.09
 Stock Consideration Section 2.01(a)
 Subject Interests
 Superior Sale Consideration plus plus plus Section
2.01(d)
 Superior Sale Transaction
 Tax
 Tax Returns
 7




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Third Party Claim Section 8.03(b)
 Title 9 Section 11.03(a)
 Trademark Licensing Agreement
 Transfer Taxes Section 9.02
 Warrants Section 2.01(d)
 Warrant Consideration Section 2.01(d)
 Rules of Interpretation
         8




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ARTICLE II  SALE AND PURCHASE
 Sale and Purchase
 Section 2.01(c) Cash Consideration Buyer Common
Stock Section 2.01(b) Stock Consideration Section
2.01(b) Section 2.01(d) Section 2.03 Purchase Price
 Stock Consideration
 Cash Consideration
 Warrant Consideration Warrants Exhibit B Warrant Consideration
 9




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provided, however Section 10.01(b) Section 10.03(b)
   Closing Closing Closing Date
 10




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Post-Closing Payments for Distributions on Class D Subject Interests
 Class D Distribution
 Section 2.03(a) provided, however
  ARTICLE III  REPRESENTATIONS AND WARRANTIES REGARDING  THE SELLING PARTIES AND THE SUBJECT INTERESTS
  Existence
 Validity of Agreement; Authorization
 11




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Consents and Approvals Schedule 3.03
 No Breach
 Ownership, Due Authorization and Transfer of Subject Interests
   12




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Litigation
 Financial Advisors
 Matters Relating to Acquisition of Buyer Common Stock
 Solvency Section 3.09 Solvent
 13




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No Other Representations and Warranties
 ARTICLE III ARTICLE IV
  ARTICLE IV  REPRESENTATIONS AND WARRANTIES REGARDING THE COMPANY
  Existence
 Capitalization and Valid Issuance of Company Equity Interests
    14




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Section 3.05(c)
 provided
 Company SEC Documents Company SEC Documents Company Financial Statements
 15




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No Material Adverse Change
 Litigation
 Environmental Laws
  Section 4.06
 Oil and Gas Properties
 Compliance with Laws
 16




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Investment Company Status
 Related Party Agreements
 Employee Compensation Matters; No Termination
 Tax Matters.
  17




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ARTICLE V  REPRESENTATIONS AND WARRANTIES OF BUYER
  Existence
 Validity of Agreement; Authorization
 Consents and Approvals Schedule 5.03
 No Breach
 18




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Capitalization; Valid Issuance of Buyer Common Stock
 Buyer Voting Preferred Stock
 provided however
  19




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Buyer SEC Documents Buyer SEC Documents Buyer Financial Statements
 No Material Adverse Change
 Litigation
 Environmental Laws
  20




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Section 5.09
 Oil and Gas Properties
 Taxes
 Compliance with Laws
 21




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Investment Company Status
 Matters Relating to Acquisition of the Subject Interests
 No Other Representations and Warranties
 ARTICLE V
  ARTICLE VI  COVENANTS
 Conduct Pending the Closing
    22




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Sections 6.01(b)(i) 6.01(b)(ii)
     Section 6.01
     23




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Section 6.01
 Investigation of Business; Access to Properties and Records; Notification of
Certain Matters
  24




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Article VII
 Article VII
 Section 7.01 Section 7.02
 Cooperation
  25




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Right to Update Article VII Article X Article
X Article X
 Listing of the Additional Shares
 26




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Non-Disclosure; Interim Public Filings provided however
 No Shop
 Acquisition Transaction
  27




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Marks
 Class A Managers
 Trading and Manipulation Section 6.10
 Buyer Stockholders Meeting
  Buyer Stockholders Meeting Buyer Stockholder Proposal
 28




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Confidential Information
       ARTICLE VII  CLOSING
 Conditions Precedent to Obligations of Buyer
  29




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Company Board Approval
 Buyer Stockholder Approval
 Section 7.03
 Conditions Precedent to Obligations of the Selling Parties
  Section 6.10
 30




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Section
7.04
 Selling Party Deliveries
   Sections 7.01(a) 7.01(d)
 31




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Buyer Deliveries
  Section 2.01
   Sections 7.02(a) 7.02(d)
   32




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ARTICLE VIII  INDEMNIFICATION, COSTS AND EXPENSES
 Survival of Representations and Warranties Sections 3.01 3.02 3.05 5.01 5.02 5.05(c) provided Sections
8.02(a)(i) 8.02(b)(i) Section 8.03(a)
 Indemnification
 Section 8.01 Buyer Indemnified
Parties Loss
   Section 8.01 Seller Indemnified
Parties
   33




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Indemnification Procedure
 provided Article VIII
 Section
8.02 Third Party Claim Section 8.03 provided provided provided further Section 8.03
 34




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Article VIII Indemnity Notice Article
VIII Section 11.03
 Limitations
 Section
8.02(a)(i) Claim Deductible Section 8.04(c)
 Section
8.02(b)(i) Section
8.04(c)
 Section 2.01(a)
  35




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Recovery
 Tax Treatment of Indemnity Payments Article VIII
 Exclusive Remedy
 Article VIII provided Section 8.02
 Section 8.07(a) Section 8.07
 36




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ARTICLE IX  TAX MATTERS
 Pre- and Post-Closing Actions; Post-Closing Assistance
 Transfer Taxes Transfer Taxes
 ARTICLE X  TERMINATION
 Termination of Agreement
  th Sections 7.01(f) 7.02(e)
 provided Section
10.01(c) Section 10.01(c)
 37




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provided Section 10.01(f)
 Sections 7.01(a) 7.01(b)
 Sections 7.02(a) 7.02(b)
 Procedure Upon Termination Section 10.01
 Effect of Termination
 provided Section 10.03 Article XI provided further Section 10.03
 38




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Section 10.01(b)(i) th Section 10.01(b)(ii)
     ARTICLE XI  MISCELLANEOUS
 Expenses
 Specific Performance
 39




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Arbitration
 Mandatory Arbitration Title 9 Arbitration Rules AAA Section 11.03
 Provisional or Ancillary Remedies; Specific Performance Section 11.03 Section 11.02 Section 11.03 Section 11.03
 Arbitration Orders Section 11.04
 Arbitrators Arbitrator
 40




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Location
 Fees and Expenses Section
11.03
 Title 9 Section 11.03 Section 11.06
 Submission to Jurisdiction; Consent to Service of Process Section 11.03
 Entire Agreement; Amendments and Waivers
 41




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Governing Law
 Notices
  
  th
  th
  
 42




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Severability
 Binding Effect; Assignment Sections 8.02(a) 8.02(b) provided, however provided further, however
 Counterparts
  43




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BUYER:
POSTROCK ENERGY CORPORATIONBy:
    /s/ Stephen L. DeGiuistiName:
    Stephen L. DeGiustiTitle:
    Executive Vice President, General Counsel
and Secretary Signature Page 1 of 2 to
Purchase Agreement




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SELLING PARTIES
CONSTELLATION ENERGY COMMODITIES GROUP, INC.By:
    /s/ Kathleen W. HyleName:
    Kathleen W. HyleTitle:
    PresidentCONSTELLATION ENERGY PARTNERS HOLDINGS, LLCBy:
    /s/ Kathleen W. HyleName:
    Kathleen W. HyleTitle:
    PresidentCONSTELLATION ENERGY PARTNERS MANAGEMENT, LLCBy:
    /s/ Kathleen W. HyleName:
    Kathleen W. HyleTitle:
    President Signature Page 2 of 2 to
Purchase Agreement